Re: Can Low Voltage kill a power supply?

Rod Speed wrote:
Sjouke Burry <burrynulnulfour@xxxxxxxxxxxxxxxxxx> wrote:

Ray Cassick (Home) wrote:

"Rod Speed" <rod.speed.aaa@xxxxxxxxx> wrote in message
news:4kp3plFcutpeU1@xxxxxxxxxxxxxxxxx


n33ck0@xxxxxxxxx wrote:



Hi, everybody. 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.

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


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.



And that depends on the quality of the supply.




.

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