I can't answer your question directly, but maybe consider these thoughts
also:

1) 150 ma "trickle" is a little high for the 8AA stack (20 hr charge
rate). To me, trickle is the charge rate you want if you leave the
charger connected indefinitely. This rate will surely cook the batteries
dry.

2) Why not set the control bias on the TIP so when the PIC is not running
(or out of the circuit) that the trickle current is the result, like a
default rate? Just a resistor across the 317 may work.

3) Since you're not really needing an exact frequency, omit the xtal and
use an RC network.

4) Your input pins on the two regulators need a 0.1uf bypass to the GND
pins of the regulators too. These will keep the devices stable and not
oscillate at some frequency that could upset the processor and also cause
improper charge rate.

5) Don't forget that your voltage divider/sampler draws a small amount of
current from the battery when the charger is unplugged. I had a customer
complain that his charger would charge the batteries fine, but when he
unplugged it, and shelved it for a few weeks, the battery would be fully
discharged.

You have a good charger circuit. I plan on doing the same pretty soon for
NiCad/NiMH.
Good luck. Rick



PICLIST@MITVMA.MIT.EDU wrote:

> Hi,
> I built this circuit for a battery charger,
> http://www.members.dca.net/kben/charger.html
> with a few modifications.
> I am using a F872, and a TIP41C for the
> transistor. I also have the 1k resistor tied to
> +5v not the MCLR pin as in the circuit. I am
> driving the base of the transistor with the
> PWM pin from the F872.
> I am using a 20VAC 800ma wall wart, and
> 2Khz for the PWM. When the CCPR1L is set to
> zero, I see 1.3A charging my 8XAA Nimh battery
> pack. If I change CCPR1L to 3 I see 933ma,
> CCPR1L to 6 I see 900ma,CCPR1L to 12 I see 750ma,
> charging, so far so good. If I set CCPR1L
> higher than 15 the charging current drops to zero.
> Any ideas why ? I thought I would be able to lower
> the current down to 150ma for a trickle charge.
>
> Thanks in advance,
> Kevin
>
> P.S. I am a programmer, not an EE, so maybe it's
> something dumb, but you got me ?
>
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