Path: ...!eternal-september.org!feeder3.eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail
From: john larkin <JL@gct.com>
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: <p5egfjlmuo9cmj5qura7daj8uhr0n2khoi@4ax.com>
References: <c5idfjp9miqru154ei6tnmg8m14qd30m6d@4ax.com> <ak1gfjdv9dlhgm2eraj7jblqimresabjnm@4ax.com>
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 <legg@nospam.magma.ca> wrote:

>On Fri, 27 Sep 2024 08:07:29 -0700, john larkin <JL@gct.com> 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.