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Driving an LED with a microcontroller should be easy. But when looking into noise, things can get complex...

As an instantaneous voltage indicator used in a variable power supply (which will be used to simulate a photovoltaic array of a student's designed satellite), I am using some LEDs controlled by PWM (~31 KHz).

schematic

simulate this circuit – Schematic created using CircuitLab

After the first revision of the PCB I have realized that each one of the LEDs is generating 200 mVpp in the 5V line:

noise screenshot

My question: which is the best way to reduce this noise? Why?

From my (small) design experience I could come with the following possibilities. Which is the most effective, taking into account real-world components (ESR in capacitors etc)? Any other suggestion? A combination of several?

(Note: changing the PWM frequency is not a good option because that same signal drives other devices)

a) Decoupling Capacitor

schematic

simulate this circuit

b) Low-pass filter

(The resistor forms an RC low pass filter with the gate capacitance of the MOSFET, removing the high frequency components of the switching)

schematic

simulate this circuit

c) Snubber

schematic

simulate this circuit

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  • \$\begingroup\$ Why are you driving the LED at such a high frequency? There is no noise to avoid as with a motor... \$\endgroup\$ – Kurt E. Clothier May 20 '13 at 1:36
  • \$\begingroup\$ That same signal is being fed to a low-pass filter to create an analog voltage, which then is amplified and buffered to simulate the power generated by a solar panel. In order to achieve good output for that part of the circuit, the frequency should be as high as possible \$\endgroup\$ – svilches May 20 '13 at 3:46
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Your a) solution is a good one, just make sure the capacitor is as close as possible to the resistor/LED/transistor-branch as possible. Start with a 100nF value and see how that works. If ripple is still too large to your liking, add a electrolytic capacitor in parallel to the 100nF. The 100nF will suppress the higher frequency components and the electrolytic capacitor will do better for lower frequency components.

As @pjc50 says in one of the comments, a gate series resistor as shown in b) is good practice too to avoid ringing. I'd personally pick a lower value, say 100Ω. It will suppress ringing and will avoid the transistor spending too much time in linear mode (=dissipating heat).

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    \$\begingroup\$ I would apply (a) and (b): (b) won't make much difference at the 31khz frequency but will suppress ringing RF noise. \$\endgroup\$ – pjc50 May 20 '13 at 9:21
  • \$\begingroup\$ A right, I didn't see the difference between b) and the original circuit before. @pjc50 you are right, though I would probably use a smaller resistor (100 ohm or so). \$\endgroup\$ – jippie May 20 '13 at 9:26
  • \$\begingroup\$ In which applications the snubber circuit is needed? \$\endgroup\$ – svilches May 20 '13 at 13:54
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    \$\begingroup\$ A good snubber would go on the load itself, not on the switch. C1/Rlim is exactly that. Can't think of a good application in this particular case to have the c) configuration. It is just wasting energy in R1 here. \$\endgroup\$ – jippie May 20 '13 at 14:13

protected by W5VO May 20 '13 at 4:48

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