Path: ...!news.mixmin.net!eternal-september.org!feeder3.eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail From: legg Newsgroups: sci.electronics.repair,sci.electronics.design Subject: Re: PSU Ripple Update Date: Mon, 18 Mar 2024 07:22:14 -0400 Organization: A noiseless patient Spider Lines: 79 Message-ID: <2s8gvitepohui9rto2opbnpd7gnb0j0m61@4ax.com> References: <3haevi1jbhf4poc4s32t99391bq4tqfc42@4ax.com> <0foevi1hd2jgmfvk9qrrlkharfipuotf5q@4ax.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Injection-Info: dont-email.me; posting-host="1c79d514c520664d74cc170e7d7dce2b"; logging-data="164484"; mail-complaints-to="abuse@eternal-september.org"; posting-account="U2FsdGVkX1+4Oh35WQiTIAUALAgnbHBn" Cancel-Lock: sha1:pRSpI5N/krc+jhrabouc+zNyJeA= X-Newsreader: Forte Agent 4.2/32.1118 Bytes: 4696 On Sun, 17 Mar 2024 21:37:15 +0000, Cursitor Doom wrote: >On Sun, 17 Mar 2024 16:49:52 -0400, ehsjr wrote: > >>On 3/17/2024 3:47 PM, Cursitor Doom wrote: >>> 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. >> >>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