There appear to be two parallel strips of metal terminating in
something looking a bit like the points in an ignition system. The
two strips of metal are separated by a bead at the input end. The
'points' appear to be fully closed.
That is the thermal switch and it will work as I described to
"regulate" the charge current.
I see now that the two main outputs from the secondaty coil supply
the twodiodes and the centre tap supplies one side of this thermal
current switch. The other side of the TCW is connected to the
negative battery clip. The outputs from the two diodes are connected
together and to the positive battery clip. The rest of the circuit,
which is hard to follow, is in the middle somewhere. If the TCW is
closed the charger should work as long as the secondary coil is OK?
But the secondary should be protected against reverse current by the
diodes and against excessive forward current by the TCW? That seems
to leave the thing which looks like a Zener. Would that be a 12.5
volt Zener? On reading all the comments and thinking about it some
more I think that I may have just connected the clips to the battery
the wrong way round! I have been using a battery charger for years
and have never made this mistake before, but I suppose that there is
always a first time. Assuming that is what I have done which
elements in the circuit would you expect to be destroyed?
The zener, if it is one, would be associated with the led circuit. For
what it's worth, using LED's to indicate charge status on such a cheap
charger is almost worthless anyway. You could simply disconnect
anything associated with the LED's and as long as the transformer, the
rectifier diodes and the thermal switch are ok the charger will work.
You can check to see if the zener is ok by using a DMM (diode range)
to measure it in both directions to see if it short circuit. If it
measures around 0.6 in the forward direction and open in the other it
might still be ok. To test it fully you need to use an adjustable
bench power supply and a series resistor (say 470 ohm 1/2W). Connect
the resistor in series with the zener and hitch it up to the psu
output - zener cathode to +ve. Now raise the voltage output while
measuring the voltage across the zener. You should see the voltage
reading plateau at its rated voltage even when the psu voltage keeps
increasing. If your PSU has both voltage and current meters you can
perform this test without an external meter across the zener. In this
case you will see the current meter jump up from near zero while the
output voltage is increased slowly. With a current of around 20mA stop
increasing the voltage and read the output voltage on the psu and that
will be the nominal zener voltage. If you dont' get this result the
zener is faulty and needs replacing.
