Reducing RFI (Radio Frequency Interference) for a POE (Power Over Ethernet) camera or wireless access point

One of the (many) banes of the amateur radio operator's existence is often found at the end of an Ethernet cable - specifically a device that is being powered via "Ethernet":  It is often the case that interference - from HF through UHF - emanates from such devices.

Figure 1:
POE camera with both snap-on ferrites installed -
including one as close to the camera as possible -
and other snap-on/toroids to suppress HF through VHF.
Click on the image for a larger version.

Why this happens

Ethernet by itself is usually relatively quiet from an (HF) RF standpoint:  The base frequency of modern 100 Megabit and gigabit Ethernet is typically above much of HF and owing to the fact that the data lines are coupled via transformers making them inherently balanced and less prone to radiate.  Were this not the case, the integrity of the data itself would be strongly affected by the adjacent wires within the cable or even if the cable was routed near metallic objects as it would radiate a strong electromagnetic field - and any such coupling would surely affect the signal by causing reflections, attenuation, etc.

This is NOT the case with power that is run via the same (Ethernet) cable.  Typically, this power is sourced by a switching power supply - too often one that is not filtered well - and worse, the device at the far end of the cable (e.g. a camera or WiFi access point - to name two examples) is often built "down to a cost" and itself contains a switching voltage converter with rather poor filtering that is prone to radiation of RF energy over a wide spectrum.  Typically lacking effective common-mode filtering - particularly at HF frequencies (it would add expense and increase bulk) - the effect of RF radiating from the power-conducting wires in an Ethernet cable can be severe.

Even worse than this, Ethernet cables are typically long - often running in walls or ceilings - effectively making them long, wire antennas, capable of radiating (and intercepting) signals even at HF.  The "noisy" power supply at one or both ends of this cable can act as transmitters.

What to do

While some POE configurations convey the DC power on the "spare" conductors in an eight conductor cable (e.g. the blue and brown pairs), some versions use the data pairs themselves (often using center-tapped transformers in the Ethernet PHY) meaning that it may not be easy to filter just the DC power.

While it is theoretically possible to extract the power from the Ethernet cable, filter it and and reinsert it on the cable, the various (different) methods of doing this complicate the matters and doing so - particularly if the DC is carried on the data pairs - can degrade the data integrity by requiring the data to transit two transforms incurring potential signal attenuation, additional reflection and affecting frequency response - to name just a few.  Doing this is complicated by the fact that the method of power conveyance varies as you may not know which method is used by your device(s).

It is possible to subject the entire cable and its conductors to a common-mode inductance to help quash RFI - but this must be done carefully to maintain signal integrity.

Comment: 

Some POE cameras also have a coaxial power jack that permits it to be powered locally rather than needing to use POE.  I've observed that it is often the case that using this local power - which is often 12-24 volts DC (depending on the device) - will greatly reduce the noise/interference generated by the camera and conducted on the cable - provided, of course, that the power supply itself is not a noise source.  Even if a power supply is used near the camera, I would still suggest putting its DC power cable through ferrite devices as described below to further-reduce possible emissions.

There are some devices (such as those sold by DX Engineering) that are essentially back-to-back signal transformers that can reduce radiation of signals from Ethernet cable, but these typically do not permit the passage of power and are not candidates for use with POE devices.

Ferrite can be your friend

For VHF and UHF, simple snap-on ferrites can significantly attenuate the conduction of RF along, but these devices are unlikely to be effective at HF - particularly on the lower bands - as they simply cannot add enough impedance at lower frequencies.

To effectively reduce the conduction of RF energy at HF, one could wrap the Ethernet cable around a ferrite toroidal core, but this is often fraught with peril, particularly with cable carrying Gigabit Ethernet - as tight radius turns can distort the geometry of typical CAT-5/6 cable, affect the impedance and cause cross-coupling into other wire pairs.  If this happens, one often finds that the Ethernet cable doesn't work reliably at Gigabit speeds anymore (being stuck at 100 or even 10 Megabits/second) or starts to "flap" - switching between different speeds and/or slowing down due to retransmissions on the LAN.

One type of Ethernet cable that is quite resistant to geometric distortion caused by wrapping around a toroidal core is the flat Ethernet cable (sometimes erroneously referred to as "CAT6" or "CAT7").  This cable is available as short jumpers around 6 feet (2 meters) long and, with the aid of a female-female 8P8C (often called "RJ-45") coupler can be inserted into an existing Ethernet cable run - just be sure that it is from a reputable source and rated for "Gig-E" service.  As it is quite forgiving to being wrapped around ferrites, this flat cable can be pre-wound with such devices and inserted at the Ethernet switch end and/or the device end at a later time.  I have found that with reasonable quality cable and couplers that this does not seem to degrade the integrity of the data on the LAN cable - at least for moderate lengths (e.g. 50 feet/15 meters or less) - your mileage may vary with very long cable runs.

As the flat cable and female-female Ethernet coupler are to be inserted into the cable run, they must be of known, good quality so it is best to test the couplers and cable that you obtain prior to installation to be sure that their use doesn't cause a reduction in signal quality/speed.

Practical examples

Best attenuation across HF

Figure 2:
Three toroids wound on "flat" Ethernet cable.  An FT114-43
is used on each end with an FT114-31 in the middle.
Click on the image for a larger version.

Using a test fixture with a VNA, I determined that for best overall attenuation across the entire HF spectrum I needed three ferrite toroids on the 2 meter long flat Ethernet jumper.  All three of these were FT-114 size (1.14", 29mm O.D.) with the first and last being of material type 43 and the center being type 31:  Both types 31 and 43 offer good impedance to low HF but 43 is more effective on the higher bands - namely 10 and 6 meters - and types 52 and 61 may be useful at even higher bands:  The three toroids, separated by a few inches/cm, offer better all-around rejection from 160 meters through 10/6 meters than just one.  Having said this, it is unrealistic to expect more than 20dB or so of attenuation to be afforded by ferrite devices at high HF/low VHF - "because physics".

One might be tempted to use the more-available FT-240 size of toroids (2.4", 60mm O.D.) but this is unnecessarily large, comparatively fragile and expensive:  While you can fit more turns on the larger toroid, one hits the "point of diminishing returns" (e.g. little improvement with additional turns) very quickly owing to the nature of the ferrite and coupling between turns.  Using the FT114 or FT140 sizes is the best balance as it may be much less expensive than a larger device, it can accept 6-8 turns with the cable's connector installed, and more than 6-8 turns is rapidly approaching the point of diminishing returns for a single ferrite device at upper HF frequencies, anyway.

In bench testing with a fixture, it was found that three toroids on a piece of flat Ethernet cable provided the best, overall attenuation across HF and to 6 meters - significantly better than any combination of FT114, FT140 or FT240 toroids of either 43 or 31 mix alone:  Figure 2, above, shows what this looks like.  Two FT114-43 and one FT114-31 toroid were used - the #31 toroid being placed in the center, providing the majority of series impedance at low HF and a #43 at each end being more effective at higher HF through 6 meters.

To construct this, the flat Ethernet cable was then marked with a silver marker in the center and four turns were wound from each end, in turn, for a total of eight turns on the FT114-31.  Placing an FT114-43 at 12 inches (25cm) and winding seven turns puts the FT114-43 fairly close to each connector, allowing the installation of one or two snap-on ferrites very close  to the connector if it is determined that more suppression is required to suppress radiation at VHF frequencies.  Small zip-ties (not shown in Figure 2) are used to help keep the turns from bunching up too much and also to prevent the start and stop turns from getting too close to each other:  Do not cinch these ties up enough to distort the geometry of the Ethernet cable as that could impact speed - particularly when using Gig Ethernet.

It is important that, as much as possible, one NOT place a "noisy" cable in a bundle with other cables or to loop it back onto itself - both of which could cause inadvertent coupling of the RFI that you are trying to suppress into the other conductors - or to the far side of the cable you are installing.

