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From: legg <legg@nospam.magma.ca>
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Subject: Re: PSU Ripple Update
Date: Mon, 18 Mar 2024 07:22:14 -0400
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On Sun, 17 Mar 2024 21:37:15 +0000, Cursitor Doom <cd@notformail.com>
wrote:

>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. :-/

Were you measuring across the filter cap terminals? No other scope 
ground connection?

RL