Path: ...!eternal-september.org!feeder3.eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail From: john larkin Newsgroups: sci.electronics.design Subject: Re: power supply discharge Date: Sat, 28 Sep 2024 10:21:46 -0700 Organization: A noiseless patient Spider Lines: 47 Message-ID: References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Injection-Date: Sat, 28 Sep 2024 19:21:49 +0200 (CEST) Injection-Info: dont-email.me; posting-host="42e606585a34e9e32b9539c830156d25"; logging-data="1424631"; mail-complaints-to="abuse@eternal-september.org"; posting-account="U2FsdGVkX19WT4ZKS6fmLLBizHmGRjdt" User-Agent: ForteAgent/8.00.32.1272 Cancel-Lock: sha1:6KAEy/eXBDNiRYyLg29yX4rbGe8= Bytes: 2823 On Sat, 28 Sep 2024 09:44:44 -0400, legg wrote: >On Fri, 27 Sep 2024 08:07:29 -0700, john larkin wrote: > >> >>Given a benchtop power supply, you can turn the voltage up and then >>down, and it goes down. Most have a substantial amount of output >>capacitance, and can be driving an external cap too. So something >>pulls the output down. >> >>I guess that there are no standards for this, but I've never seen a >>supply that just hangs high when it's cranked down. >> >>I'm designing some programmable multi-channel power suplies and that >>is one of many tangled issues in the project. > >Twiddling the adjustment knob on a bench supply doesn't >represent a dramatic change - and most adjustible >supplies don't load their output terminals with a >lot of capacitance. I've measured a few, and got output terminal capacitance of a few hundred to maybe 2000 uF. People here might measure some random power supplies. I leave them off and connect to a 50 ohm sinewave-output function generator and find the -3 dB point. One could use a square wave and scope the slopes too. Keeping the amplitude low will avoid turning semi junctions on. A square wave source driving a cap illustrates C, ESR, and ESL on a scope. C-meters don't usually separate the components so trend to lie, especially with big electrolytics. > >DC coupled programable supplies, or bipolar programmable >supplies are made to drive loads in the first and third >quadrants. > >There are issues in the second and fourth quadrants, where >the supply is expected to absorb power. > >An amplifier driving a pure reactance experiences the same >losses as driving a dead short. I don't understand that. An audio amp driving a 1 pF cap or a 1K henry inductor would surely cause less amp losses than a short.