Re: PSU Ripple Update

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. :-/