Best attenuation at VHF and HF

If you are experiencing interference from HF through VHF, you will need to take a hybrid approach:  The use of appropriate snap-on and toroidal ferrite devices.  While snap-on ferrite devices are not particularly useful for HF - especially below about 20 MHz - they can be quite effective at VHF, which is to be expected as that is the purpose for which they are typically designed.  Similarly, a ferrite toroid such as that described above - particularly with type 43 or 31 material - will likely have little effect on VHF radiation - particularly in the near field.

Figure 3:
A combination of a snap-on device with an extra turn looped
through it and two ferrites to offer wide-band suppression
from HF through VHF.
Click on the image for a larger version.

Figure 3 shows such a hybrid approach with a snap-on device on the left and two toroids on the right to better-suppress a wider range of frequencies.  In this case it is important that the snap-on device be placed as close to the interference source as possible (typically the camera) as even short lead lengths can function as effective antennas at VHF/UHF.  You may also notice that the snap-on has two turns through its center as this greatly improves efficacy at medium/low VHF frequencies but may be counter-productive at high VHF/UHF frequencies owing to coupling between turns.

Doing this by itself is not likely to be as effective in reducing radiation at VHF/UHF from the cable itself, often requiring the placement of additional ferrite devices.  Figure 1 shows the installation of several snap-on devices placed as close to the POE camera as physically possible - mainly to reduce radiation at VHF and UHF as at those frequencies where even a few inches or centimeters of cable emerging from the noise-generating device can act as an effective antenna.

Determining efficacy

During the installation of these devices on my POE cameras I was interested in how much attenuation would be afforded at VHF:  Since I'd already used the "chokes on a flat cable" approach like that in Figures 2 and 3 I knew that this would likely be as effective as was practical at HF - but because the VHF/UHF noise could be radiated by comparatively short lengths of "noisy" cable - and that the 43 and 31 mix ferrites were probably not as effective at those frequencies - I needed to be able to quantify that what I did made a difference - or not.

Figure 4:
The cable in Figure 3 installed, but not yet
tucked into place as depicted in Figure 1.
(This does not show the snap-on ferrites installed
where the wire exits the camera housing.)
The female-female RJ45/8P8C "splice" can be
seen in the upper-left corner of the picture.
Click on the image for a larger version.

For HF this was quite simple:  I simply tuned my HF receiver - connected to my main antenna - to a frequency where I knew that I could hear the noise from the cameras and compared S-meter readings with the system powered up and powered down.  This approach is best done at a time during which the frequency in question is "dead" or at least weak (e.g. poor propagation) - 80/40 meters during the midday and 15/10 meters at night is typical.

For VHF this required a bit more specialized equipment.  My "Go-To" device for finding VHF signals - including noise - is my VK3YNG DF sniffer which has extremely good sensitivity - but it also has an audible "S-meter" in terms of a tone that rises with increasing signal level:  This allowed an "eyes and hands off" approach in determining efficacy of the installation of a ferrite device simply by hearing the pitch of the tone..  Switching it to this mode and placing it and its antenna at a constant distance fairly close to the device being investigated allowed me to "hear" - in the form of a lower-pitched tone - whether or not the application of a ferrite device made a difference.

Slightly less exotic would be an all-mode receiver capable of tuning 2 meters such as the Yaesu FT-817, Icom IC-706, 703 or 705.  In this case the AM mode would be selected and the RF gain control advanced such that the noise amplitude audibly decreased:  This step is important as not doing this could mean that if the noise decreased, the AGC in the receiver would simply compensate and hiding the fact that the signal level changed.  By listening for a decrease in the noise level one can "hear" when installing a snap-on ferrite made a difference - or not.

One cannot use a receiver in FM mode for this as an FM detector is designed to produce the same amount of audio (including noise) at any signal level:  A strong noise source and a weak one will sound exactly the same.  It's also worth noting that the S-meter on a receiver in FM mode - or an FM-only receiver - are typically terrible in the sense that their indications typically start with a very low signal and "peg" the meter at a signal that isn't very strong at all which means that if you try to use one, you'll have to situate the receiver/antenna such that you get a reading that is neither full-scale or at the bottom of the scale to leave room for the indication of change.

Of course, a device like a "Tiny SA" (Spectrum Analyzer) could be used to provide a visual indication, using the "Display Line", markers and stored traces to allow a quick "before and after" determination.  As mentioned above, one would want to place the antenna and the receiving device (an actual receiver or spectrum analyzer) at a fairly close distance to the device being investigated - but keep it and its antenna in precisely the same location (or connected to a fixed-location antenna) during the entire time so that one can get meaningful "before and after" readings.

Conclusion

With the use of ferrites alone, one should not expect to be able to completely suppress radiation of RF noise from an Ethernet cable - the typical maximum to be reasonably expected is on the order of about 20dB (a bit over 3 "S" units) and this can vary wildly with frequency.  In a situation where the POE device is very close to the antenna, it may not be possible to knock the interference down to the point of inaudibility in which case relocation to place the two farther apart - or trying similar devices of different models/brands to try to find a combination to reduce it..

The most effective use will be for noise sources will be at some distance from the receive antenna - particularly if a long cable is used that may act as an antenna.  Additionally, these measures can be effective in situations where your transmitter causes problems with the device itself due to ingress of RF energy along the Ethernet cable.

Be prepared to install appropriate ferrite devices at both ends of the cable as it's often the case that not only does the POE device itself (camera, wireless device) radiates noise but also the POE switch itself:  No-name brand POE power supplies and Ethernet switches are, themselves often very noisy and the proper course of action would be to first swap out the supply or POE switch with a known quiet device before attaching ferrite.

As every interference situation is unique, your mileage may vary, and the best road to success is being able to quantify that changes you have made made things better or worse.

This page stolen from ka7oei.blogspot.com

[END]

On the winding of power chokes and transformers: Part 1 - A power choke

There are follow-up to this posts to this one:

• The Filament Transformer (link)  - Designing and building a transformer with two 11.5 volt, 11 amp secondaries.
• The Plate (High Voltage) Transformer (link) - Designing and building a transformer with high voltage and low voltage, medium current secondaries.

There are projects that homebrewers undertake that seem not to make sense when one considers the amount of time that it takes to do that thing - but then again, this is often the case when one builds projects at home, from "scratch".  The case in point for this discussion is the winding of power transformers and chokes.

Figure 1:
The finished choke with a rating of about 50 Henries at
200 milliamps, described below.
Click on the image for a larger version.

I am not an expert in the design and building of power transformers and chokes so what follows is, among other things, the documentation of the learning experience.  No doubt I will make mistakes along the way, but analysis of these mistakes and observations will (hopefully) be enlightening.

Why?

For some reason I have decided to go "old school" and, along with a friend (Bryan, W7CBM) we will each construct a two-channel single-ended triode audio amplifier using some WWII vintage tubes (the details of this amplifier to be described in a later post.)  To be clear I'm not of the sort that really believes that the "tube sound" is best for various intangible reasons - in fact, I'm expecting that if it works better than expected, its performance will be worse than practically any other modern audio amplifier that I have in the ways that matter to most people (e.g. power efficiency, noise, hum, distortion, frequency flatness, phase - just name it!) That being said, the goal is definitely to end up with an amplifier that at least sounds "nice."

As it is often the case with a project like this, practicality is somewhat irrelevant:  It is the experience of doing something that you have never done before - and learning from it - that will be the reward whether or not the project is ultimately successful.  To be sure there will not likely be a cost savings in doing this and there certainly will not a time savings!

Diving into this project we decided to do something that I'd not done for many years:  Wind my own power transformers and chokes.  As for the audio output transformers for this audio project we decided to forgo that task and get "store bought", proven transformers given our current level of experience.

Where to find useful information:

Interestingly, scant useful and credible information seems to be available on the GoogleWeb on this topic, but two sources that are useful are:

1. Turner Audio (link) - These pages contain much practical advice on power and audio transformers and chokes.  (Refer to the link "Power Transformers and Chokes" (link) and related pages linked from that page.)



