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Noise filter for ATX power supply

D

daveatfernie

Jan 1, 1970
0
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
 
M

Mike Monett

Jan 1, 1970
0
daveatfernie said:
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
 
T

Tim Williams

Jan 1, 1970
0
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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