Re: Can Low Voltage kill a power supply?

David Maynard <nospam@xxxxxxxxxxx> wrote
Rod Speed wrote
David Maynard <nospam@xxxxxxxxxxx> wrote
Rod Speed wrote
Sjouke Burry <burrynulnulfour@xxxxxxxxxxxxxxxxxx> wrote
Ray Cassick (Home) wrote
Rod Speed <rod.speed.aaa@xxxxxxxxx> wrote
n33ck0@xxxxxxxxx wrote

2 weeks ago ther was a case of low voltage in the neighborhood. After calling the
utility co. it was determined there was too much demand on the grid. not anything
specific in the house. So here is my question can a low voltage situation kill a
computer power supply.

A properly designed power supply should shut down in that situation.

A badly designed power supply can be killed in that situation.

Just a bit curious as to how you think a power supply can be damaged by a low
voltage situation?

Since all the regulation in a PS is designed to convert AC to DC
and then regulate those output voltages down to useable levels within a specified
tolerance I don't see how a low voltage could result in a dead PS. I DO see how it
can cause flakiness on the low voltage
side since all outputs are based upon a properly regulated input voltage tolerance,
but all that should result from a low input voltage is
proportionally low output voltages. Personally I would think that
most of the damage would have been
caused by a backlash of higher voltage that can often occur after
low voltage situations. This higher than normal voltage inrush
could happen faster than the regulators are prepared to handle
and cause a very quick spike to get through the filters and fry
the lower voltage side of the regulation circuit perhaps.

A switching supply will draw a bigger current from the
mains to compensate for less voltage, the output voltage
wont drop ,power wont drop, so you need more current.
When input is low enough ,and current becomes big enough,
either the safety cuts in, or the supply blows.

That increased current isnt what kills a badly designed supply.

Very well can.

Nope, not when it didnt drop enough to cause the other system to turn a hair.

According to that kind of logic it didn't fail at all, yet it did.

Wrong again. One PSU is properly designed and the other isnt.

The only current that goes up is from the mains and the diodes
that rectify the mains arent that marginal current capacity wise.

The problem is the flyback regulator. Low voltage on the filter
caps means an increased PWM duty cycle to compensate,

Yes.

stressing everything: drive transistors, flyback transformer, flyback diodes, etc.

That shouldnt kill anything in a properly designed power supply.

In theory nothing should... but it failed.

Wrong again. One PSU is properly designed and the other isnt.

What actually kills a poorly designed power supply in that
situation is the inevitable associated mains surges as loads
trip out due to the low mains voltage, particularly motor loads.

Of course, that shouldn't make it past the input suppressors and filter caps.

Shouldnt and didnt are too entirely separate matters.

That's funny considering you've just been singing the "properly designed power supply"
mantra.

Yes, clearly the one that did die isnt properly designed.

MUCH more likely than your scenario killing the power supply.

Frankly, no.

Fraid so.

Surge suppression of the type you're arguing is relatively simple and fairly static from
design to design. I.E. the power mains don't 'change' when one goes from, say, a 400W
design to a 450W.

Yes, but you dont know that the badly designed power supply
would have survived that surge even in the 400W version.

Likely it never did the surge suppression properly.

The flyback circuit, however, is a whole different situation.
Get that off so the core goes into saturation with an unpredicted scenario and you've
got fried flyback.

It is however trivial to design the supply so that that cant
happen by monitoring the rectified mains voltage and shutting
the supply down when its low enough to produce that result.

And it's no very hard to do, especially when plagiarizing, 'revising', or designing to
power line 'specs' (meaning normal tolerances).

It is however trivial to design the supply so that that cant
happen by monitoring the rectified mains voltage and shutting
the supply down when its low enough to produce that result.

Not a shred of rocket science required at all.


.

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