Re: power supply discharge

On Sat, 28 Sep 2024 18:25:47 -0000 (UTC), piglet
<erichpwagner@hotmail.com> wrote:

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.

That goes without saying for linears. SMPAs are increasingly low-cap
as the conversion frequencies rise.
 
Smaller switching amps - think mp3 portables - don't bother filtering
anything, they juat figure the load will do it for them.

The $2 regulators from offshore are a role of the dice (or carefull
visual inspection), whether an output filter is there, or not. Some of
the cheapest even expect a common + rail.

I've seen unfiltered high power stuff that lets a lossy LF isolation
transformer do the job, but you shouldn't be able to smell (or hear)
a power supply in operation, if it's properly designed.

Another instance of 'Why Can't I Do That'.

You WERE saying 'good', so I expect you're in a different ball park.

RL