Gel Cell Charger Schematic

Parts List:
Resistors are carbon, 1/4 watt, 5% tolerance, unless otherwise indicated.
 R1 = 22 ohm, 1W            C1 =  0.1µF, ceramic
 R2 = 270 ohm               C2 =  0.1µF, ceramic
 R3 = 220 ohm               D1 = 1N4001
*R4 = 715 ohm, 1%           Q1 = TIP31A, B, C (or equivalent)
*R5 = 3.57K, 1%             U1 = NE555V (or equivalent)
*R6 = 1.40K, 1%             S1 = Toggle switch, ON-OFF
*R7 = 1.47K, 1%
This circuit needs a regulated 10V-DC front end capable of supplying 2 Amps. Starts the charge cycle at 240mA and at full charge switches automatically to a float condition (trickle charge) of 12mA.
The capacitors are the ceramic 50V (or better) type. Switching transistor Q1 is a NPN, Si-Power Output/SW, with a TO-220 case and can be replaced with a suitable substitute like the NTE291, ECG291, etc.
Timer/Oscillator U1 is a 8-pin NE555V and can be replaced with a NTE955M or ECG955M.
Resistors R4, R5, R6, and R7 are 1% metal film types. They may not be available at your local Radio Shack/Tandy store and have to be ordered in. Try Electro-Sonic or Newark Electronics supply stores.

NOTE: For 6-volt, 1.2Ah Gel Cell type batteries only!

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Copyright © 2001 by Tony van Roon