Re: Challenging 2465 PS repair


Thanks for the added info Harvey. That's one of the reasons why I always
start my Variac at very low voltage (~1 VAC) and watch the Ammeter for a
good minute before I start *slowly* start increasing the voltage. If I get
any current that approaches the rated operating current (at 110VAC or
whatever) then I KNOW that there's a problem but as long as the current
draw stays *below* the rated current then it shouldn't cause any damage.

On Mon, May 3, 2021 at 2:14 PM Harvey White <> wrote:

The undervoltage damage theory isn't a theory under certain circumstances.

It all depends on how the switching is done.

If the pass element always switches, and has a current limit, then it's

Some supplies don't. I've see designs where there is a minimum required
voltage to start switching, and the transistor is full on until the
switching starts. If there's no protection, then you can get damage.
Bad design? Yes, but it's possible to do and you may not be aware that
you have one.

That's what I'd say would be the problem. Pass transistor on
(especially if it's a boost supply and not a buck supply).

Be aware, just in general, that some of the inexpensive boost or buck
modules coming out of China may behave like this. Running them on a
current limited supply will let you know.

Does your supply do this? Don't know. Could it? Maybe...

If I run a scope or piece of test equipment off a variac, I'm watching
the input current, and also a bit of the outputs to see what the
regulator is up to.


On 5/3/2021 1:47 PM, - wrote:
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
IMO there is a big difference between "it's theoretically possible to"
"likely too". I've *operated* literally thousands of smaller switching
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
"operated" loosely, since at very reduced voltages (< ~60%) they will
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
attempting to operate their equipment at both reduced and excess
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


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


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