Today I got a knock on the door, it was a neighbor asking if I could find out why his computer would not power on. I asked him if he has been paying his electricity bills. But seriously, I agreed to go to his house and take a look at it.
This neighbor is not a do it yourself kind of guy, so I figure the problem might be as simple as a blown circuit breaker. On first getting to his computer room, I noticed a scent of burning electronics. I asked him what happened and if he noticed anything. Condensed version of the story: He said it worked yesterday and today when he turned it on the screen flicked on just for a second or two, then he heard a small pop. He did check the power strip and there was power, but the computer would not turn on.
After sniffing around I located the problem to the power supply, I opened the supply and found that there was a blown diode. I did replace the diode with one I had but it was not the exact replacement part. I told him and he decided he would go out and buy a new power supply and he will give me the old one for my efforts. I also scored 2 cups of coffee.
A power supply is a circuit that converts AC power to DC power. In order to do this, the AC is applied to a rectifier. A rectifier in this case is a full wave bridge composed of four diodes. The diodes are one way valves that only pass electricity when it is polarized correctly. When applying an AC source the diode will only pass the positive wave or negative wave but not both. If the circuit has only one diode, it is likely a half wave rectifier and the power will be about 1/2 the input.
A full wave will pass both the positive and negative wave. The circuit shown is a full wave bridge rectifier. It operates by taking the input voltage which is applied to the primary winding of the transformer, the electricity is passed to the secondary winding by induction. The secondary voltage in the case of a computer power supply is much lower than the primary voltage. The output from the transformer’s secondary is still AC and can’t be used to power the computer circuits. The lowered AC is applied to the bridge rectifier and it is converted from AC to DC which can be used to power the computer.
In the defective power supply, one of the diodes blew and the DC output was too low to supply enough voltage to the computer.