Re: Challenging 2465 PS repair


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The equipment that I usually work on is usually too old to use a
switching PSU. Also I've NEVER had a switcher fail by using it on a Variac.
IMO there is a big difference between "it's theoretically possible to" and
"likely too". I've *operated* literally thousands of smaller switching PSUs
at extremely reduced voltages and never seen one that damaged itself by
drawing "excessive" current. Sure, they will draw more current at reduced
voltages but, in my experience, *never* enough to damage themselves. I say
"operated" loosely, since at very reduced voltages (< ~60%) they will not
operate and their current draw drops to near zero. Let me also point out
that as part of their Performance Tests, HP and others frequently call for
attempting to operate their equipment at both reduced and excess voltages.
Also I don't ever recall seeing a warning in a HP manual saying that
attempting to operate their equipment below XXX voltage *would* cause
damage.

YMMV

PS does anyone have a few switching PSUs that they're willing to
possibly sacrifice, in order to test and see how low their input voltage
can go before they stop operating and if very low input voltages will
damage them? Maybe we can finally prove or disprove the "under-voltage =
damage" theory.

On Mon, May 3, 2021 at 11:59 AM Tom Gardner <tggzzz@gmail.com> wrote:

Since a *switching* PSU will attempt to deliver a constant output
power, a low input voltage means a high input current flowing through
transistors, diodes and some capacitors.

An abnormally high current might cause damage.

On 03/05/2021, Milan Trcka <milan.v.trcka@gmail.com> wrote:

In an attempt to keep in as much smoke as possible when powering up
suspect
or failed PS, I use an adjustable transformer (Variac) to slowly bring up
mains voltage while monitoring for vital signs. Use isolation
transformer or
battery powered isolated scope when monitoring waveforms.

M









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