Hi All,
I use a number of ATX power supplies as bench supplies. However the voltage changes when the load changes and there is quite a bit of noise accross all frequencies on it. I plug in and disconnect devices from the PSU randomly.
I'd like a way to clean up the noise from the supply. Can anyone suggest the circuit to make a nice clean powersupply from it. I would like to get rid of as much noise form as many frequencies as possible.
Many thanks in advance
Daveatfernie
Switching power supplies are great for high power and digital work, but if you are having problems with noise, you are probably working with low-level analog signals.
In this case, once the switching noise is generated, there no good way to get rid of it.
Filtering, grounding and shielding have limited effect. Without completely shielding everything, the noise simply goes around them. So you need a linear supply with special attention to eliminating the switching noise from diode turnoff in the bridge rectifier.
Even if you use a linear supply for dc power, simply connecting the circuit to a pc to transfer the data can often introduce large amount of noise into the circuit. Optoisolators can be used to reduce the noise, but the inexpensive ones are slow.
When working with low-level circuits with reasonable bandwidth, the amount of effort you have to spend reducing noise increases as the signals get weaker. Often, you can amplify the signal at the point where it is generated, and only have to shield and filter a small area. The resulting high amplitude signals won't give as much problem with noise.
Everyone has their special tricks and approaches. Listen to them carefully, since they are telling you things that cost them dearly to learn:)
Regards,
Mike Monett
I don't see why it can't be fixed with filtering. I mean, you do worse with an analog supply with crud down to 120 or 60Hz. You just need smaller filters to get rid of the 1MHz-ish harmonics without coupling it back in by parasitic capacitance.
I would use a common-mode choke, then a pi filter or two to kick HF noise. Then another pi filter if I need low hum too.
Tim
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