This weekend we had a power flash at work, and my second machine was seemingly dead – All connections/plugs in place, I switched the outlet the plug was in to a known working one, and I got nothing. This happened once before, and on a suggestion, I unplugged the power cord from the back of my PC for about 10 minutes and tried it again. It then powered up with no problems.
Anyone know why this happens?
In short, the power supply has some protective circuits in it. Removing power to the supply allows those circuits to reset.
They are known as crowbar’s, if to much (current or voltage depending on design) power is drawn, the crowbar snaps shut sending the power supply into a shutdown state, rendering the it dead until fully reset. On some machines I have, if the supply is in shutdown, the LED’s glow dimly.
Dan
works for tvs, vcrs, etc…Its the first thing I suggest to people to do-unplug from wall for a full minute.because it resets the electronics & this gets numerous appliances working again.
Funny thing is, is that this gives the impression that the tech’ fixed’ the unit.
resetting the electronics in this manner has fixed my problems in the past… but if you allow me to expand on mouthbreather’s question, i’d like to ask:
What exactly do you mean by “resets the electronics” ? Which components get reset ? and in what manner ? is there mechanical movement of any parts ? are memory bits erased ? What exactly happens in that unplugged 10 minutes ? would anybody be able to give examples of an electronic component being reset ?
thanks so much…
Note: i would have started this question in another thread, but since this is directly related to the OP’s question i thought it would get it’s audience here… also thought it could save on the server load…
Mechanical movement, no. Memory erased, yes.
I believe the components involved are called “capacitors”, so named because they have the capacity to store a small amount of electricity for a short period of time. This is useful for short-term memory tasks. But it is short-term, and it will lose its electric charge very quickly unless refreshed with new electricity.
A very common usage of this is in electronic clock-radios. Many models can be unplugged for a few seconds, and then when you plug it back in, it still knows what time it is. But if it is unplugged too long, it forgets the time and you must set the time anew.
Similarly on a computer: Even if you totally unplug it from the wall for a few seconds, those capacitors are still thinking, and when you plug it back in it will keep on trying to do what it was doing before. Or maybe it will try to do something different than before because the circuitry is now somewhat messed up. But if you allow the capacitors to fully discharge, it will totally forget what was going on before, and when the electricity comes back on it will start up fresh.
I might need Eng_comp_geek to assist in this, as his electronics knowledge is greater than mine, however I’ll attempt to explain what I ment by reset the circuit.
The switching power supply in your computer (and TV) have protection circuits called crowbar’s. When a “watched” voltage goes over a set point, the crowbar kicks in, and prevents the supply from eating itself. Once the crowbar is brought down, it remains down until it’s reset. One design of a crowbar uses a SCR (Silicon Controled rectifier) The SCR has 3 leads, and anode, cathode, and a gate. No voltage flows through the anode to cathode path until sufficient voltage is applied to the gate. Once current flow starts anode to cathode, the voltage on the gate is a mute point. IE, the only way to shut it off is remove voltage from the anode / cathode gate.
A simple crowbar is like this. You hook up your gate to your watched voltage, (through a zener diode) your cathode goes somewhere earlier in the circuit, where bringing that point to ground, will either shut the supply down, or blow a fuse. The Anode goes to ground. Now when the watched voltage goes over the threshold voltage of the gate, the SCR triggers, current flows through the SCR, and the curcuit is rendered void either through a blown fuse, or in this case more like the switching transistor is forced to stop doing it’s job. This causes the power supply to quit working. Remember the SCR will not reset until the voltage difference between it’s anode and cathode is removed, so when you unplug it, and leave it unplugged for a bit whilst waiting for the capacitors to discharge, the crowbar is reset.
The only other thing I can thing of is your power switch is not the same as unplugging something
Dan
Thanks Keeve, i always suspected the role of capacitors in resetting the electronics of a device, but your post made their function clear.
Danvanf, that is one of the most amazing answers i have received for any question that i have posted here… or anywhere… i’m saving it to re-read 
Thank you, Dan.
So we have identified two such components that take active part in the resetting the electronics… would you guys be able to identify any more such components or do SCRs and capacitors alone cover the range ?
Both my brothers were computer healers for the military; one fixed radar for the USNavy, the other fixed missile guidance, radar, and other stuff for the USArmy. My naval brother referred to the disconnect-for-60-seconds fix as the “Raytheon Reset.” My older, army brother often used a less elegant standard procedure. He’d lift the device about three inches and drop it. The success rate was surprisingly good.