Finding the Shorted Component on a Printed Circuit Board

Once in a while, you find yourself staring at a printed circuit board with a shorted power rail.  It could be any of the components, capacitors, diodes, ICs.  Sometimes, you have a hunch which component might be bad and remove that component to see if the short goes away.  But you might take off one component after another, still could not located the bad component.  

One way to quickly identify the faulty component is to apply current to the shorted rail and look for the hot part.    You may have to apply a few amps of current, but not too much current to damage the traces.  Sometimes you can actually see a component starting to smoke; other times, you have to feel the heat.  If you have a thermal imager, you can easily see the hot spot.

A Ni-MH Battery Charger by Panasonic

 Here we'll look at Panasonic Ni-MH battery charger, rated as 0.3A for AA and 0.15A for AAA.  It can accommodate 1-4 AA or AAA cells.

The AC to DC conversion uses SC1142D1 by Power Integrations.  No datasheet is available, but an application schematic is found, which gives the idea how the circuit might work.  A third transformer coil provides the feedback; no opto-coupler is used.

A slot in the PCB increases the creepage distance from the AC primary.

The charging control is based on a microcontroller EM78P418N by ELAN Microelectronics, a Taiwanese company.  EM78xxx is called  "a RISC-like single chip microcontroller" by the manufacturer.  But its instruction set is akin to PIC or 8051.  It has an internal oscillator of 4MHz.

Pulse charging method is used:  power is applied on and off and open-circuit voltage is measured when the power is off.