Path: ...!eternal-september.org!feeder3.eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail From: Cursitor Doom Newsgroups: sci.electronics.repair,sci.electronics.design Subject: Re: PSU Ripple Update Date: Sun, 17 Mar 2024 19:47:52 +0000 Organization: A noiseless patient Spider Lines: 47 Message-ID: References: <3haevi1jbhf4poc4s32t99391bq4tqfc42@4ax.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Injection-Info: dont-email.me; posting-host="6b68b13d0f8cd75cb4b90b640cffe83e"; logging-data="3888390"; mail-complaints-to="abuse@eternal-september.org"; posting-account="U2FsdGVkX1/+rLAWUCVGrNgnsZNJHWXM+ZrIMEQZyiQ=" User-Agent: ForteAgent/8.00.32.1272 Cancel-Lock: sha1:AOakILt+Yl1yipiUu7h7aBRkhiw= Bytes: 3127 On Sun, 17 Mar 2024 20:20:58 +0100, Jeroen Belleman 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.