Figure 2:
The NZ-1 "Hand Shake" winder (e.g. "hand cranked") winding
machine, before it was mounted to a firm base.
Click on the image for a larger version.

2. Homo-Ludens - Practical transformer winding (link) - While mostly about power transformers, this page also contain practical advice based on hands-on experience of winding, re-winding and reverse-engineering/rebuilding transformers.  There is also another linked page "Transformers and Coils" (link) that has additional information on this topic.

A few other bits and pieces of information have been found on the web, but the above two seem to be some of the best, both written by people who have years of practical experience, and the pages include additional references to other sources.

Rummaging through some old amateur radio and electronics books I have been able to divine other details as well, but much of this information is rather generic and doesn't speak much about the use and capabilities of modern types of wire, insulation or core materials.

Getting the gear together:

Figure 3:
 A simple wire spool holder made from pieces of "1x2"
(actually 0.75" x 1.5")  poplar.  While it works, the variable
feed rate of the wire due to winding it onto a square bobbin
can cause the momentum of the supply spool's rotation to
occasionally "overspool" some wire.  I hope to (soon) 
add additional wire features (e.g. guides, wire-unspool
prevention, etc.) to prevent the wire from getting
tangled as it is unreeled during winding.
Click on the image for a larger version.

Before laying a large number of turns on a transformer bobbin one must have a minimum of gear on-hand.  Starting out with nothing, I decided to buy an inexpensive ($40, delivered) Chinese-made "NZ-1 Hand Shake" (probably a mistranslation of "Hand Crank") coil winding machine (seen in Figure 2) via EvilBay.

Made mostly from cast steel, this device is about as simple as it can get:  It has some gears that provide an 8:1 ratio for the hand crank (e.g. one revolution of the crank = 8 turns on the coil), a 5-digit turns counter (0-99999) and a spindle on which one would mount the bobbin, and it seems to be built "well enough" for the purpose, likely to last many hundreds of thousands of revolutions.  This device has no wire-handling capability (e.g. nothing to hold the spool, guide the wire or provide tension) so those pieces would have to be constructed and/or improvised.

Figure 4:
 A simple bobbin holder made from scraps of "1x2"
poplar wood and paneling.  The left and middle pieces
are pinned to each other using finishing nails and
are clamped together when mounted on the winder.
"Face identification" marks allow one to keep track
and note the configuration of the wires and the visible
screw on the middle block - and its mate on the back-
side - allow wires attached to the various windings
to be kept out of the way.  To the right, in the background,
are two blocks that are used to help compress the
windings onto the bobbin as they bulge out when
many layers are added.
Click on the image for a larger version.

After the NZ-1 arrived I bolted it to a block of wood and for wire handling of the supply spool I made the simple wooden "A" frame holder seen in Figure 3, placing it on the workbench behind the winding machine. 

By default the crank is connected to a lower shaft that, via gears, results in eight turns on the coil with one turn of the crank. For practical reasons, winding a core like this needs a bit more precision and control and a 1:1 ratio is desired and the obvious place to relocate the crank is the upper shaft with the pulley, visible in Figure 2 on the upper-right corner of the winding machine.

While the shaft size is the same, attaching the crank at this point caused the crank's handle to hit the lower shaft to which the crank is usually connected!  Fortunately I could attach the crank to the very end of the shaft and have it just barely clear the lower shaft where the crank would normally be attached for the 8:1 ratio:  At some point I'll have made a metal piece made that will extend the shaft by a half inch (approximately 1cm) or so.  (Addendum:  After winding this choke Bryan did make a short extension piece so the crank can now be attached easily to achieve a 1:1 ratio.)

Also very important is the bobbin holder shown in Figure 4, constructed from scraps of paneling and "1x2" pieces of poplar.  Because the core size is "E150" - which stands for 1.50 inches, two pieces 1x2 wood (which are really 0.75"x1.5" or approximately 19x38mm) fit very nicely within the bobbin.  Glued to the end of the bobbin holder on one side is a scrap of paneling as an end-stop while on the other side, oriented by two finishing nails used as pins, are two more pieces of 1x2 glued together as the "other" end stop.  Both the bobbin and the removable end stop are marked to provide a means of orienting the four faces of the bobbin and the removable piece also sports screws around which wires that are brought out from the bobbin may be wound to keep them out of the way.

Two more blocks of wood, cut to the internal length of the bobbin, are also available to compress the "bulge" with a clamp that inevitably forms as wire is wound.  Through the center of the bobbin holder was drilled a hole of the size appropriate to allow it to be slipped over the shaft of the winding machine.  When mounted on the bobbin holder is held in by compression of the nut and it does not easily turn on its own.

While not the most elegant of solutions this arrangement seems to work - provided that one takes care to prevent "overfeeding" and the subsequent tangling of off-spooled wire caused by the inconsistent feed rate as one winds onto a square bobbin where the supply wire occasionally tries to wind itself around the piece of "allthread" on which the spool hangs!

The most important aspects of this (minimal) arrangement are:

1. A means of automatically counting the turns laid down.  While inductors are somewhat less critical in terms of keeping track of turns with absolute accuracy, when winding transformers, you do not want to lose count!
2. The bobbin itself being mounted on a stable, rotating platform about its winding axis.
3. Being able to manually gauge the tension and guide the wire into the bobbin.

While it would have been pretty easy to make a simple 1:1 gear ratio winding machine from parts laying about, the $40 price for the NZ-1 was hard to beat!

The goal:

Because a choke is "simpler" to wind than a transformer, I decided to start with that.

The design goal of the choke was to provide at least 10 Henries of inductance with a current capacity of at least 200mA.  The general rule of thumb for a modest-sized transformer or choke is to size the wire to 0.3 to 0.4 mm² per amp (from references 1 and 2, above) which would imply that for 200mA the minimum wire would need a cross-sectional area of around 0.06mm² which corresponds to #29 AWG.

Now we need to figure out (approximately) how many turns can fit on our bobbin.

The E150 bobbin that I would be using has an inside dimension of 41.5mm on a side and an outside dimension of 75.4mm on a side with a "width" of 53.3mm and based on this we calculate that the area of the bobbin's window is approximately 53mm wide and 17mm high, or about 901mm².

Because the cross sectional area of #29 AWG wire, based on a 0.33mm diameter with insulation, is 0.086mm² this implies that we could theoretically put around 10477 pieces of wire (or turns) in this 109mm² space.  Practically speaking, this is simply not possible since there is always going to be some "wastage" based on the fact that we will be laying round wires next to each other and there will be gaps in between.  Typically, one would scale this calculation by a "fill factor" to accommodate such wastage with this factor typically being around 0.4 (e.g. 4191 turns) or as high as 0.5 (5239 turns) if one winds extremely carefully and neatly - and assuming no added insulation:  Less careful winding can easily result in "fill factors" lower than 0.4.

Breaking this down differently we can calculate that across the width of the bobbin we should (theoretically) be able to put (53mm/0.33mm = 161) turns on each layer, but taking into account the practicalities of being able to get wire to lie snugly next to its neighbor we can more reasonably expect to achieve around 95% of this for wire of this rather small size, - about 153 turns per layer, maybe slightly more if we are fastidious.

Since our bobbin window is about 17mm "tall" we could theoretically expect to be able to stack 51 layers of 0.33 thick wire, but this also assumes both "perfect" stacking efficiency and the lack of any inter-layer insulation.  If we presume that each layer contains a 0.33mm thickness of wire and 0.05mm thickness of insulation - a total of 0.38mm - we can then recalculate that it would take about (17/0.38) 44 layers of windings to fill the bobbin - again, assuming perfect stacking and no need for additional insulation beyond that.

For practical reasons we would want to fill our bobbin to only 70-90% "fullness" in order to be able to add the final covering insulation and connecting wires, so this would take our maximum down to between 30 and 39 layers - again, assuming that we didn't need extra insulation anywhere.

