Charge sensing

[linw]

I'm trying to devise a charge indicator but am running out of ideas. The indicator is needed because the charged item is connected to the charger via touch contacts and I want to know whether the contact is good. I am trickle charging a 4 cell AAA NiMH bty pack using a 6.6V source via a current limiting 5 ohm resistor. The voltage across the resistor is too low to bias a LED directly.

Any ideas?

[David T]

One way would be to measure the voltage drop across the resistor to determine current flowing through it. The voltage will be current x resistance (eg: 0.050A * 5ohm = 0.25v). If the value is too small it could be amplified with an op amp or probably measured direct with a Pic but these increase the complexity.
David.

[Acetronics]

Hi, The simple way is to use a PnP transistor that drives a led.

just use this scheme :

I use it for my table vaccuum cleaner ( Black & decker w/2 Nicads ) ...

Alain :)

PS: the BD 436 is oversized ... but it's for its base current capacity !!! ;)

[linw]

Thanks for replies, guys. Much appreciated.

Alain, Is the charging current supplied via the base-emitter path? Would a TIP42 do this job?

[Dan Baldwin]

In Alain's circuit, he is charging throught the emitter-base junction of the transistor. It's not a method that I would use, but it should work fine. The Tip42 has an absolute maximum base-emitter current of 2 amps, so it should work in your application.

Dan