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From: Cursitor Doom <cd@notformail.com>
Newsgroups: sci.electronics.repair,sci.electronics.design
Subject: Re: PSU Ripple Update
Date: Sun, 17 Mar 2024 21:37:15 +0000
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On Sun, 17 Mar 2024 16:49:52 -0400, ehsjr <ehsjr@verizon.net> wrote:

>On 3/17/2024 3:47 PM, 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:
>>>> Gentlemen (and others)
>>>>
>>>> I only get a few spare minutes a week to look into this, hence this
>>>> update. Hopefully my latest finding might ring a bell for some of you
>>>> and assist in pinpointing the fault with this (linear) PSU.
>>>> So, I've carried out a few more tests and discovered that there is a
>>>> total absence of ripple on the storage caps when all the downstream
>>>> circuitry has been disconnected. So it's totally fine with no load.
>>>> However, as I re-connect all those downstream circuits, the ripple
>>>> commences and the more connectors I re-attach, the worse it gets. This
>>>> is a screen shot showing over a volt of ripple at only about 66% of
>>>> the full supply voltage applied:
>>>>
>>>> https://disk.yandex.com/i/vgxfpXgNp-F4Yg
>>>>
>>>> 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?
>>>
>>> Jeroen Belleman
>> 
>> 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.
>> 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.
>
>OK, we need to know more. Your scope picture shows about
>1 volt ripple (assuming an X1 probe). Is that correct?
>(As I see the image it indicates 500mV per division, but
>the units - mV - are an assumption on my part as the image
>is not 100% clear.) 1 volt ripple measured across the
>filter caps is not a problem.
>
>Next, what is the DC level on the input to the regulator,
>and what is the DC level on the output of the regulator?
>The ripple should be eliminated by the regulator if it's
>working properly. Can you descri9be the regulator circuit?
>
>Ed

I'm unable to answer very much until the next time I have an hour to
spare to look at the issue again (which will probably be next Sunday).
In the mean time, I'll be noting any questions such as yours and will
answer them all in a single post after the next inspection. It's a
pity I don't have more free time available for this, but that's just
the way it is at present. :-/