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I'm in the process building a linear lab PSU, the fan which is supposed to cool the heatsink is causing 6 mV peak to peak noise at the output of PSU. It's a 4x4 cm brushless chinese fan with unknown brand.

I tested adding a wide range of capacitors from 0.1uF to 100uF, close to the fan circuit or on the supply and it didn't make any difference. also tried adding an LC filter (47uH with 220uF) with no luck.

Adding an overkill LC filter on the fan input smooths the output though, 100mH inductor with a 100uF capacitor is causing a huge drop in the current (fan voltage drops to ~4.5V).

PCB trace:

The fan is being supplied from a different secondary than the main secondary:

schematic

simulate this circuit – Schematic created using CircuitLab

The secondary that is supplying the fan is also used to supply the op amps in the PSU circuit and has a common ground with the main supply.

How do I eliminate the fan noise?

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    \$\begingroup\$ Is it just me not seeing the whole picture or did you only draw 1% of your schematic, without the capacitors and fan where the whole question is about? I can guess from the PCB there are relays involved as well. Could you please show the relevant parts as well? \$\endgroup\$
    – Huisman
    Jan 30, 2020 at 18:48
  • \$\begingroup\$ @Huisman In the picture of the PCB those two connections on the right bottom of the picture are for the fan. the capacitors is also in the diagram. all relevant circuit is in the question. which part is missing? \$\endgroup\$ Jan 30, 2020 at 18:55
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    \$\begingroup\$ Where is the fan in the schematic? Where did you connect the LC filter? And which capacitor is which? I'd advise using reference designators in the schematic as well as in the PCB. As the schematic is shown now, I think hardly anyone will be able to answer/help. The more you show, the higher the chance of someone being able to help you in the right direction. \$\endgroup\$
    – Huisman
    Jan 30, 2020 at 19:05
  • \$\begingroup\$ I'd seperate completely the fan in this type of situation. Make it run off a standard 12V supply. If it still induces noise, you've got more serious issues. \$\endgroup\$
    – Natsu Kage
    Jan 30, 2020 at 19:12
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    \$\begingroup\$ If you physically move the fan away from the rest of the circuit, does the ripple decrease? \$\endgroup\$ Jan 30, 2020 at 19:19

1 Answer 1

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The fan current goes through the same ground trace as the PSU output. This will produce a voltage across the ground trace. The fan draws pulses of current during operation, so the voltage it produces will look like AC noise.

As you haven't shown us the whole PCB layout I can't tell how long this trace is, but the part shown looks quite long and thin so it probably has significant resistance.

You should route the fan ground directly to the rectifier ground or close by (not past the regulator ground). You probably should route the relay driver transistor ground to there too, to prevent noise when the fan voltage is changed. Best place to cut the track would be between the output capacitor and the transistor next to it.

Also try to make the PSU output ground trace as wide as possible, using a copper pour if your PCB design software has that feature.

enter image description here

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  • \$\begingroup\$ The ground trace goes to another PCB which bridge rectifier and regulators are located and it's 9 cm long. I cut the trace as you suggested and wired the fan negative line directly to bridge rectifier ground, it did reduced the noise to ~4 mV peak to peak. I connected a separate supply with common ground to the fan and it didn't cause any noise on the output. maybe the noise is going via both positive and negative line, any idea? \$\endgroup\$ Jan 31, 2020 at 7:17
  • \$\begingroup\$ The problem was from the common ground, after doing more investigation I managed to fix to it. thanks for giving me a clue on what to look for. \$\endgroup\$ Jan 31, 2020 at 8:45

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