Friday, December 1, 2017

Containing RF noise from a sine wave UPS

An amateur radio friend of mine (WA7X) has a cabin in the mountains.  It is not a particularly "rustic" cabin as it is festooned with radios, antennas, propagation beacons, computers and cameras and has an internet connection, but because it is in a remote location it occasionally has 3-6 hour power outages and thus it also has a UPS (Uninterruptible Power System) to keep many of these things online in the interim.

This "sine wave" UPS is a 1.5kVa unit that was cast off from by someone for the same reason most UPSs are cast off:  Its internal battery went bad.  Rather than simply replacing the battery, its previous owner simply got another UPS and asked the question "Do you want this?"

Instead of replacing the internal battery, the DC connections for the batteries were brought out and a bank of six 12 volt lead-acid batteries was wired up (two sets of three parallel batteries connected in series amounting to a nominal 200-ish amp-hours at 24 volts) to provide the needed 24 volts and a DC circuit breaker was added for safety, this battery capacity allowing the unit to run for far longer than it could have on the original battery.

While this UPS was more efficient than a previous unit and produces a fairly nice sine wave rather than the typical, ugly "modified sine wave" there was a price to pay:  RF Interference (e.g. RFI) that was present whether the unit was active or on standby.

But first, a few weasel words:
  • This project involves high voltages and/or currents:  Do not attempt to construct a similar device unless you are thoroughly familiar with electrical safety and the wiring of such devices.
  • If you use an external battery bank with a UPS it is imperative that you include some sort of current liming, such as a fuse or circuit breaker rated for both the expected current and battery voltage.  Such devices are available from auto-parts stores.
  • If you use an external battery bank with a UPS you must determine if this battery bank is "mains referenced" internally by the UPS or not.  If it is mains referenced (e.g. connected directly or indirectly to the mains AC voltage) then the low-voltage DC terminals will pose a line voltage safety hazard and care must be taken - the least of which is enclosing the battery and any exposed DC terminals to prevent accidental contact.  This UPS's battery terminals were isolated from the mains and given that the room in which the UPS resides has restricted access, the low-voltage battery connections themselves were deemed to be "adequately safe" left exposed.
  • YOU are responsible for the safety and for any liability if you choose to do something similar to what is described on this page.  You have been warned!
    Figure 1:
    The completed AC/DC filter.  This
    box contains a "brute" force L/C filter
    for the battery (DC) leads as well as
    separate filters for the AC mains in/out
    power connections.
    Click on the image for a larger version.


The radio frequency "grunge" from the UPS manifested itself on the HF ("High Frequency" or shortwave) bands in several ways.  Most obvious was a loud "buzz" every 40-50 kHz on the lower (80 and 40 meter) bands, but there was also more subtle interference that pervaded these and higher bands:  A background "hiss" that might initially escape the notice of the casual listener until one realizes that this noise masked signals that should have still been perfectly audible.  If one switched the receiver to AM it would be observed that this "hiss" was subtly modulated at twice the mains frequency, 120 Hz.

To determine the extent of this sort of interference the typical procedure is to start turning things in the house off one-at-a-time (or, more reliably, the reverse:  Turn everything off at the breaker panel and then turn on one thing at a time) until the culprit is found.  This was done with the UPS and the magnitude of its "radio interfering" nature was determined.  Clearly, this "grunge" was being conducted from the power leads going in and out of the UPS.

Further experimentation revealed the true extent of the noise:  Even with everything disconnected from the UPS - that's to say, with it running on its battery, unplugged from the mains and the load disconnected - there was still detectable noise getting to the antenna and a quick check with a portable shortwave receiver proved that the remainder of this noise seemed to be being radiated by the physically-large battery bank and the wires that connected it.

While the proper application of a "brute force" AC line filter would likely quash the noise conducted in and out of the UPS on the mains power leads, something else would be required to minimize/eliminate the noise emanating via the DC lines.

"Brute force" line filters:
Figure 2:
A typical "brute force" L/C line filter typically found on devices
to minimize conduction of extraneous RF energy on the
AC mains.  For a 1500kVa UPS the filter would need an
appropriate current rating - particularly the fuse!
In some filters, two sections are used, with the components
L1, C1, C2 and C3 repeating.  This same filter
topology is used for the described DC filter.
Click on the image for a larger version.

