>> 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 think you're looking for something that isn't there..

How well the circuit does its job and how the circuit works 
are reasonably independent.

I think the "problem" beginners are liable to have with it 
is that meter readings make no sense, whether SC or DC.

He has 21V in and about 15V accounted for using meter 
readings.  Realising that the half wave DC  utterly 
discombobulates the meters is the key.

> This is a
> simple "trickle charger" which modern NIMH cells hate, and 
> NICAD cells
> tolerate.

Yes. It's not a nice circuit.
Even a NiCd should really be 1000 mAh or more (so "trickle" 
rate is C/10 or less) and even then not good.

> The charger can overcharge if left on too long, which
> causes free oxygen and hydrogen inside the cell. The 
> hydrogen escapes,
> and the oxygen combines with things.

All that

> The resistor value is set to give a charge current that's 
> roughly a
> tenth of the amp-hour capacity of the cells used.

Probably. But in this case it was probably set empirically 
(ie cut and try). OR the transformer could be energy 
limited, as is sometimes done.

Best use of the device that this is a circuit of is 
"something else".

As Dave suggests, for NimH cells to last well they need a 
decent charger.
Or a conservative one.



        Russell



-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist