Re: [OT] Fried motherboard
On Tue, Dec 10, 2002 at 03:01:02AM -0600, Rich Puhek wrote:
> > probably. I've never had a power supply die on me. maybe I'm lucky.
> > I've even plugged a power supply that was set to 110V into a 220V
> > circut, tripped a breaker, but the power supply itself was fine(haven't
> > tried going the other way around).
> >
>
> I've done that (220V setting on a supply, plugged into a 110V outlet).
> Just acted like a dead supply.
>
> Had the darn thing swapped out of the PC and a new one in place before I
> thought to look at the switch on the back.
>
> I believe the supply was alright.
>
> Hard to believe you didn't fry more in your case... the transformer
> should have dumped 2X the normal voltage into the regulator/switching
> portion of the supply, which can be very bad.
Here's why...
There's no transformer involved in the 110/220V selection. The mains
input comes into a conventional bridge rectifier, across the output of
which is connected, not one reservoir capacitor, but two in series.
For 110V operation, the centre point of the series-connected
capacitors is connected to one of the AC input lines. This converts
the bridge rectifier into a voltage doubling rectifier, so the
switching department is fed with the same voltage as for 220V.
Across each reservoir capacitor is a VDR. This is a surge protection
device which goes low impedance if the voltage across it exceeds a
certain value.
As the first half-cycle of mains input tries to charge one of the
reservoir capacitors above its normal working voltage, the VDR across
that capacitor goes low impedance. This discharges the capacitor. The
same thing happens when the next half-cycle tries to overcharge the
other reservoir capacitor. The result is that the voltage-doubling
action never gets off the ground, and the switching department never
sees any excessive voltage.
The VDRs going low impedance also represents a short circuit across
the mains input, so after a few cycles a fuse will blow or a breaker
trip. As long as this doesn't take too long, you'll probably get away
without damaging anything. The components most at risk are the bridge
rectifier diodes, but these usually have a goodly surge rating anyway
to cope with charging the reservoir capacitors from cold. Sometimes
there is a wirewound resistor of a few ohms to limit the switch-on
surge. Under fault conditions this tends to act like a fast-acting fuse.
Note that this only applies to computer PSUs. A dual-voltage
switch-mode PSU in something like a VCR has no VDRs. This is
deliberate, so that faults, surges and setting the voltage selector
switch wrongly cause the thing to blow up catastrophically so you have
to buy a new one.
Pigeon
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