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From: piglet <erichpwagner@hotmail.com>
Newsgroups: sci.electronics.design
Subject: Re: power supply discharge
Date: Sat, 28 Sep 2024 18:25:47 -0000 (UTC)
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john larkin <JL@gct.com> wrote:
> 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.
> 
> 

Good bench power supplies should have minimal built in output capacitance;
it compromises current limit performance. Anyone wanting supply decoupling
caps on their test load is going to have them already at the load side
anyhow.

I am a fan of the HP / Harrison Labs supplies of 1960s that have no output
caps and are stable.

-- 
piglet