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From: Bill Sloman <bill.sloman@ieee.org>
Newsgroups: sci.electronics.repair,sci.electronics.design
Subject: Re: PSU Ripple Update
Date: Tue, 19 Mar 2024 00:31:54 +1100
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On 18/03/2024 6:47 am, Cursitor Doom wrote:
> On Sun, 17 Mar 2024 20:20:58 +0100, Jeroen Belleman
> <jeroen@nospam.please> wrote:
> 
>> On 3/17/24 18:48, Cursitor Doom wrote:

<snip>

>>> Now I did check to see if there was anything downstream which had
>>> shorted or gone low-resistance which could possibly account for this,
>>> but found nothing amiss. So the question is:
>>> What could cause ripple to arise when even very light loads are
>>> applied to the output of a pretty substantial linear PSU?
>>>
>>> BTW, the bridge rectifiers were fine and have been exonerated from any
>>> culpability in this fault.
>>
>> Isn't that what you'd expect? The storage capacitor gets topped
>> up twice per mains period. In between top-ups, it's the sole source
>> of the output current, so its voltage drops until the next top-up.
>>
>> You could measure the rate of voltage drop and check if it has the
>> expected slope for the output current and the storage cap's value:
>> dV/dt = -I/C.
>>
>> You could also check if the pass transistors still have enough
>> voltage across them at maximum current and just before the next
>> top-up. Do you see ripple on the regulated output?
> 
> Not only am I seeing ripple on the regulated output, I'm seeing it on
> the final output of the signal generator this PSU powers.

That sounds as if the linear regulators aren't regulating.

You need to compare the voltages applied to the inputs of the 
regulators, which can be expected to show appreciable ripple with the 
nominally regulated output, which shouldn't. Any residual output ripple 
should be a couple of orders of magnitude smaller than the input ripple.

> It's quite a beefy PSU, but as little as 90mA draw gives rise to an
> unacceptable level of ripple which permeates through the whole of the
> downstream circuitry.

It may be "beefy" but it doesn't seem to be working as it should.

-- 
Bill Sloman, Sydney