so im trying to design a simple trickel charger for a 4.8v 7.95Ah NiMH battery pack powered by a wall wart. im shooting for 5v at about 200mA. im doing this on multisim first to work out any bugs. it doesnt seem to wanna work the way i want it. im using 2 LM317 regulators. one as an adjustable voltage regulator and the second as a current regulator.one thing i dont get is, the LM317 constant current is sposed to keep constant current at whatever load, but its not working that way. ive been playing around with different value pots and resistors. any help or advice is appreciated!

 

Like I said on the other website ... never mind.

 

Try something like this instead:

Decrease R3 to somewhere around 330 Ohms to get you in the ballpark for terminal voltage. R2 is your "fine adjust".

R4 controls the current. It's roughly I=0.62/R4. Since you want 200mA, change R4 to around 3 Ohms; 0.62 / 3 = 206.667mA.

The stuff to the right of R1 is a very rough approximation of a small battery; just for the sake of simulation. If you want to protect your battery from accidental discharge in case the mains power gets unplugged/turned off while a battery is being charged, add a diode (Schottky preferably, like a 1N5817) in the B+ line to the right of R1.

Note that when you are measuring the output voltage, you need to measure with respect to B-, not ground - or you'll be about 0.62v off in your measurement.