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From a previous post of mine, you can see that I am trying to test the efficacy of some AC line filters. I got it to connect properly this time without frying my load, but we cannot seem to replicate the noise seen in the field (where our load is powered off the same input as compressors, air purifiers, other household appliances, etc. and is disrupting the measurements of our onboard analog and digital sensors).

In our lab, we've tried plugging in a compressor to the output of the 24VAC transformer, which is the same supply that is going to our small sensing PCB. However it doesn't seem to be inducing much noise so we cannot tell if the filters are doing anything.

Any suggestions?

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  • \$\begingroup\$ you could be getting conducted and radiated cable noise . best bet is an impulse current like a loop antenna discharging dI/dt=1A/ns from a big plastic charged cap. and small spark plug gap 0.1 to 1mm \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Aug 13 at 1:16
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Brush type AC / DC motors produce a lot of low-band RF and audio noise. The best I have found is a Dremel tool motor running at full speed with no load. Isolate it from the AC line with heavy LC filters and tap noise from the motor with 1 uF 630 VDC polypropylene capacitors. If you need more isolation insert a 1" to 2" diameter ferrite toroid, N30 material. Use 30 turns 22-24 awg on both primary and secondary.

Not sure what your bottom roll-off frequency is but the capacitors will remove much 50/60 Hz content and the added transformer should remove noise below 2 KHZ. Add more turns for more LF content, but N30 cores saturate easily.

If isolation not needed a 100 uH inductor could remove LF content, else you will get all the noise the brushes put out. Note that the capacitors will NOT prevent a shock hazard if power source is a AC outlet. If you need noise in the low MHZ range a spark-gap device or Jacobs ladder may work just fine.

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