If we take an average of 153 turns per layer we could reasonably expect be able to put between 4590 and 5957 turns on this bobbin.  Taking a median per-turn length of wire on the bobbin to be around 234 mm based on the size of the bobbin itself we can estimate that we'll need between 1074 and 1394 meters, or between 28 and 37% of our 5 pound spool.

Comment:

At this point in the design I should have referred more closely to link #1, above, to determine the amount of inductance that the calculated number of turns would yield in a typical situation.  To do this, I would have wound a known number of turns onto the bobbin and assembled the coil into a core and made inductance measurements so that the permeability - particularly that when an air gap was added (more on that later) - could be measured, but instead I simply bulldozed ahead and started winding turns!  What this means is that I probably would have wound a suitable coil with fewer turns and heavier-gauge wire.  More on this later.

Starting the wind:

With the necessary parts (bobbins and core material being "E150" size from Edcor, the wire and polyimide insulating tape from other sources) I began to wind a choke - the simplest of the devices and (hopefully) hardest to screw up!

Figure 5:
The start of the wind with the ending wire (orange) having been insulated and
secured with polyimide (a.k.a. "Kapton" (tm)) tape.  Note how the end wire
is secured to screws on the wooden bobbin holder to keep it out of the way.
I now know that his method of starting the winding and connecting the
end wire is not the best way:  I should have brought the joint into the
middle of the face of the bobbin and then folded the 29 AWG
wire back to the edge.  While this would have produced a lump in the
middle of the face of the winding, the edge of the bobbin and the
corners would have remained flat.
Click on the image for a larger version.

The first step in the wind was to drill a hole in the bobbin through which one of the external connecting wires would be attached.  Carefully scraping - to avoid nicking and weakening the copper - tinning, twisting and then soldering the start of the coil's wire (29 AWG) to the heavier lead that emerged from the bobbin, I insulated it with several layers of polyimide tape, secured it in place on the bobbin, laid down another insulating layer of tape on the bobbin itself and began to wind.

The general advice on winding transformers seems to be that it is best to assure that all wires are laid side-by-side on nice, even layers.  By doing this one avoids one turn from crossing another and putting tremendous point-pressure on the insulation of the two wires as they cross.  This advice hales from the days when wire was insulated with simple, comparatively fragile varnish that could be easily penetrated from vibration due to hum and thermally-related movement and by its softening at higher temperatures.

According to the Turner Audio web pages (reference #1) modern wire, with much more durable insulation, it is permissible to wind chokes that allow wires to cross at shallow angles, saving the need for expending time and effort on assuring a very consistent, parallel lay of wire on the bobbin.

Meanwhile, on the Homo Ludens web page (reference #2) he makes note that if the wire is of good quality and that one keeps to a very shallow angle of crossing - that is, slowly moving across the face of the bobbin as several layers "neatly" build up - and avoiding excess wire tension that even with transformers it is acceptable to forgo the painstaking "side by side" winding, provided that one insulate insulate each "layer" of 2-3 wire thicknesses before starting across the face of the bobbin in the other direction.  All of this assumes that one has enough room on the bobbin to accommodate the reduced "packing efficiency" of this method and will be satisfied with being able to fit fewer turns.

Both authors allude to having seen rather poor transformers that seem to haphazardly throw wire onto bobbins with the author of the Turner Audio pages being particularly critical of the methods that he'd observed on badly-built transformers.

No matter who you believe there are several things in common between these two sources of advice:

• Use good quality wire:  NEVER re-use wire from another transformer as it will likely have nicks and scratches in its insulation.  Modern, high-temperature wire (e.g. that rated for 200C) is more likely to handle a little bit of abuse.
• When wires are crossed, a very shallow angle is necessary.  This automatically rules out any "scramble wound" (a.k.a. "random wound") windings which can cross at sharper angles.
• While one can apply fairly significant tension when laying wires side-by-side with each layer being individually insulated, more care should be taken to avoid excess tension when doing the "not as neat" method of winding where wires can lay atop each other:  They should still be tight enough to not easily move around and wear in each other with thermal cycling and the inevitable "buzzing" from magnetic fields, but not so tight that the insulation of one turn will eventually "bite through" to the turn underneath!
• It is arguably more important that windings that are not carefully parallel-wound be immobilized with appropriate varnish - particularly on a transformer in which the windings are subject to constant vibration.
• It would seem to be more "acceptable" to forgo the neat, side-by-side winding on very small tranformers - particularly when very fine wire is involved.  While I suspect that at least some of this is due to practicality (e.g. it is difficult to machine-wind very fine wire in a nice, neat side-by-side lay) but it may also be due to more carefully-controlled wire tension, that these are typically vacuum-impregnated with varnish and the fact that a smaller transformer is more likely to have a more consistent thermal profile through its volume.  This is another way of saying that a small transformer has less opportunity to trap heat deep within the middle of some windings that it cannot easily dissipate.

Since this was my first large power choke I decided to take the "neat, even layer" approach for the most part, but what this meant was that during each and every turn I would have to watch exactly where the wire turn lay, often scrunching it close to its neighbor if it shifted away and "un-crossing" it if it happened to overlay another.  This also meant that after I finished winding every layer  of approximately 150-156 turns I had to stop, temporarily tape the source wire to the bobbin to keep in from unraveling, apply a layer of 2-mil (0.05mm) polyimide tape over the top of the layer and then resume winding.

As is the case with such things, it is a bit more difficult to do this than one might at first expect.  For the first several layers I had to contend with the "lump" and the "gap" at the "start" end of the bobbin where the external wire was attached and I did this by filling in the void with very thin strips of cardboard laid down to match the width and height of the heavier, connecting wire. After several layers, the layers equaled the height of this "lump" and with the matching thickness of the cardboard strips, it disappeared, allowing me to more easily wind across the entire width.  As noted in the caption of figure 5, I should have placed the splice in the center of the face of the winding rather than at the edge.

Figure 6: 
Pausing at the end of winding a layer, securing the end with a piece of
tape and small, plastic clamp to avoid wire damage.  Already one can see
ripples and uneven-ness starting to appear in the surface of the
winding - something that I had to continually correct as I continued on!
Click on the image for a larger version.

After a dozen or so layers the windings started to form "ripples" caused largely by the fact that the polyimide tape used to insulate between layers overlapped in approximately the same place each time - a problem exacerbated by the fact that I had, at the time, only one width of tape that limited the degree to which I could "stagger" the overlap. The biggest problem with this was not necessarily asthetics, but the fact that the as the wire was wound it tended to slip into the trough of the ripple making it very difficult to maintain evenness and the nice, side-by-side wire lay for which I was striving - and with each layer, these irregularities grew!

What I had to do was fold over some of the tape in half (sticky side facing outwards) to fill in some of the dips, and on the square outside "corners" between the faces of the bobbin I started to strategically add small bits of tape to level it out as it was there that the slippage was most likely to occur.  As I got into the 10s of layers, some of the troughs seemed to resist the attempts at leveling so I carefully resorted to putting several turns of wire atop each other at shallow angles to help even the surface:  Only a tiny fraction of the turns were wound in this way.

The lesson here is to have on hand multiple widths of tape so that the overlaps may be staggered.  What would have also been handy would have been to have on-hand some thick (10 mil, 0.5mm) pieces of MNM ("Nomex" (tm)) insulating sheets to cut into strips to fill the voids.  It would have also been very helpful to have some tape that was exactly as wide as the bobbin, but such a specific width is not likely to be commercially available.

The results:

Figure 7: 
The winding of the bobbin is finally complete!
At this point the windings are covered with several layers of polyimide
tape with the soldered and insulated end of the coil captured
underneath.  The uneven-ness of the coil's surface can be seen, but
the degree was just manageable.
Click on the image for a larger version.

Finally, after 4905 turns and 33 layers, the bobbin had mostly filled up:  This number of turns and layers ended up being within the range that I'd calculated above.