One of the best ways to eliminate or minimize the amount of RF energy that might be conducted out of a potentially interference-generating device is to apply a combination of inductance and capacitance to that line as depicted schematically in Figure 2.

With the noise coming from the power supply (the UPS in our case) capacitor "C4" effectively "shorts" this RF noise to both sides of the power line, leaving the AC signal (pretty much) unchanged.  Inductor L1 consists of two equal windings on a ferrite core and it is practically invisible to signals that are equal and opposite, but it acts as a series choke for signals that are "common mode" - that is to say, equal on both sides of the power supply's mains leads - such as the RF noise energy.

On the "mains" side of the filter capacitor C3 reinforces the common mode again while capacitors C1 and C2 shunt any remaining RF energy from the power supply - its impedance now made much higher by inductor L1 - to ground - which would be the metal enclosure in which everything was mounted.

As it happens, these filters are available on the surplus market, and we would need three of them:
  • One for the AC mains connection to the UPS.
  • Another for the UPS's AC output
  • A third one for the battery connection to the UPS.
With the UPS being rated for 1.5kVa, some surplus AC mains filters, rated for 16 amps, were obtained while a filter designed specifically to filter DC lines rated for at least 50 amps at 60 volts was found (at The Electronic Goldmine - item G21652 - link) to filter the power connection between the UPS and the battery:  50 amps at 24 volts is not quite 1.5kVa, but there was nowhere near this amount of power being drawn from the UPS and the added circuit breaker/disconnect would provide the required safety - and the filter should be able to handle a brief overload, anyway.

Putting the filters in a box:
Figure 3:
Inside the filter box.  Along the top edge is the main AC input.  On the bottom edge - and just below the outlet on the left side - are the short leads that conduct the "dirty" AC power to/from the UPS, each through its own, separate filter - the two black boxes.
On the far right is the high-current DC filter with stud (bolt) connections being used to make the connections between the battery and the DC input of the UPS.  Barely visible along the bottom, one of the three studs is used to connect a piece of heavy wire or braid to bond this box to the chassis of the UPS to minimize conducted/radiated RF.
In the middle is a circuit board that contains a mains transformer, a high-current mechanical relay and a small solid-state relay.  This board - added later in the design - allows one pair of the outlets to be turned on and off with a simple contact closure of an internet-connected remote power switch.  On the control cable for the relay (the thin white wire) is a ferrite device which minimizes the amount of RF energy that might possibly be conducted in or out of the box on that control lead.
Click on the image for a larger version.

Ideally, one would have put the UPS and the batteries in a large metal box and passed the power leads in and out of this box only via the filters, but this simply wasn't practical, so the next best thing had to be done:  Put the filters in a single, metal box that would be electrically bonded to the UPS chassis and make the connections in and out of the UPS using short leads.  By keeping the leads short and bonding our new filter box to the UPS, we'd do our best to limit the number and length of conductors that carried the RF interference and, most importantly, preventing RF circulating currents from finding their way on external connections.
Figure 4:
The end of the box with the mounted outlets.  As noted,
one pair of these outlets is connected to a relay to allow the
connected devices to be remotely controlled.
Click on the image for a larger version.

To that end a power distribution box was found at a home improvement store (Lowes Depot, I think) for less than $25 and the "guts" removed (the box with "guts" was cheaper than just an empty box by itself!) and the filters mounted inside.

With the short-as-possible conductors between the UPS and the filter, they will have minimal ability to directly radiate RF while the RF conducted on these lines will be filtered by the circuitry in the box itself with the bonding of the two boxes minimizing differential RF currents.  The power "to" the UPS was made with a short length of "SO" cord with a female connector on it while the power "from" the UPS is via a short cable with a male connector, plugged into one of the UPS's outlets - and being the ONLY thing plugged into the "dirty" AC output of the UPS.


Figure 5:
Perhaps a bit cluttered, this is the UPS sitting atop the filter, installed
and working.  In the lower-left corner of the picture, above two
batteries may be seen the DC circuit breaker/disconnect that protects
the DC circuit for short circuits.
Click on the image for a larger version.
Being that this is a remote location, the filter unit was installed a few weeks after it was constructed, having been tested (as best as could be done) on the workbench:  Power flowed through the various filters and the load control relay worked properly.

