In this circuit a 26.5 volt mains adapter charges a 22volt 1300ma/hr lithium cell.
I found the Varistor had failed to a short circuit and cannot identify the 7mm disc device marked
xxx x40
ZTjB (I marked it wrong in the schematic)
I find the circuit, extracted from a Hoover Charger, rather strange in that the break point of the varistor is probable above the 26.5 working voltage and the rating of the Mains Adapter is only 300ma. It maybe that the charge current uses the non linear varistor impedance below the breadown point to linearise. However a zinc oxide device probably provides a very high resistance below the breakdown point whilst a silicon carbide device might better sustain the charge current?
I would be interested to hear from anyone who indentifies the varistor - to enable me to replace it.
Whilst an unusual use of a varistor the circuit could not be described as an ideal charger.
I would also be interested in comments on the circuit. If a battery is connected the wrong way round D2 would probably explode and if not the battery might explode instead. At full charge the charge voltage remains although a LED circuit monitors the voltage across R2 and the \led is extinguished if this falls below about 1 volt.
Very many thanks,
Docwat.