Based on the DC resistance - around 277 ohms - this represents approximately 3384 feet (1031 meters) of wire - a bit more than a quarter of the 5 pound spool.  Carefully drilling another hole in the bobbin I attached the "finish" wire to the outside of the coil, insulated it and taped it securely into place.

By itself (no core) the inductance of the 4905 turns on the bobbin measured out at approximately 884 millihenries and when the core was installed - butt-stacked, but not carefully aligned - this increased to over 110 Henries:  If I had carefully interleaved all 109 E-I laminations and properly seated each piece this value would have likely been much higher!

Preventing core saturation:

Because this is intended to be used as part of a "choke input" high voltage power supply one must take into account the fact that DC current will flow through the choke, magnetizing the core material.  One property of practically any ferromagnetic material - including the steel used in the core of this choke - is that if the magnetic field exceeds a certain point, the core will saturate and the permeability (and the inductance) will drop.  While there are chokes that are designed to do this (e.g. "swinging" chokes - those that have inductance that, by design, drops with increased current) it was desired that this choke's inductance be more consistent over the current range.

The most common way to control this effect is to intentionally reduce the permeability of the core overall by the introduction of an air gap which breaks up the magnetic lines of force:  The larger the gap, the greater the reduction.  The trick here is to reduce the permeability - and thus the level of magnetization - to the level that the core does not saturate at the desired current, but not too much more.

Fortunately, there are some equations that help us calculate things like this.  Knowing that the core material that I am using ("M6" Grain-Oriented Silicon Steel) will operate at 1.6-1.7 Tesla (16000-17000 Gauss) and still be well below its 2+ Tesla saturation flux I could crunch some numbers based on what I already knew.

Taking the following equation from link #1 (the Turner audio web pages), above:

uE = (10⁹ * ML * L) / (T² * Afe)

Where:

• uE = The permeability of the core
• ML = Iron path length in mm (approx. the width plus length of a single assembled E-I section for a standard "economy" E-I core set.)
• L = The measured inductance in Henries
• T = Number of turns
• Afe = The cross-sectional area of the core material inside the bobbin in mm²

Since I was using standard "economy" E150 cores with square bobbins, Afe = 38²mm and ML = 209mm.

With the temporary butt-stacked arrangement (e.g. full of small gaps due to pieces not aligning perfectly) yielding approximately 110 Henries the calculated permeability was 525:  If I'd very carefully interleaved and stacked each lamination, I would have expected the permeability to be considerably higher.

Taking this into another equation, we can calculate the magnetic field strength using this equation, also from link #1:

Bdc = (12.6 * uE * T * Idc) / (ML * 10000)

Where:

• Bdc = Magnetic field, in Tesla.
• uE = The permeability of the core.
• T = Number of turns.
• Idc = DC current, in amps - the target being 0.2 amps for this design.
• ML = Iron path length in mm, as above

Crunching the numbers, above, we find that Bdc is approximately 3.1 Tesla - a bit too high for our "1.6 Tesla" M6 material so we need to add more gap than is present due to the haphazard stacking of the material.  All things being equal, since the magnetic field strength is proportional to permeability and since inductance is proportional to permeability, we now know that if we can add enough of a gap to reduce the inductance by approximately half we will be in the ballpark (e.g. a permeability around 225-260).

Figure 8:
The arrangement of the E-I laminations and the insertion of insulating
strips used to add an air gap to reduce the overall core permeability
and prevent core saturation.
In the middle and on the ends are single "E" sections that are interleaved
to help hold the other sections in place via compression when the screws
are installed and tightened.  As it is the most common type, the "E-I" core
segments used are of the "economy" type - that is, two "E" cores placed
facing each other will yield a pair if "I" cores from the material that is
punched out to make the "E"s, wasting the least amount of material.
Click on the image for a larger version.

Placing some 10 mil (.25mm) thick "fish" paper between the "E" and "I" pieces I observed that the inductance was now around 51 Henries which meant that the magnetic field was approximately 1.44 Tesla - within reasonable range of the M6 core material that I'm using.

After the original assembly of the core, I had to disassemble the choke's laminations again to mount the end bells as shown in Figure 1, above.  In the process I used a wooden block to more evenly seat the laminations:  Between this and cinching the screws tightly, the gap between the E and I sections was slightly reduced and the measured inductance went up to around 75 Henries - a bit too high to prevent saturation at 200 mA according to our previous calculations.  Adding a second piece of 10 mil insulating paper - for a total of 20 mils (approx. 0.5mm) of thickness (or 40 mils overall in the magnetic path) reduced the inductance to around 54 Henries as can be seen on the impedance bridge in Figure 9.

Final thoughts:

For the time being - and since this is a prototype - I'll leave the design alone, but it brings up some points for the next time that I might wind a similar choke:

• Now that I have some actual, practical numbers and a bit of a "feel" for this particular core material and the effects of gapping, I should be able to predict ahead of time with reasonable accuracy the outcome of a particular winding configuration.  Using a different core (size, material) I would be certain to do a "test winding" to divine its aspects prior to finalization of the design.
• 51-54 Henries is a bit higher than is typically used for a power supply choke, providing an inductive reactance of around 38-40k Ohms at 120 Hz, the AC ripple frequency for full-wave rectified 60 Hz mains.  (It would be about 32-34k for 100 Hz on 50 Hz mains.)  In general, the higher the inductance, the better - provided care is taken to avoid resonances related to the filter capacitance and the load.  The higher inductance will reduce the ripple - particularly important for single-ended triode amplifiers.
• The self-capacitance of a choke of this inductance is likely to be significant.  While likely not much of a factor at the mains-related frequencies (100 and 120 Hz) it would likely be an issue for so-called "parafeed" tube-type amplifiers (e.g. those that use the inductance for decoupling the plate from the power supply and audio is extracted from a transformer effectively in parallel with this choke, typically via capacitive coupling) where this reactance could quash high frequency audio components unless the design were modified accordingly.
• The design goal of the choke was to handle at least 200mA.  With a DC resistance of 277 ohms approximately 55 volts will be lost across the choke due to this resistance at the design current, implying a heat load of approximately 11 watts (resistive only and not including core losses) - an acceptable amount for a core this size.  The voltage drop - while higher than desired - can be simply accepted and/or taken into account when designing and building the high-voltage plate transformer and associated supply.
• A more conventional inductance for a choke-input power supply is in the area of 3-10 Henries, so this design is likely an overkill - but then again, reducing the ripple on the power supply of a single-ended triode amplifier is not a bad thing! 

Were I to have more carefully measured the "typical" permeability of the core in its various configurations and gap sizes with a small number of turns I would have had a better "feel" as to its characteristics and cut the number of turns in half.  In so doing I would have (theoretically) ended up with about one quarter of the inductance - around 12-13 Henries - and half of the resistive loss.

Based on the equations, if I did that I could also have decreased the size of the gap to increase the overall permeability and, again, boost the inductance without fear of saturating the core which could permit a further reduction in the number of turns and winding loss.

Figure 9:
The completed choke (upper right) being measured on my old General Radio GR-1650A impedance bridge.
The measured inductance is approximately 54 Henries with a Q of approximately 4.1 at a frequency
of 1 kHz.  By the time I was able to take the picture, the inductance had drifted very slightly due to
change in temperature, moving the meter's needle away from a good null.
Click on the image for a larger version.

Doing this would allow the use of a slightly heavier gauge of wire to be used on the same-sized bobbin to further decrease ohmic losses and increase current-handling capacity.  In short, I suspect that something in the range of 1600-2000 turns of 27 AWG and a reduced air gap would yield something fairly close to a 10 Henry inductor with significantly reduced DC resistance (e.g. in the range of 55-75 ohms) with the current rating being restricted by the desired minimum inductance rather than the heat dissipated due to Ohmic losses.

At some point on the near-ish future, another such inductor will be wound, but we have yet to determine the form it will take:  Will it be as neatly wound as this, or will it be done so that the wires are allowed to cross at shallow angles.  Will it have about as many turns as this choke or will it use a heavier conductor with fewer turns for less - but adequate - inductance?

