Re: power supply discharge

On Sat, 28 Sep 2024 10:21:46 -0700, john larkin <JL@gct.com> wrote:

On Sat, 28 Sep 2024 09:44:44 -0400, legg <legg@nospam.magma.ca> wrote:
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On Fri, 27 Sep 2024 08:07:29 -0700, john larkin <JL@gct.com> wrote:
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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.
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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.
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I'm designing some programmable multi-channel power suplies and that
is one of many tangled issues in the project.

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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.

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I've measured a few, and got output terminal capacitance of a few
hundred to maybe 2000 uF.
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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.
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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.
>

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DC coupled programable supplies, or bipolar programmable
supplies are made to drive loads in the first and third
quadrants.
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There are issues in the second and fourth quadrants, where
the supply is expected to absorb power.
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An amplifier driving a pure reactance experiences the same
losses as driving a dead short.

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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.

Capacitors and inductors are as 'short' as their reactance allows.

Just don't be surprised when it happens - that's all I'm saying.

RL