RF noise from "grow lights" - the same phenomenon
Several years ago, there was some noise (pun intended) in the Amateur press about LED power supplies being sold that caused a tremendous amount of RF interference - and many of these stories also included anecdotes of many of these interference sources having been tracked down and found to have been "grow" operations. Later, some stories surfaced where law enforcement officers were able to locate some of these "grow ops" simply by finding the source of RF interference.
The LED power supply described on this page is of the same type that was found to cause these very high levels of RF interference.
Even though these lights aren't turned on very often, he decided that their flaws went firmly against his eternal crusade against RFI-generating devices at his house. After all, when it comes to RF interference, one should remember this cardinal rule:
Most RFI begins at home!
To be sure, there are many cases in which there are noisy power lines or a neighbors plasma TV - just two in a long list of things that can cause interference, but the worst offenders in generating interference are likely in one's own house. The main reasons for this are simple and (for the most part) obvious:
- They are nearby. If a noise generating device is in your house, it's very close-by - and the closer it is, the more your antenna is likely to intercept "grunge" from that device.
- They are connected to the same wiring as everything else in your house. There's nothing like a piece of copper to convey RF all over the place with minimal loss, and if a noise generator is powered from the mains, it's likely conducting much of that noise into the same mains connections that power your radios.
- If you are like most amateurs, you probably have radiating feedlines on your HF antennas. By their very nature, almost all HF antennas tend to radiate a bit of RF on their feedlines. For some antennas (e.g. dipoles, yagis, loops) this is incidental - often due to inadequate balun design, but other antennas (offset-fed antennas like Windoms, end-fed antennas) this is often by nature or design. If the feedline of your HF antenna isn't very well-balanced (often using a "current mode" choke) some of your "noise" from the devices in your house wiring is being conducted from your shack, onto the feedline and then into your antenna. Fixing this problem certainly warrants a series of articles itself, but suffice it to say, "noisy" devices will seem worse because of this issue than they would normally be.
The constant-current LED driver with added filtering. This LED driver
is typical of what is seen in these devices: A rather generic, potted module
of likely-questionable lineage and quality.
Click on the image for a larger version.
As is typical with these inexpensive LED lamps, the power supply is a constant current module that uses PWM/switching techniques to regulate the current applied to the LED array to some value. As can be seen Figure 1, this is simply a box with two sets of wires: The AC (main) input on one side and the DC output to the LEDs on the other.
Because these are constant current supplies, they can be used over a wide range of LED module voltages: 22 to 36 volts, according to its label of that in Figure 1. Noting the official "50 watt" power rating, we can do the math and see that with a constant 1500mA, the power being delivered to the LED array can vary from 33 to 54 watts, depending on its actual operating voltage. Depending on the design, these supplies may or may not have their DC outputs isolated from the mains input via an internal transformer, so it is best to assume that they are not isolated and that the DC outputs will be line-referenced and hazardous (even lethal!) to touch.
In this example, the red and black DC leads disappeared into the body of the case where it would connect to an LED module that is (presumably!) insulated from the lamp's case. Because you can't be sure what to expect, one must always make sure that the safety ground of these lamp housings is actually connected to the case (the ground wires in these devices are often not connected to the case at the factory!) and that it is plugged into a GFCI-protected outlet.
How bad was it?
In the case of these LED floodlights, the only connection that they had to the rest of the universe was via their power connections, so it was clearly via its power leads that they were radiating their "grunge". To determine in some quantitative way how noisy this device was, a simple test fixture was constructed to measure the energy imparted on the mains power lead, represented schematically in Figure 2, below:
The result of this measurement can be seen in Figure 3, below, covering the range from nearly DC to 1 GHz, with the cyan trace being with the unit turned off and the yellow trace with it turned on:
Refocusing on a smaller frequency range with different analyzer settings, let's take another look at how bad it is over the lower HF range:
Some board-mountable Shaffner mains filters from the Electronic
Goldmine, item G21844 (no longer available - sorry...)
Click on the image for a larger version.
"Fixing" the problem:
One solution to this problem (aside from not getting cheap, uncertified devices in the first place - but even then, one is never sure what one is really buying!) is to add known-to-be-effective filtering to the mains leads.
At about the time my friend brought these lamps to me, I noticed that the Electronic Goldmine had, on sale, some small, board-mount mains filters, so I suggested that he buy at least two for each of his three lights (for a total of six) - so he bought 10 of them. These particular devices were attractive because they were relatively inexpensive, potted (helpful, because this will be mounted outdoors where moisture ingress could be a problem) and small enough to fit in the limited-space enclosure in the back of the floodlight. Being that the lamps were only "50 watt", the 1.6 amp rating of these filters would be more than adequate.
