Should I put capacitors on my LEDs to reduce possible RF interference? - Page 1
Forum section: electronics / beginners

author: SlowBro
author_rank: Regular Contributor
timestamp: January 13, 2018, 06:25:54 am
content: Any issue if I use capacitors on LEDs like the attached picture? The LEDs will be on approximately 10cm leads of stranded wire. The LEDs would not expected to be lit for any shorter than 5Hz.This question issimilar to anotherin which I asked about push buttons: I am being paranoid about my design producing extraneous emissions (and passing the FCC unintentional radiator lab measurements) so I want to put on low-pass filters on these LEDs to prevent stray noise leaking from the RPi to the outside world. My board has some 400KHz I2C devices and I could see there being a (very slight) potential for the LED leads turning into antennas.It is for me a matter of some two cent capacitors vs $500 an hour for lab time

author: TimNJ
author_rank: Super Contributor
timestamp: January 13, 2018, 07:24:13 am
content: The FCC radiated emissions regulations are concerned with frequencies in the megahertz range. You switching on an LED 5 times a second isn't likely to cause any issues.What is more likely to cause a radiated emissions issue is the power supply powering the LEDs. If it is a high frequency switching power supply, then you have the potential to radiate some energy in the 30-300MHz band.Also, every time you try to turn an LED on or off, you'll have a big inrush current into/out of the 10uF caps, i.e. a current spike. If emissions is your concern, large peak currents probably isn't a good idea. If anything, use a small cap like in the picofarads range. But I am not suggesting you use any caps, really.

author: BrianHG
author_rank: Super Contributor
timestamp: January 13, 2018, 09:59:44 am
content: You making things worse. Get rid of the 10uf caps.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: January 13, 2018, 10:18:13 am
content: Consider what happens when that driver turns on - you will get a big current spike from the charged cap, which could even damage the driver.If you want to drive LEDs softly,use a N-channel MOSFET with 10K in series with the gate.But chances are you'll have more common-mode noise on the LED cable from other parts of the system than anything the LEDs do, as the LED current is returned down the same cable.

author: IanMacdonald
author_rank: Frequent Contributor
timestamp: January 13, 2018, 10:39:40 am
content: Caps are on the wrong side of the resistors. Plus, larger than needed.Also, bear in mind that a ferrite ring with all the wires looped through it is a good way to stop RFI.Few years back, designed a timing device (glider launch signaller) which had to work next to an airband base station, and must not glitch when the PTT was pressed. A generous placement of same solved any problems. As a test, a 5W handheld with its antenna right against the box had no effect at all.

author: SlowBro
author_rank: Regular Contributor
timestamp: January 13, 2018, 06:25:14 pm
content: Quote from: TimNJ on January 13, 2018, 07:24:13 amAgreed. My concern is that my board has some 400KHz I2C devices and I could see there being a (very slight) potential for the LED leads turning into antennas.Quote from: TimNJ on January 13, 2018, 07:24:13 amThat's what concerns me, the on-board frequencies leaking outward. The supply is a USB wall wart on a 15' USB cable.Quote from: TimNJ on January 13, 2018, 07:24:13 amOkay good, thanks for the info. What do you use instead, or do you bother? I am reading now about ferrite chokes and would love help selecting the right type.

author: SlowBro
author_rank: Regular Contributor
timestamp: January 13, 2018, 06:34:01 pm
content: Quote from: BrianHG on January 13, 2018, 09:59:44 amOkay. Replace them with anything? Ferrites? Or nothing, because it's not usually a concern?

author: SlowBro
author_rank: Regular Contributor
timestamp: January 13, 2018, 06:35:02 pm
content: Quote from: mikeselectricstuff on January 13, 2018, 10:18:13 amAgreed. My concern is that my board has some 400KHz I2C devices and I could see there being a (very slight) potential for the LED leads turning into antennas.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: January 13, 2018, 06:37:30 pm
content: Quote from: SlowBro on January 13, 2018, 06:25:14 pmIt's not about the 400KHz, but the spectral content of the rise/fall timesQuotePull all the ferrites etc. on which might help, then when you get to the lab, do a quick scan, and if things look OK, start taking them off, in descending order of cost & see what the effect is.Or take several protos with different levels of filtering.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: January 13, 2018, 06:41:29 pm
content: Quote from: SlowBro on January 13, 2018, 06:34:01 pmIf you have a problem it will most likely be common-mode, not differential mode. A ferrite round all of the LED leads ( as one bundle, not individually) is a portential fix, but a better one is to understand where that CM noise is coming from and deal with it at source. Things like solid crounding and good decoupling around the MCU and any switching regulators, and minimising loop area of any noisy current paths is the first thing to get right.The LEDs themselves will not be an issue - any radiation from the leads will be the same whether or not there are actually LEDs at the end.

author: rstofer
author_rank: Super Contributor
timestamp: January 13, 2018, 06:49:17 pm
content: You can also add series resistors to slow down pulse transitions. Something like 300 Ohms between the pin and the world helps a lot in terms of eliminating reflections.Those large capacitors on the output pins is a really bad idea.I like the ferrite ring approach although I have never tried it.

author: SlowBro
author_rank: Regular Contributor
timestamp: January 13, 2018, 07:11:10 pm
content: Quote from: rstofer on January 13, 2018, 06:49:17 pmThere are already some 130 ohm resistors to reduce the voltage, is that sufficient?Here is the updated schematic. Not pictured is the ferrite ring. What size/spec ring are we talking about?

author: SlowBro
author_rank: Regular Contributor
timestamp: January 13, 2018, 07:22:47 pm
content: I have some ofthese clip-on ferrites. No spec given except "7mm+9mm inner diameter". Useful or too difficult to say?