# Why did my power supply fry itself?

I am in Europe. There was a switch on my PSU that could switch the voltage between 230v and 130v (not 100% sure). In Europe we use 230. I switched the PSU to 110 and turned it on. Several sparks and a power failure resulted, the PSU was fried.

Can someone explain why this happened. I was assuming that because the system was using 230 and the PSU only draws 130 it would be safe cause it's less. I guess I was wrong.

Can someone explain me the physics behind this.

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WHY WHY WHY would you do this???? You took a working component that you didn't understand and altered it for no good reason? – micmcg Apr 13 '10 at 3:11
Don't criticize me you don't know the circumstances. – Tomasi Apr 13 '10 at 7:58
@micmcg - it can happen by simple mistake, not everyone knows everything about electricity. And anyway, the point here is to answer a question, not to judge or criticize what happened. You can keep that kind of intervention to yourself. – Gnoupi Apr 13 '10 at 9:07
@micmcg, most commonly it happens simply by accident, or if the paint based label has worn off, the user may have no idea it is in fact a working switch. – mctylr Apr 13 '10 at 18:04
The smart thing to do would have been to ask this question BEFORE he messed around with something he didn't understand. – micmcg Apr 14 '10 at 2:45

Here is a simplified explanation of what's going on in the power supply. The power supply divides the input voltage (230V in your case) by a fixed amount to give the voltages needed by the motherboard and other components. There are various outputs, but one of them is 5V. To get this you have to divide 230 by 46, or divide 110 by 22.

Power supplies can work on 230V (most of the world) or 110V (North America). The switch selects which divisor is used inside the power supply. In your case you set the switch to 110. This selected the 22 divisor. However, you applied 230V to the input. This resulted in a voltage of about 10.5V on the 5V circuits. This voltage was too high and something blew. It may not have been on the 5V circuit, but the story is still the same. You applied too high a voltage for the setting of the divisor. If you are lucky only the power supply was damaged and all the other components are still OK.

If that didn't make sense, then consider this. Suppose your car is stuck in the snow (I'm from Canada; somebody's car is always stuck in the snow). If you push the car out by hand you won't damage it. If you push the car out with a tractor you will probably dent the metal. The tractor pushes too hard. The voltage is like that push. Voltage pushes electrons around. When you applied 230 volts to a circuit designed for 110 volts, you pushed too hard.

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Nice explanation thanks. – Tomasi Apr 13 '10 at 6:47

One of the intermediate stages of the switch-mode power supply (SMPS) use in a PC power supply most likely exceeded the voltage ratings of its components, which caused them to dramatically fail.

Depending on the topology of the SMPS design, the intermediate voltage may reach about 400 Volts, so if you accidentally switch the incoming AC mains voltage switch from 230-240 to 110-120 Volts (AC) (roughly multiplying from 2x to 4x), the intermediate stage could raise the voltage to over 800 V (240 x 4 rather than the intended 120 x 4), which if the components were only rated for 600 V maximum the components would fail (complete with sparks). Roughly speaking 600 V may be a reasonable safety margin for component selection, if there was little to no risk of having the wrong voltage switch setting (and it is fairly uncommon).

Now in newer PC power supplies (many/most in the past couple of years) are "universal" input, so they run from about 90-265 Volts AC (Vac) input and there is no more switch to accidentally flip. The requires more complex (intelligent) circuitry, but the complex circuitry also is more efficient, reducing overall power consumption of newer PCs with these newer power supplies.

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The first stage in the power supply is the transformer which (in this instance) reduces the AC line voltage to a lower voltage suitable for the next stages. With two particular poles connected it drops 230V to (say) 48V, and with another two it drops 110V to 48V.

What happens is that this divisor is fixed, so setting it to 110V and then hooking it up to a 230V line will cause it to produce 96V at the output. This may be too much voltage for further stages in the power supply, causing them to burn out.

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Actually no, in a switch-mode power supply (SMPS) the first component is not a transformer. In fact most don't contain any transformers, though they likely contain inductors (aka chokes or coils). You describe a linear power supply, which is much simpler, but far less efficient and have not been used in personal computer power supplies since 1985 or so. (e.g. Apple I) – mctylr Apr 13 '10 at 2:35
Sorry, most PC power supplies do not contain a transformer, due to the cost and weight of a transformer, all generic PC power supplies eliminate them unless they need isolation (e.g. medical equipment, some harsh environments). – mctylr Apr 13 '10 at 2:45