You may notice something else about the layout: The wires going in and out of the LED driver are bundled together with plastic wire ties and routed to the "far" side of the power supply, as distant from the mains filters and wires as possible - this to minimize the amount of RF energy that might be coupled from these "noisy" wires into the power cord - something that would surely "un-do" some of our hard-won efforts in minimize the amount of conducted RF noise.
The results of this effort can be seen in Figure 7, below:
- The Cyan (blue-ish) trace is our baseline measurement with the LED driver module powered down. The signals below about 1.5 MHz are ingress from strong, nearby AM broadcast stations, some of which are nearly as strong as the noise at specific frequencies.
- The Yellow trace is with no filtering of the LED driver module, showing the relative energy from the LED driver module over the 0-10 MHz range.
- The Magenta (purple-ish) trace is with the LED driver module powered up with the added filtering.
"Could you have just snapped ferrites on the power cable?"
In reading this article, one might wonder if we could have solved the problem simply by putting snap-on ferrites on the power cord.
I doubt it.
Snap-on ferrite devices are very good about reducing the amount of RF conducted on wire, but with the extreme nature of the interference of these devices, it would never have been enough at HF. The reason for this is that in order to adequately quash the QRM to the "point of undetectability" it would take at least several k-ohms of impedance on the power cable to solve the problem.
While it is possible that one can do this, it would take several large-ish cores (probably mix 31) with a dozen or more turns on each just to add that much reactance - but that material and winding topology would only work to the high end of the HF spectrum, so you'd need another core or two with windings on different materials - say 43 and 61 mix.
To make matters worse, you'd have to keep these chokes well-separated physically or else RF energy would be conducted around them - or even radiate directly from this rather large structure: You certainly wouldn't have been able to easily fit it in the back side of the lamp's enclosure.
Self-contained filter modules like the ones used are specifically designed to quash RF over a very wide frequency range: Not only are bifilar inductors used, but capacitors are also used to force the interfering energy to common mode so that the inductors can best do their job, plus there are other capacitors that do an excellent job of shunting RF to the case to "completely" contain that energy.
In other words: In such an extreme case, you'd be far better off using an L/C filter like that depicted in Figure 5 without even bothering with ferrite chokes.
When this plot was taken, the circuit depicted in Figure 2 was very close to the filter networks and it was believed that some energy was directly coupling into it from the LED driver module. After the lamp was assembled (the cover put on and the power cord fitted) another test was done and no difference at all could be seen in the "on" and "off" traces - except at frequencies below about 1.5 MHz: I somehow managed to omit capturing this trace.
Did it help?
My friend reinstalled these lights and was happy to report that upon listening on various HF bands from 160 through 10 meters, he was unable to detect when the lights were on or off, indicating that the modification was successful. It is possible that within a few feet/meters of these lights that some low-level direct radiation of noise could have occurred on VHF/UHF frequencies, but this energy was demonstrably not being conducted via the power cord, and emissions would not likely be detectable more than a few feet/meters away, anyway.
Would just a single filter have done the job?
Probably - but since the lights were a bit of a pain to take down and put up again it was decided to use two of these filters just to avoid the possible hassle of having to take them down (and apart) again if just one filter hadn't been enough!
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Links to other articles about power supply noise reduction found at ka7oei.blogspot.com:
- Containing RF noise from a "pure" sine wave UPS. Even when it is not operating your sine wave UPS may be producing a lot of HF radio interference!
- Completely containing switching power supply RFI - link. Sometimes it can be difficult to quiet a switching power supply, so it may be necessary to put it in a box with strong filtering on all of the conductors that enter/leave.
- Minimizing VHF (and HF) RFI from electronic ballasts and fluorescent tubes - link. Electronic light ballasts, like many switching power supplies, operate in the LF frequency range so "cleaning them up" at VLF/LF/MF frequencies can be a challenge.
- Quieting high current switching power supplies used in the shack - link. This page describes techniques that can be used to reduce the amount of RF energy produced by switching power supplies that you may be using to power your radios. Again, higher-inductance chokes may be required at VLF/LF/MF frequencies.
- Reducing switching supply racket - link. This describes techniques that can be used to beef up the filtering for switching supplies in general.
This page stolen from ka7oei.blogspot.com