One of the ways to determine what exactly your sons PSU is doing is by going through it with the proper multimeter and test for AC on the DC output of the PSU.
You would have to really know what you are doing before you would attempt such tests on a device such as a power supply, because the voltages found inside a PSU can at the very least cause electric shock or serious burns.
The circuitry found inside a power supply, is designed to remove the AC Sine-wave, which is Alternating Current and changes it to DC which is Direct Current.
It also uses a transformer, by which the current is reduced through a network of wrapped wires around a core. The output of a transformer is a lessor sum of the original, but it still contains AC.
Note: A DIODE is a small round cylinder, it can pass current one way like a valve, but it can not pass current from its other direction, a diode is a one way valve, one way it conducts, the reverse it does not conduct. I will mention the word diode below and how the power supply uses diodes to convert AC house current 115 Volts AC to DC current that is useable for home electronics.
The reduced AC from the down-step transformer is fed into a series of diodes put together to create what is called a Bridge Rectifier, four Diodes are usually used, their polarity is ~ +~ -
The two ~
symbols are the two wires coming from the transformer's output, which is still a pure AC sine-wave. The +
symbol is where your DC + or Positive output is created. The -
is the negative or ground output.
and the -
go to a network of filtering, by use of very large electrolytic capacitors, these are designed to hold a charge, so that when the DC current, passes through them they keep the DC wave in a straight non alternating line, it contains no ripples like this line here -------------------------. This is an example of a pure clean DC wave, it is a straight line. Now here is an example of an unclean wave here ---v^----v^. The wave is NOT straight here. It is dirty DC current with noise in it. (Not a good example of a graph, but it gives you an idea that the line is NOT straight)
If a short circuit develops or if AC by-passes the bridge rectifier circuit, damage can result to circuits down the line that are designed for DC only.
A short circuit can be created by diodes failing and conducting in both directions, or someone has placed a coin or another small metal object into the power supply, where it creates a short, allowing dangerous AC to pass onto the main board.
Above I gave a brief easy to understand description of how a basic power supply works, I have not included the fact that one power supply can output several outputs with different voltages ratings such as +3.3 Volts, +5.0 Volts or +12.00 Volts etc.
Edited by MrBruce1959, 20 July 2010 - 12:39 PM.