When installed, the filter box was placed underneath the UPS as it had a slightly larger footprint - and to minimize the length of the "noisy" DC and AC power leads from the UPS, along with the lead used to bond the two cases together.

To connect the DC, the cable coming from the batteries was effectively cut so that there was just enough of it emerging from the UPS to connected to the "output" side of the filter:  On both sides of this cable heavy "ring" lugs were attached and these were connected  to the studs of the DC filter.  To eliminate the probability of accidental shorting, the ground stud on the "input" side of the filter was removed and near both connectors a plastic wire clamp (one may be seen in Figure 6, below) to keep the positive wire in a fixed position and rotating into and shorting to the other stud.

Figure 6:
The back side of the UPS, showing the coiled power cord and the
(green) bonding wire that connects the chassis of the UPS and filter
box firmly together.  At the bottom of the picture can just be seen the
plastic clamp the keeps the positive wire lifted and away from the
the negative wire, to prevent shorting.
The ONLY thing plugged directly into the "dirty" AC output
of the UPS is the cord going to the filter:  Plugging anything else
directly into the UPS would at least partially negate the filtering! 
Click on the image for a larger version.
The AC input and output of the UPS was simply "plugged in" to the cables and the excess cordage was neatly coiled and stowed at the back end of the UPS using plastic "zip" ties:  It was important to do this because these cables are unfiltered and are "noisy" with RF meaning that they should be kept as small and as close to the metal of the cabinet as possible and kept away from any other conductors to minimize cross-coupling which would defeat the purpose of this filter.

To the remaining ground stud of the DC filter was attached a short piece of heavy (8 AWG) green wire with a ring lug on each end, this wire being visible in Figure 6.  The other end of this wire was attached to a marked grounding screw on the UPS chassis to bond the two boxes together and minimize RF circulating currents and to prevent the UPS chassis itself from being a source of radio frequency interference by direct radiation.

The result:

Taking the 20 meter (14 MHz) amateur band as an example, the UPS caused an extra 2 "S" units or so of noise above that of the typical ionospheric noise floor when it was powered up, before the filter was installed - this, being detected on a Carolina Windom antenna lofted between two trees high above the cabin's roof.  After this filter was installed the noise from the UPS was completely undetectable on any HF band, revealing other weaker low-level noises from other devices - the quieting of some of these will be discussed in later installments.

Figure 7:
A general block diagram of how the parts are interconnected.
The external UPS battery bank is protected with a DC-rated circuit breaker/
disconnect switch.  This particular UPS operates from 24 volts, hence the two
series-connected 12 volt batteries.  If the battery is inside the UPS's metal
cabinet, the DC filtering and connections are not needed.
Note that the "output" side of both AC line filters are both connected to
the UPS:  This is done because many filters are designed such that the
"output" side is that best-suited for connecting to RF-noisy circuits.
This diagram does not include the remote relay described.
Click on the image for a larger version.
Before the installation of the filter I'd placed my FT-817 (a small, portable HF transceiver) across the small room from the UPS, receiving with a short antenna and tuned to the 20 meter band.  When the UPS was operating and connected to its loads its noise was clearly audible, causing several S-units of indication on the signal meter.

After the filter was installed and the unit was powered up again with the loads connected, the noise was barely audible in the FT-817 and from across the room, it went away completely when the green wire was connected, bonding the UPS and filter chassis together.  If the radio was moved to within a foot or two of the UPS I could start hearing the "hash", but it seemed to be emanating only from the coils of AC cables "zip"-tied to the back end of the unit.   Because of the short length of the wires - and their being close to the metal case and not near any other wires into which this noise could be coupled it is unlikely that these short conductors will radiate any detectable noise at a distance greater than a few feet.

