Ozito 14.4V Li-On charger circuit

[ray merrick]

had a friend here at work with ozito cordless drill. Battery not charging. So i checked the battery and charged it on a power supply. Came up fine. 18 volts.
So the problem is in the charger.
Simple thing, 2 transistors and LM431. and the series pass transistor shorted Base/Collector

I traced it out, attached here.

 

[Colin Mitchell]

The 270n will only allow 7mA charging

 

[(*steve*)]

I suspect the power supply is switchmode and is not grounded.

These can give a little tingle, but are not (theoretically) dangerous.

Interesting that the transistor was shorted from base to collector. Was the emitter junction still OK? this should have left the transistor to basically operate as a diode.

 

[ray merrick]

yes it may be a switcher. I have designed many myself. i found it strange the short B/C..confused me for a while. The pcb was discoloured.. its been hot. I replaced it with a TO-220.. 10watt device.
1.8ohm B/C and 10ohm C/E.. cooked.

 

[KrisBlueNZ]

Hi Ray

Can you add a heatsink on the transistor? And drill some small ventilation holes?

There's no need for the incoming supply voltage to be as high as 32V. 24V DC should be plenty. Perhaps the adapter has a problem. Is 32V the marked output voltage on the adapter?