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cheap chinese adjustable lab power supply kit on ebay ( 3AG1 , xiaolin )
exe:
Just to be sure, what power supply are we talking about? Because if it's a linear psu with a diode bridge and a capacitor then it's not a waveform I'd expect. I'm also not entirely sure that small caps across diodes do much due to very slow voltage frequency in mains (50-60Hz).
floobydust:
It's this linear PSU.
In the video, VoltLog mentions he does have noise on mains in his lab, so he added a CM choke after the PSU transformer secondary for his measurements to help filter some out.
But as you can see there is still some present. Nothing to do with this PSU kit.
exe:
If it's a linear psu, then why does it have 72mV p-p 100HZ noise? :) It's also strangely shaped. I'd say something wrong in there. Not entirely sure the noise comes from the outside. At least not all of it. I've built like 5 linear power supplies, never had issues like that in the final product (did have stability issues, etc, but that were problems in schematics, wrong parts, etc). I'd expect noise and ripple to be well below 10mV under full load and no high-freq components (although, there are some nasty loads...).
So, there are two problems: 1) 72mV p-p noise 2) ringing at the top of waveforms. I'd check drive of pass transistor to be sure there is no stability issues.
Cliff Matthews:
He should have measured those caps before installing. Maybe there's cheap 470uF caps inside of bogus cans.
mikerj:
--- Quote from: exe on April 03, 2019, 09:46:59 am ---If it's a linear psu, then why does it have 72mV p-p 100HZ noise? :) It's also strangely shaped. I'd say something wrong in there.
You should expect to see 100Hz ripple on the main filter apacitor of any linear supply that uses a full bridge rectifier. I don't understand why you think this is wrong? The person that posted the waveform didn't mention anything about how much load was on the output of the supply when it was measured so it's impossible to say if the amount of ripple is too high or not.
Capacitors across bridge rectifier diodes certainly are helpful. Despite the mains frequency being low, the rectifiers still switch on and off quickly, generating high frequency harmonics. As I said, you will find these capacitors in linear supplies where low noise is important.
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