Noise from linear regulator output?

Thread Starter

eem2am

Joined Jul 22, 2012
24
Hi,

Today I was scope probing the output of an offline 300W linear regulator. (Mains transformer/rectifier/capacitor/regulator). Output was 69.5V at 5A.

When I probed from Vout to GND I got the attached trace, which was noisy.

I wanted to see if the noise was just pickup from the surroundings, so I attached both probe tip and GND to the Vout node only, and looked again…but this gave a much quieter trace (as attached), which kind of proves that the noise was actually coming from the linear regulator output?
 

Attachments

DickCappels

Joined Aug 21, 2008
8,686
Linear regulators do make noise, but I can't tell from your scope images how much noise you are looking at or what the frequency content is. The clipleads are probably not helping you any if much.

When measuring noise try to find or make a spring clip like this so the ground connection is less likely to pick up noise.
1643398999604.png

If the power supply has a switching pre-regulator that may be the source of most of the noise. Does the supply have a noise specification?
 

LowQCab

Joined Nov 6, 2012
1,929
Schematic and Part Specifications please.

Your Regulator may be going into Oscillation due to improper Filter-Capacitor selection.
.
.
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Thread Starter

eem2am

Joined Jul 22, 2012
24
Thanks, it seems strange that the noise pickup happened much more when the probe was connected across power supply output and ground.....though maybe this made the pickup loop bigger due to it traversing the power supply power and ground tracks.

Spec: its just a replacement power supply we've built for someone, ...they asked for <10mV ripple at 100Hz......but ive been told not to build a special noise probe for the scope, and that if it appears to be less than 10mV with the "bad probe" shown, then we can assume it'll be ok if we used a "good probe"....it is a bit unfortunate though, that all this high frequency noise is kind of fuzzing it up.
 

Thread Starter

eem2am

Joined Jul 22, 2012
24
Hi,
Do you agree, when measuring power supply output ripple voltage, x1 scope probe setting should be used?
 

MrChips

Joined Oct 2, 2009
25,928
Hi,
Do you agree, when measuring power supply output ripple voltage, x1 scope probe setting should be used?
No.
There are reasons for using x1 and x10 probe settings. This is not one of them.
As a general rule, I always set the probe to x10 attenuation.
 

MrChips

Joined Oct 2, 2009
25,928
You need to learn how to use your test equipment and how to take measurements correctly.
You got lucky on this occasion. In most circumstances you would have blown your power supply and the oscilloscope by doing what you did.
 

MrChips

Joined Oct 2, 2009
25,928
You need to learn how to use your test equipment and how to take measurements correctly.
You got lucky on this occasion. In most circumstances you would have blown your power supply and the oscilloscope by doing what you did.
Normally, your oscilloscope is grounded.
If your power supply output is also grounded which is very common, connecting the probe's GND to the output of the power supply would have created a short circuit between the power supply's output and the scope's GND. Something would have gone up in smoke!

5A @ 70V is 350W!
 
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