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Hi All,

I'm hoping someone can give me some help understanding this circuit- i'm
just a hobbyist with no training in electronics and i'm struggling with this
one.

I have a simple "wall wart" battery charger. It's designed to charge an
8-cell NiMH/NiCad battery, at 100mA.

I've taken it apart and deduced the following circuit, which i'll describe
because i'm not sure if you can attach pictures on the list. I've tried,
maybe it'll work, maybe it won't.

The secondary of the transformer: one leg is tied to the negative terminal
of the battery. The other leg goes through a diode (can't see what it is,
just a standard 1N4xxx series I presume, but i don't think that's
important).  then the diode connects to a 27 ohm resistor, and then the
resistor connects to the positive battery terminal.

Across the 27 ohm resistor there is a 680R in series with an LED - this is
just a charge light and I think has negligible impact on the functionality
of the circuit.

So, we have a diode performing half wave rectification, and a 27 ohm current
limiting resistor.

I've measured some voltages whilst everything was connected: the secondary
of the transformer is about 21 VAC. There is a drop of 2.96 VDC across the
27R, and a voltage of 10.5VDC across the battery. At the time, the current
was 103 mA.

I want to understand the math behind the circuit. I'm confused because I
don't know how the voltage of the battery being charged comes into things.
I'd like to know a formula for the current in terms of the input voltage and
the value of the current limiting resistor.

Any explanation would be very greatly received.

Thanks

Jon

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