Sujet : Re: PSU Ripple Update
De : jeroen (at) *nospam* nospam.please (Jeroen Belleman)
Groupes : sci.electronics.repair sci.electronics.designDate : 17. Mar 2024, 21:20:58
Autres entêtes
Organisation : A noiseless patient Spider
Message-ID : <ut7fo4$3mbrg$1@dont-email.me>
References : 1
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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