3D printer Ramps controller board PWM interference on 12V line

I have a ramps 1.4 board which among other things, drives a 12V cartridge heater for the extruder hot end. The controlling arduino mega regulates the desired temperature via PWM through a simple mosfet driver.

I have connected a short (8 leds) LED strip to the board’s onboard 12V aux connector. The mosfet and the aux connector share the same 12V line which has basic filtration via a parallel 100uF/100nF arrangement.

The problem I have is that when the PWM algorithm is active the LED strip starts to flicker (I know this as the board has an LED to indicate power status to the heater output and it flashes when PWM is active). The LED strip flickering happens exactly and only when the onboard LED flashes.

I’m guessing the power supply capacitors aren’t quite up to the task? If so should I simply be increasing them, and what value would be appropriate? I’m also beginning to suspect this PWM ripple is also causing a couple of minor issues with the quality of my prints.

Yes, the power supply is struggling - it may be that it is not happy to drive a fluctuating load or that it is being overloaded. Or both!

It's a modified ATX PSU which has a current rating of 11A on the 12V line.

I connected another LED strip directly to the 12V terminals of the PSU, and strangely this did not flicker at all yet the (shorter) strip attached to the ramps board was flickering.

ATX supplies are not really designed for fluctuating loads... The 12V outputs just power disk drive motors. Suggest you either push the heater PWM frequency up to kHz range where decoupling caps can help, or live with the flickering.

Maybe your power wires are too thin, and the voltage loss us too high, try thicker wires.