Re: PSU Ripple Update

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.