Stay tuned!

Note:  There is a follow-up to this post linked here that discusses the design and building of a filament transformer.

[End]

This page stolen from "ka7oei.blogspot.com".

Minimizing VHF (and HF) RFI from electronic ballasts and fluorescent tubes

Several years ago we started replacing the old, "iron" ballasts with T-12 fluorescent tubes at my work with T-8 tubes and electronic ballasts.  The program to do this was along these lines:

• When a T-12 tube or ballast failed, immediately retrofit the fixture.
• As soon as practical, retrofit those lights that are on all of the time, or "most" of the time.
• Gradually retrofit other fixtures as needed/convenient.

The lamp sockets (known as "tombstones") for T-8 and T-12 sockets are, for all practical purposes, interchangeable.  (Yes, there can be some voltage rating differences...)  Since, in most cases, we were working with what had originally been 4-tube fixtures - two of which (e.g. one ballast) had been disconnected years ago to save energy (we didn't miss the extra light)  we had extra tombstones on-hand to replace those that were broken, discolored or damaged and in some cases we even replaced the bracket and tombstone with those of the brand new "official" T-8 variety.

Figure 1:
Typical "iron" ballast used for T-12 tubes.
Click on the image for a larger version.

We were also able to get T-8 ballasts of known-good brands (Advance, Triad, Sylvania, etc.) for very good prices from an number of sources - far lower than "retail" - and we were sticking only with these brands because we felt that they would offer good reliability and efficiency.  In the years since we have started, the majority of fixtures have been retrofitted and we have yet to replace even a single electronic ballast and we've only replaced a half-dozen tubes - most of those within a week or so of being installed - all of this while providing at least as much light with lower power usage and heat load.

Before we go on, here are a few "weasel words" of warning:

• The devices discussed present hazardous/fatal shock hazards:  Do not even think about doing any such modification unless you have experience with such things.
• There are no guarantees, expressed or implied, that the modifications will work for you, are suitable or will meet electrical code requirements in your area.
• If you do such modifications you take any and all risks related to it, including damage to person or property, injury, fire, or liability.
• All RF interference situations are unique:  There is no guarantee at all that even when performed as described that the steps described will mitigate problems that you might be experiencing.
• Please use care and common sense!
• You have been warned!

The rise of interference:

There is one place where we have had problems with the T-8 tubes and accompanying electronic ballasts and that is the electronics shop.

Soon after retrofitting the fixtures in the engineering shop we noticed that we had difficulty hearing local 2 meter and UHF repeater when listening in that room, the noise floor having risen up by 10-20dB - depending on frequency and exact location within the room - with 120 Hz modulated noise and the fact that it disappeared when the lights were turned off pointed directly to the cause.


Differences between magnetic and electronic ballasts:


For a typical 4-foot (approx. 102 cm) tube, good, old magnetic ballasts are, at least when powered from 120 volts, transformers that boost the voltage to that high enough to sustain conduction on each cycle of the 120 Hz sinusoid.  When starting up there are special windings that will activate the filament and/or boost the voltage such that this conduction - which is higher when the tube is "cold" (both physically and has not been lit) but when the current is established as the tube lights, these diminish. To limit the current through the tube to a safe value it is typical that there is a large amount of series inductance which, by virtue of its reactance, sets the maximum current consumed by the lamp.

For more general information about how fluorescent tubes and ballasts work, go to "Sam's F-Lamp FAQ" - link.

For various reasons (core, skin-effect, magnetic and resistive losses, etc.) these old "iron" ballasts are somewhat inefficient, but even so, this efficiency of the linear, fluorescent tube made the combination far more efficient than practically any other tungsten (incandescent) light source.  Nowadays, the skinnier "T-8" tubes are used along with electronic ballasts and these skinnier tubes can, for various reasons, produce at least as much light as the larger T-12 tubes, but in order to maintain such efficiency and to provide reasonable longevity they must be driven with an electronic ballast.

Unlike the old-fashioned "iron" ballast, modern, electronic ballast are essentially high-power oscillators that produce a lot of voltage (perhaps much as 1kV peak, open-circuit and while starting the tube) at a fairly high frequency - say, 30-60 kHz.  Using this higher frequency allows much smaller, lower-loss magnetics to be used and driving the tube with a higher frequency than that of the mains improves its "power to light" conversion efficiency even more.

Electronic ballasts and interference:

Even the old, "iron" ballasts with the fluorescent lamps could cause RFI (Radio Frequency Interference), largely due to significant nonlinearities in the conduction of the tube with respect to the applied voltage resulting in the generation of harmonics.  Furthermore, a malfunctioning ballast could generate interference in other ways, such as internal arcing. Typically, such interference, if present, occurred on the mediumwave (AM broadcast) frequencies and, occasionally, in the HF (shortwave) range and, most typically, either very short range - within a few feet/meters of the lamps themselves - or conducted via the power connections.  Usually, replacement of the defective tubes and/or ballast remedied this situation.

With electronic ballasts the situation is quite different.

With a high-power (10's of watts) electronic oscillator on-board there are already problems at hand:

• With frequencies in the 10's of kHz, harmonics appearing in the mediumwave and HF are of much lower order.  In other words, the 30th harmonic of the switching frequency in an electronic ballast puts you squarely in them mediumwave range while the 30th harmonic from an iron ballast is still within the frequency range of hearing!
• The rise/fall times of the waveforms from an iron ballast are quite slow in comparison with those from an electronic ballast which, by necessity of design, can be in the order of 100's or 10's of nanoseconds.  Having these high switch rates on the transistors inside the electronic ballasts puts energy squarely in the MF, HF and even the VHF or UHF range!
• The length of a common 4-foot (1.2 meter) tube/fixture is commensurate to the wavelengths within the VHF/UHF range.  This means that the fluorescent tube itself is perfectly capable of acting as a radiator at many frequencies - not to mention various other parts of the fixture and contained wiring.

Unless one were to scrap the electronic ballast altogether and go back to using the old, iron ballast, if you experience interference issues there are a number of things that you will have to do.

Conduction of high frequency energy onto conductors and radiators:

In the case of our electronics shop the biggest problem appeared to be direct radiation of energy at VHF frequencies, apparently from the tube and connecting wiring.  One way to minimize this effect would be to reduce the harmonic energy emerging from the ballasts and finding their way onto the tube and wiring.

The most practical way to do this with minimal risk of deleterious effects (e.g. loss of efficiency, electric shock, fire, reduction of operational lifetime of the ballast and/or tubes) is to apply a bit of series inductance to the leads that feed the fluorescent tube.  While we are at it, we might as well apply some common mode filtering to the AC (mains) leads as well to minimize conduction of "grunge" that might find its way out via that path, as well.

Figure 2 shows the typical connection of the tubes and electronic ballast.

Figure 2:
Electronic ballast and typical connection to tubes.  In typical "room temperature" installations the filament heater is not used, except as a simple emitter of electrons:  In the example, above, "load sharing" is afforded by the individual, series capacitor for each tube and starting ionization for each tube is provided solely by high-voltage excitation from the ballast.
Not depicted, ballasts are available that have four wires to each tube to provide heater current for starting the tube(s) in cold environments, but most "indoor" ballasts have only a single wire connected to each end of the tube.
Click on the image for a larger version.

As can be seen, the tubes are in parallel with the output of the high-power oscillator, typically using a series reactance (capacitor) to limit each tube's current and to provide a mechanism for load-sharing and offer operational stability in the presences of the tubes' negative resistance characteristics.  Many electronic ballasts allow different numbers of tubes to be connected:  3-tube ballasts will allow 2 or 3 tubes while 2-tube ballasts will usually allow just one tube to be powered, and using these series reactances is a simple way of keeping the tube current more-or-less constant, despite the number of tubes connected - either at the time of installation or due to later tube failure.