Doing the same on another UPS:

As part of his "noise abatement" strategy, WA7X did some RF sniffing around his house and discovered that a major contributor of RF noise was a sine wave UPS of a different brand.  While the UPS itself was completely different, the same things needed to be done to it as was
Figure 8:
Line input filter.  This was mounted externally, using very
short wiring, as it was difficult to insert a filter directly
into the AC mains path via the connector that was
directly mounted to the UPS's circuit board.
Click on the image for a larger version.
done to the UPS at his cabin:
  • A "brute force" line filter on the AC input.
  • Another line filter on the AC output.
  • Because the batteries on this one were, like the other one, external there needed to be a filter on the battery DC line as well.
This UPS was a bit smaller - 750VA (about 600 usable watts assuming an average 0.8 power factor) and it, too, had been modified to use a much larger set of external batteries to maximize run time.   Like the other UPS, it produced copious quantities of RFI whether it was "UPSing" (providing back-up power) or idle.

Figure 8 shows the input filter - and its mounting represents a slight complication:  The AC (mains) input went directly to the UPS's circuit board - and no on-board RF filtering was seen.  While we could have probably removed the board, cut some traces and soldered wires to "intercept" the AC flow between the input mains cord and the circuitry, we did something a bit "cheesier" - but still effective:  Attached a filter to the outside.  Going to our local electronics surplus store we found a suitable line filter with an overkill rating of about 10 amps (6 amps would have sufficed for a 750 kVA UPS) with a built-in IEC power connector.  As can be seen from Figure 8 we cut up an IEC power cord and with minimal length, soldered it directly to the terminals of the line filter and insulated them
Figure 9:
Output line filter.  This was mounted in the empty battery
compartment.  The "dirty" AC power from the inverter
is via the black/white wire on the left and the "clean" AC
power to the outlets is on the right.  The "clean" power
was wired using green wires with black/white markings
as that was the only solid wire - used to push into the
outlet's rear connector - that was on-hand.  These two
sets of wires are kept physically separate to prevent RF from
capacitively coupling from one to the other.
Click on the image for a larger version.
with heat shrink tubing and copious quantities of RTV sealant.  Because this filter was intended to mount via screws to the back panel of a piece of equipment, it had no mounting lugs, so we soldered some to the tin-plated case and used these for the ground connection as well.  It looks a bit kludgy, but its "pretty safe"!

For the AC mains output we disconnected the wires from the rear-mounted power socket and spliced short lengths of wires to it, sleeving the splices with several layers of heat-shrinkable tubing, connecting the output of the UPS's inverter to one side of the filter mounted in the battery compartment.  Connected to the AC outlets were short pieces of wire which were then soldered to the other side of the mains filter.  Although not seen in the pictures, there were no chassis holes that permitted the passage of these two sets of wires, so a pair of holes (protected with rubber grommets) were drilled with the unit tilted so that metal shavings (e.g. "swarf") would not end up in the circuitry - one hole for the "dirty" AC power from the inverter and another hole for the "clean" AC power to the socket on the output.  Having these two sets of holes and routing the two sets of wires away from each other prevents the RF grunge from coupling from one set of wires to the other.
Figure 10:
The DC line filter.  Too big to fit in the battery compartment,
this filter was mounted to a metal partition present in the UPS,
holes in that partition and in the chassis below to pass the
metal studs that carried the DC power.  Underneath, through
the battery compartment, was later wired the cabling for the
24 volt, fused DC input.
Click on the image for a larger version.

Finally, there is the DC filter.  Using the same surplus filter obtained from Electronic Goldmine as was used in the other UPS, this filter was mounted in an empty space adjacent to the circuit board.  There, a metal divider provided a handy mounting point for the "dirty" side of the DC connection (e.g. the input to the inverter).  A metal hole punch and a nibbling tool was used to fashion the holes necessary to pass the power studs through them with a similar hole being punched through the case into the battery compartment.  Before the filter was installed we declared which terminal was the positive side and clearly indicated it with a red permanent marker.

In Figure 8 one can see a grommet through the panel to the right of the filter:  This was the original passage of the DC power cables which were not installed at the time the pictures were taken.  When these cables were added later, appropriate DC fusing was included and the wires were carefully routed through the battery compartment, keeping them away from the "dirty" side of the filter to prevent coupling of RFI.

The result:

As with the other power supply, a portable AM/shortwave radio held near the UPS was not able to detect any noise unless it was held within an inch or two (couple of cm) where it was likely that magnetic coupling occurred.  When checked, there was no noise heard on the HF spectrum that was attributable to the inverter - only a few more "new" noises that will need to be chased down now that this UPS has been quieted!

Links to other articles about power supply noise reduction found at this site:


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