Learn how to use a DC regulated power supply to diagnose main board shorts

The main board of a switch mode power supply (SMPS) is designed to automatically shut down and prevent any further damage from occurring in the event of a short circuit. It's easy to blame the power supply when, in reality, the issue is on the motherboard if you don't know how to troubleshoot a switch mode power supply.

If there is a short circuit in the secondary side (either in the secondary diodes or on the main board), the current drawn would increase, and the PWM IC would stop generating output to the power fet, causing the power supply to shut down. This is because the PWM IC has a current 12v to 120v dc dc converter sense circuit (if you look at pin 3 of the UC3842 PWM IC, it states I-sense which mean current sense).

In the event of a secondary-side component short, some SMPS power supplies may shut down automatically even if they do not use a PWM IC. The printer's power source is a great illustration of this. Commonly, a printer's power supply board and main board hv power supply are separated by a cable. When there is a short circuit on the main board, the power supply won't work. Connector removal from the power supply board is required for fault isolation.

If you find that the voltage at any of the supply (VCC) pins is zero, you know that the power supply www.hvmtech.com is faulty and may devote all your attention to fixing it on this board.