Figure 2 also indicates something else:  On the wires marked "A" and "B" are carried the high-frequency currents from the ballast's oscillator.  As we discussed before, not only are these same currents very non-sinusoidal and contain many harmonics, but the tubes themselves tend to badly distort the currents, further-increasing the harmonic content on these leads.  Finally, at VHF and UHF frequencies, the very lengths of wires "A" and "B" and the tubes themselves, "T", can make them capable radiators in their own rights!

The distortion added by the tubes themselves is likely not significant one gets above MF and HF frequencies as the mechanism is generally too slow in its own right, but we still have the harmonics from the oscillator itself to contend with which can be carried, as current, by the ionization of the gas within the tube itself which will form an antenna and as we know, as long as there is some current flowing, we will have electromagnetic radiation.

Inductors used for reducing the harmonic content on the tube current:

Our best bet is to reduce the harmonic content on the tube current itself and the easiest way to do this with minimal intrusion and risk to the user is to insert a small amount of inductance onto the connecting leads "A" and "B".

Figure 3:
A pile of toroidal inductors pulled from some scrapped power supplies.
In the upper-left is a bifilar choke used for mains filtering, also found
in higher-quality switching supplies.
Click on the image for a larger version.

We had a number of junked switching power supplies in the "boneyard" and we pulled a pile of toroidal inductors (see Figure 3) from some of them and did a quick analysis.

The most common type of toroidal inductor is the sort typically found in PC power supplies, usually used for output filtering, wound in either a yellow or yellow-and-white core.  The permeability of these cores isn't extremely high, typically achieving somewhere in the area of 20-30 microhenries with 10-15 turns, but they are not particularly suitable for use much above the range of 100-200 kHz:  Higher than this they get quite lossy - but we don't really care too much about that since we are trying to quash this energy, anyway.

The "Yellow-White" cores appear to be "26 Mix" Iron Powder types that are specified for use from DC to approximately 800 kHz.  The aforementioned "Yellow" cores would have the color code for "6-mix", but when they were measured, their permeability was closer to that of "26 mix" indicating that whoever made them wasn't precisely following color convention!

The sizes (outside diameters) of these inductors varied, from approximately 1 inch (25.4mm) in diameter to over 2 inches (50mm).  If you were to translate these to "store bought" toroids these could equate to sizes from "T94-26" to as large as "T200-26" or "T225-26".

Another inductor that appeared to be useful was one that was simply painted light gray and it, too had permeability roughly equal to that of the yellow/yellow-white ones - that is, 10-20 turns yielded something in the area of 20-100 microhenries.  I've found no obvious reference to "light gray" painted toroids in the typical references so I don't know what "mix" they might be, but they appear to be vaguely similar to the "Yellow-White" ones in general properties.

When either the gray or the yellow/yellow-white inductors were broken, it appeared that they contained some sort of metallic powder, pressed and molded into shape and covered with paint. 

The third type of inductor, often used for filtering the the AC mains input, appeared to be some sort of very high permeability ferrite and putting 10-20 turns on these yielded hundreds of microhenries or even millihenries of inductance:  These are not suitable points "A" and "B" and should be set aside for now - but we will be using them later.  When broken these particular cores were either a black compound (e.g. ferrite) or, in a few cases, wound with some sort of thin, ferromagnetic tape as in the case of the red-cored inductors depicted in Figure 3.

Which toroids to be used where?

Let's take a look at a redrawn version of Figure 2:

Figure 4: 
A fluorescent ballast/fixture with added ferrite to minimize RFI by virtue of "isolating" the fluorescent tubes at
radio frequencies.
For "cold weather" ballasts that use filament current for starting, each "pair" of wires to each tube would be treated as a single conductor through L1-L3.
See text for explanations!
Click on the image for a larger version.

First, let us take a look at inductor L1 which is in series with the "common" lead "B" that connects to one side of both tubes.  With the inductors that we had we are able to put approximately 15 turns of the original wire into this core which yielded an inductance of 10-15uH (value not critical!)  In running the numbers we can see that at 30 kHz, a typical operating frequency of an electronic ballast, this inserts approximately 3 ohms of reactance into the circuit.  Considering that for a 32 watt, T8 fluorescent tube that the average current will be around 300 milliamps (see reference, here - link) we can calculate that if the current were perfectly sinusoidal (it is not!) the effective loss would be a volt.  Because this is a reactance, we would not experience I^2R losses in the same way that we would if we were dropping the same voltage with an ohmic loss (a resistor) but even if we were, this would amount to only 1/3rd of a watt or so.

If we were to translate this same inductance (15 uH) to the middle of the AM broadcast band (1 MHz) we would see a reactance of around 94 ohms - significantly higher than at the operating frequency, reducing the potential current through the tube, at that frequency, by a factor of around 30 which corresponds to 10's of dB reduction.  Since only a fraction of the oscillator's energy is at harmonics of this magnitude a negligible amount of power is being blocked by this inductance.

Taking this to an extreme, let us consider a frequency of 150 MHz with this same 15uH of inductance where we find the reactance to be approximately 14000 ohms - extremely high and, for all practical purposes, completely "blocking".  In reality the effective series resistance would not be nearly this high as not only would this particular core material ("26" mix) not be effectively offering such inductance at this frequency, but there would also be capacitive (shunt) coupling across the inductor/windings itself and between various conductors (wires) located under the cover of the ballast shield.  Nevertheless, the impedance would be significantly increased due to the inductance present and the core losses at this frequency.

Taking another look at Figure 4 we can see that we'd need to install similar filtering in the other leads connecting to the tubes as well (e.g. the "A" leads) and the similar math applies.  By installing such inductors in both leads we effectively "isolate" the tube at high frequencies by virtue of the series inductance while leaving the frequencies at which the tube is powered - around 30 kHz - virtually unaffected.

How do we know that this does not affect normal operation?  A bit of emperical measurement can (and did) verify this assertion:

• A "Lux Meter" placed below the fixture showed no discernible difference in light output before and after modification.
•  The added inductors did not get perceptibly warm:  If they did, this would indicate power loss!

A core not to use at points "A" and "B":

In an experiment I placed one of the high permeability ferrite cores (not one of the yellow cores) in place of L3:  It immediately got too hot to touch, indicating both high losses and a significant amount of inductance.  When checked on an inductance meter I found that approximately the same number of turns as I'd placed on this core when tested in the fixture (15 or so) yielded around 200 microhenries - a reactance of around 38 ohms at 30 kHz - a significant amount at 300 milliamps of tube current!

The upshot:  When trying an core, verify that it does not warm up by an appreciable amount before deciding to use it!

If it gets (noticeably!) warm after several seconds/minutes, it is not suitable and could pose a hazard due to heat:  This heat could, in theory, melt insulation which could pose an electrical/fire hazard or the heat itself could melt or cause combustion in its own right!

Don't worry - we still plan to use this core!


Additional modification:

Figure 5: 
Added inductances on the leads marked "A" in Figure 4,
above.  The inductor on the far left is a multifilar choke -
see the text below for an explanation.  There are actually three
small yellow/white toroidal inductors, one being obscured.
Click on the image for a larger version.

If you look at Figure 5 you will see, on the right side, the three, smaller yellow/white inductors (one is obscured.)  In this particular case this is a 3-tube fixture, the third tube used to provide extra light as it was placed over the workbench area.

Because it was directly over the workbench, it was typically much closer to the gear being used and tested which made the noise emitted by it a bit more problematic by proximity.  To reduce this noise still-further an additional inductance was added in the form of the large, yellow inductor on the left.  This particular core was taken from a scrapped PC power supply (almost all of them have such a device) and the three wires were wound, in parallel - keeping the same number of turns - through the core to fill it up to make, in this case, a "tri-filar" winding.  Doing this forced an additional series inductance on the "A" leads depicted in Figure 4 as "F1" but it forces the currents through the tubes to be equal - not too difficult a job considering that L1 and L2 (depicted in Figure 4) have already presented a high impedance (at high frequencies) on those leads already.

