Power Interruptions and PSUs

[AmberClad]

You know those momentary power interruptions that happen during a lightning storm? Why is it that some devices like lights and monitors will flash off and then back on, but the computer itself stays fully on (no reboot or power down)?

Is it the PSU capacitors (I'm really not familiar with how it all works)? I think I've noticed before when I unplug the PSU power cable that the motherboard lights take several seconds to go out.

 

[Mark R]

Yup. The PSU capacitors are designed to store energy.

Because the mains power is AC, it shuts off very briefly 120 (or 100) times a second. So some store of energy is required if you want to power something electronic which requires a constant supply.

The capacitors are oversized so that they don't get overstressed with the normal 120 Hz fluctuations even at maximum load. On most PCs, the PSU is running at barely 30% capacity - so there is a lot of spare capacity which can be called upon for a brief interruption - perhaps enough to keep the PC going for 1/10 or 1/20 of a second.

 

[AmberClad]

Originally posted by: Mark R
Yup. The PSU capacitors are designed to store energy.

Because the mains power is AC, it shuts off very briefly 120 (or 100) times a second. So some store of energy is required if you want to power something electronic which requires a constant supply.

The capacitors are oversized so that they don't get overstressed with the normal 120 Hz fluctuations even at maximum load. On most PCs, the PSU is running at barely 30% capacity - so there is a lot of spare capacity which can be called upon for a brief interruption - perhaps enough to keep the PC going for 1/10 or 1/20 of a second.

Sweet. Thanks for the explanation :thumbsup:!

 

[BrownTown]

Yeah, its the capacitors providing momentary voltage support. Although if you have a cr@ppy computer like mine it ends up rebooting more times than not during those mometary power losses.

FWIW, those are caused by faults on the lines opening the circuit breakers which have delayed reclosers. When a fault (like lightning hitting a line or squirrels bridging two conductors) occurs on the line the upstream circuit breaker will open and clear the fault and then reclose. If the fault condition has ended (lightning bolt over or fried squirrel fallen off) then the power is restores, if not the breakers open again and lock out since the fault is likely permanent (like a tree knocking down a line).

 

[AmberClad]

Originally posted by: BrownTown
FWIW, those are caused by faults on the lines opening the circuit breakers which have delayed reclosers. When a fault (like lightning hitting a line or squirrels bridging two conductors) occurs on the line the upstream circuit breaker will open and clear the fault and then reclose. If the fault condition has ended (lightning bolt over or fried squirrel fallen off) then the power is restores, if not the breakers open again and lock out since the fault is likely permanent (like a tree knocking down a line).

Thanks! I've always wondered why sometimes the power outages are only momentary and only last a split second, and other times, they last for hours.

All of this reminds me that I really need to get a bedside clock with a backup battery :laugh: . Sometimes I let the thing blink for days before finally setting the correct time again.

 

[futuristicmonkey]

Originally posted by: Mark R
The capacitors are oversized so that they don't get overstressed with the normal 120 Hz fluctuations even at maximum load. On most PCs, the PSU is running at barely 30% capacity - so there is a lot of spare capacity which can be called upon for a brief interruption - perhaps enough to keep the PC going for 1/10 or 1/20 of a second.

If you have a decent power supply you may be able to find it's specs on the net which'll tell you how long it's actually rated for. My Enermax 420w psu is rated to provide within-spec voltage for I think what was 17ms at full load. It's not a whole lot of time, but incandescent bulbs tend to exaggerate voltage fluctuations in mains power.

Oddly enough the damned thing was taken out by a nasty brown/blackout here in northend Winnipeg a few months ago. The voltage dropped down to 70/80 volts for a few minutes and surprisingly the computer stayed on. I forced it off, however, and I don't know how but the psu got buggered in the whole ordeal. The weird thing is some older power supplies in a couple of p3 servers were untouched by the event. I got a warranty replacement, tho, so in the end all I'm out is an explanation -- which I'd love, considering the older supplies were undamaged...

-ben