Howdy


I didn't describe this fully.

Dissipation protection... Put the diode of an optocoupler
with a resistor in series across the pass device's emitter
and collector. Then connect the opto's transistor to the
reference feeding the base of the pass device. Adjust the
resistor in series with the LED so that when there's too
much voltage across the pass device the led lights and
decreases the reference voltage which reduces the current
in the pass device.

Another way to do this requires a zener, two transistors
and three resistors. Optos are cheap enough and it's
less parts to wire up.

I fused the output in case something breaks, to keep the
circuit from being slagged. In a short the thermal breaker
on the transformer took about 45 seconds to trip. I've no
idea what the fault current was but the needle of the lazy
ten amp full scale meter that's built into the charger was
slammed forcefully against the stop.


73,
Grumpy


Grumpy The Mule wrote in
:

You'll need some sort of short circuit protection. During a short
the ten amps times the supply voltage will be dissipated in the pass
device. Could be a fuse or breaker or active protection. I left
this out of my first attempt and the pass device didn't like being
shorted for even a moment. Luckily I tested this before putting it
all back together rather than finding out later on in a snow storm.
The existing thermal breaker was much too slow.