While doing the initial work on the first fixture to be modified, a Yaesu FT-817 - an all-band, all-mode amateur radio transceiver - was laying on the workbench immediately below it and it was noted that by grabbing a wire of the unmodified fixture I could couple some of the noisy RF (at around 144 MHz) into my body which would then be re-radiated as evidenced by the the noise from the FT-817 increasing.

As a rough indicator of the efficacy of the added inductances I observed that after installing L1-L3 I could grasp the wire "after" these inductors (on the "tube" side) and hear no difference, but if I did so on the "ballast" side of the inductors I heard the noise increase, indicating to me that they were doing their job in removing a significant amount of "grunge".  While I was doing this I noted that I could also hear "grunge" if I grabbed around the AC mains wires as well, indicating that some of this energy was being conducted via that route.  Being a commercial building, the mains input is routed via metallic conduit and it was noted by poking around in the "drop" ceiling and placing the antenna of the FT-817 near-ish the power conduit that relatively little of the noise was emanating from the conduit itself, but since I had a plethora of chokes on-hand I decided to try an experiment.

Using the high-mu (lossy) cores:

Using one of the same "lossy" ferrite cores that got hot when tested, I parallel-wound, in bifilar fashion, as many turns of the AC mains wire of the ballast as I could fit on the core.  As with the milti-filar core, above, it is important that these wires be kept parallel and that the exact same number of turns be used for each of the two mains conductors so that the "common mode" attenuation be maximized.

When wound in a bifilar fashion - and on leads that have only a residual amount of RF energy - we are using these ferrite cores in the manner intended with no fear of their getting hot:  Because both wires of the AC mains go through in parallel, their magnetic fields cancel and the core does not "see" it - it is only that small amount of RF energy that is not supposed to be there that is being blocked by the bifilar winding on the core!  It is important to note that in order for this type of winding to work, you must wind equal numbers of turns of both wires, preferably in parallel or gently twisted together:  It is by virtue of the equal inductance and coupling between these two wires that this sort of filter works.

After doing this I did the "grab" test with the wire again:  If I grasped the wire on the "ballast side" of the newly-added bifilar inductor I could hear an increase in the noise on the FT-817, but if I did so on the "mains" side I could not.  In other words, this choke was keeping the "grunge" from the ballast out of the mains.

While it may not have been particularly important on VHF/UHF to add this extra bifilar inductor on the mains power leads, it was pretty easy to do since I had the fixture "open" already - and I decided that every little bit helps!  Also, it is worth noting that on MF and HF that it will likely be via the mains power leads that the majority of noise will be conducted while at higher frequencies like VHF and UHF, the wavelengths are small enough that energy can be emitted directly from the lamps and fixtures themselves as their very size is a significant portion of a wavelength.

Efficacy of the modifications:  Did they work?

Of the 6 fixtures in the shop, only the two directly above the workbench where they are closest equipment under test now have both the small chokes in each of the leads in series with the tubes (e.g. L1 and L2).  All of them have an choke on the "common" side of the tube (L3), the common-mode choke on all of the tubes (F1) and the bifilar choke on the mains side (F2).

Did this modification have any effect?  The quick answer is yes and here are the ways that it was measured.

• A portable HF/VHF/UHF all-mode transceiver with a whip antenna (an FT-817) was placed underneath/near the fixture, the distance of the plastic diffuser.  The "S-Meter" reading of the noise of the modified fixture was compared with an unmodified fixture with the same complement of tubes and ballast type/model and the modified (filtered) fixture was found to radiate far less than the unmodified one based on observations of the S-meter and audible noise.
• Two spectrum analyzers were set up on the workbench in the electronics shop connected to small whip antennas and sweeping in the range of 100-200 MHz.  After all of the fixtures in the shop were modified, the amount of noise indicated on the analyzers was 10-20dB lower than before the modification, depending on frequency.  It was not completely gone, but significantly reduced.
• Using a portable handie-talkie transceiver, several VHF (2 meter) repeaters were checked for receive signal quality within the shop before and after fixture modification.  All but the strongest were inaudible in the shop prior to modification, but all were easily audible afterwards.  A slight amount of extra noise is apparent when the lights are turned on, but there is no trouble in finding a location that provides a suitably noise-free signal for testing/monitoring now.

Final comments:

The addition of screening

Prior to the installation of the chokes in series with the lamps some experimentation was done with metallic screening of the fixture.  The particular fixtures that we are using are light-gauge Lithonia fixtures that have spot-welded seams and appear to be likely to provide a reasonable RF-tight seal on the back side:  It was via the open front, through the plastic diffuser that RFI was radiating - apparently from the tubes themselves.

In an experiment we decided to cover the front of the fixture with aluminum foil.  While blocking the light, it would serve to help us determine if this was a viable means of containing the RFI within the enclosure.  We placed, lengthwise, two parallel sheets of foil over the diffuser, but it happened that there was a very narrow (1/4" inch, approx. 6mm) gap between the two strips running along the long dimension of the fixture.

We noted that with the foil in place that the interference actually got worse until we bridged the narrow gap with small pieces of scrap foil in 3-4 places.  We surmised that the two pieces of foil were acting as independent radiators until we connected the two, at which point the noise level was reduced - but it was still significant.  Clearly, the foil itself was coupled to the the noise energy within the box and re-radiating a significant percentage.

We then bridged the foil to the steel box itself, causing a further reduction in noise - but the amount of reduction depended strongly on where, exactly, we made this connection.  If this was done in each corner of the box (e.g. four places) there was a significant reduction.

Practically speaking one would not use aluminum foil to provide such shielding as it would clearly block the light.  Aluminum window screen would also not be recommended as there is not guarantee that there is an electrical connection across its entire face as each wire is not "bonded" to its neighbor - plus it, too, blocks a significant amount of light.

This leaves the use of so-called "expanded metal" or "hardware cloth" - both of which are electrically-connected across their plane as options.  Of the two, the "hardware cloth" - typically galvanized steel mesh is, by far, the cheapest and least "light blocking" alternative - plus, it is possible to solder to the zinc coating with relatively little difficulty.  This material is available in different mesh sizes and for VHF/UHF frequencies the larger sizes (around an inch or several centimeters) would be adequate.

The problem would be providing a firm, electrical bonding of this screen to the case of the light fixture.  On the "hinge" side of the diffuser cover one would use very short, wide metal straps to make the connection to the case, but for best efficacy it would also be preferable to make a similar connection on the side opposite the hinge where the cover opens - but aside from the installation of "finger stock" or very short plug/receptacles, this is rather complicated to do!

About LED replacements:

I am aware of LED drop-in replacements for T-12 and T-8 fluorescent tubes, but has, for the time-being, dismissed these owing to issues of cost, reliability, efficiency, and "light quality" (e.g. color rendition or "CRI").  For an in-depth report on this subject see "Performance of T18 and T8 Fluorescent Lamps and Troffers and LED Linear Replacement Lamps" - link.  Since this report was published significant improvements have been made in LED efficiency, but similar issues - particularly those related to quality and longevity - remain unless one uses devices made by reputable manufacturers.

As far as EMI/RFI issues related to LED replacements for T-12 and T-8 tubes:  Based on reports in the amateur radio press and on forums such as the EEVBlog (examples here - link and another link) I would expect similar - or worse - problems to occur, depending on the manufacturer - as they would also have issues with high slew rates on switching regulators and/or semiconductor-related transients.  While it is very likely that common-mode mains filtering would also be recommended on these devices as well, it may be more difficult to remove noise along the entire length of a series or series-parallel fed array of LEDs that was nearly 4 feet (1.2 meters) long as this is a significant portion of  a wavelength at VHF/UHF frequencies and could easily radiate on its own right!

If you do wish to upgrade an existing "troffer" or similar fluorescent fixture to LEDs, I would strongly recommend that you obtain JUST ONE and carefully analyze it before you risk ruining your HF, VHF and/or UHF reception with little means of mitigation!


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This page stolen from "ka7oei.blogspot.com".