solved: Placing a capacitor across an PWM driven LED for EMI reasons? - Page 1
Forum section: electronics / beginners

author: incf
author_rank: Frequent Contributor
timestamp: April 03, 2025, 03:55:35 pm
content: Attached is a doodle.I've got a diagnostic LED that is driven directly from an H-Bridge. The H-Bridge switches very quickly and has fast rising edges - dozens of volts peak to peak. The LED is located a long distance away from the H-Bridge.In order to limit any potential EMI, I am considering placing the LEDs current limiting resistor and a parallel capacitor at the H-bridge. This will leave a long high impedance trace (with almost no AC content) going to the LED.A small part of me has qualms about having something like a capacitor in parallel with an LED, but so far as I can tell, it won't damage anything and 'should' operate as intended. (the LED should illuminate without shortened lifespan)Can you spot any issues with this circuit?I am trying to use as few components as possible. I don't really care about flicker, or average brightness, or anything like that.

author: BennoG
author_rank: Frequent Contributor
timestamp: April 03, 2025, 07:48:30 pm
content: without the resistor your led will be at full brightness (or even burn) when you only use the elco.With the resistor you can make a low pass filter, but the linearity of your pwm is probably gone.For the C I would use something liken 10 or 100 nF and the R about 100 ohm, depending on the LED current.Don't do it without resistor then you will probably burn you H bridge.Benno

author: Andy Chee
author_rank: Super Contributor
timestamp: April 03, 2025, 09:06:49 pm
content: Consider what happens when the duty cycle is 99-100%What will happen when dozens of volts are placed across the LED?

author: mikerj
author_rank: Super Contributor
timestamp: April 03, 2025, 09:26:43 pm
content: Quote from: Andy Chee on April 03, 2025, 09:06:49 pmDozens of volts will not be placed across the LED, the resistor limits the LED current so the voltage across the LED will be the Vf at whatever current is pass through it. Additionally I am assuming the "dozens of volts" that the OP is referring to are the spikes caused by the PWM edges driving into a long wire rather than the nominal output voltage of the bridge. Could be wrong on this though.The capacitor across the LED is fine assuming you are only interested in changing the brightness of the LED. If the LED needs to be modulated (e.g. an infra-red remote system) then the capacitor will likely stop it working.

author: Gyro
author_rank: Super Contributor
timestamp: April 04, 2025, 08:44:59 am
content: The OP indicates that it is a diagnostic LED, presumably monitoring one half of the H-Bridge - there would be no point in driving a unipolar LED from the bipolar output of the full H-Bridge (although it is worth checking, as the LED wouldn't survive significant reverse voltage).No, there is no problem with putting a small capacitor to ground after the series LED current limiting resistor. As you are driving it from quite a high voltage, the series resistor will be quite high in value, so the capacitor can be corresponding small if you are just wanting to slow the edges for EMI reasons. Just calculate for the desired RC time constant.

author: Vovk_Z
author_rank: Super Contributor
timestamp: April 04, 2025, 10:01:47 am
content: In short: if LED curent is small enough then RC filter is OK (as usual).And, as usual, if current is large enough then LC filter is more OK.

author: Andree Henkel
author_rank: Regular Contributor
timestamp: April 04, 2025, 01:01:47 pm
content: what kind of H-Bridge do you have?in case of diskrete Transistors, better add a resistor or a ferrit at the gates, this slows the switching speed and reduces the EMI at cost of slightly more power loss in the Transistors.But with the RC you just have this power loss in the R to achieve the EMI improvement.