PWM filtering - Page 1
Forum section: electronics / projects

author: orinocopaul
author_rank: Contributor
timestamp: August 25, 2021, 09:45:54 pm
content: I am trying to dampen the noise created by a PWM LED dimmer. With help on the forum, I successfully eliminated the PS whining using a pi filter (see pic below) btw the dimmer and the SMPS. However, there is some noise left coming from the LED strips and the housing they are sitting in. These are my two failed attempts to filter the dimmer's PWM output:Attempt 1Made a low-pass RC filter. It cleaned up the signal and eliminated the audible noise. However, the lights got significantly dimmer (because of the resistor I guess). Increasing the PS output voltage to compensate for the voltage drop fries the resistor completely. I would need to pass 4.5 amps through the resistor. I don't know if that is even possible.Attempt 2Tried filtering the PWM using a CLC pi filter. The PWM straightens pretty well, though a little noisier than the RC filter above. For reasons I cannot perfectly explain, one leg of the first capacitor turns red hot and the dimmer becomes unresponsive.I am open to any suggestions.

author: Tom45
author_rank: Frequent Contributor
timestamp: August 25, 2021, 09:59:33 pm
content: It would be a big help if you gave some specifics:What current are you trying to drive the LEDs with?What PWM frequency?What component values for the first attempt?What component values for the second attempt?4.5 amps through a resistor can be no problem if the resistor's value is low enough. Or a big problem for a higher value resistor.Try computing the power dissipated in a 0.01 ohm resistor and 4.5 amps. Then do the same for a 100 ohm resistor.

author: John B
author_rank: Frequent Contributor
timestamp: August 25, 2021, 10:08:30 pm
content: If you run a circuit sim of a CLC filter, you will notice that the current though the capacitors is not equal. The one subjected to the highest AC content is going to have a lot of current flowing through it, and so the internal resistance of the cap is going to cause the heating. You may need parallel low ESR caps to give the best results.

author: orinocopaul
author_rank: Contributor
timestamp: August 25, 2021, 10:16:52 pm
content: Quote from: Tom45 on August 25, 2021, 09:59:33 pmThanks for the quick reply.I need to drive the LED's with a maximum of 4.5A.The PWM frequency is 7.8 kHz. Matches the audible noise. PWM high/low state 12/3.5 V.RC filter: 10 ohm + 1000 uF (low ESR)CLC filter: 1000 uF + 7,5 uH + 1000 uF

author: orinocopaul
author_rank: Contributor
timestamp: August 25, 2021, 10:50:34 pm
content: Quote from: John B on August 25, 2021, 10:08:30 pmThis is what I get with parallel low ESR caps. It does work now, but still kind of noisy. Maybe the RC filter would do a more effective job. (the extra caps also make the dimming super slow)

author: Zero999
author_rank: Super Contributor
timestamp: August 27, 2021, 12:15:33 pm
content: The only way to do this is to increase the PWM to above 20kHz.

author: orinocopaul
author_rank: Contributor
timestamp: August 30, 2021, 07:37:41 pm
content: Quote from: Zero999 on August 27, 2021, 12:15:33 pmThat's a great idea. The thing is that I'm using a prefab dimmer with included RF remote control for intensity and temperature. I have no idea how to re-program this dimmer.

author: orinocopaul
author_rank: Contributor
timestamp: September 10, 2021, 09:46:49 pm
content: In conclusion, I did filter the 7.8 kHz PWM output of the LED dimmer. While the output was pretty clean, the vibration migrated from the LED strips to the dimmer itself. This made the whining localized, thus more audible. For this reason I removed this low pass filter in-between the dimmer and the LED strips.The only filter left, that does a great job at keeping the switched mode PS quiet, is a pi filter with beefy caps in-between the PS and the dimmer.