Simple Constant Current Source

Simple Constant Current Source
by John Nooyen


This simple constant current source uses an LM317 regulator IC.
N.B. The pin-outs of the IC are not a mistake, the output is taken from the adjustment pin.
I found this circuit to charge all my NiCad batteries, and I use a Kambrook mechanical light-timer (or equiv.) for 110vac to time the charge.

The reason I use a mechanical timer is:
A: You can remove all the “ON” pins, and just have one pin in an “OFF” hole.This allows you to set the timer for 14 hrs, then it will switch off and never switch on again.

B: If you get a power failure, the timer will stop.When the power is restored, the timer will continue and the pack will get its 14hrs charge. (Digital timers have a battery and you will miss out on charging for the time the power is interrupted, so you cannot guarantee that the pack has received its full charge time.)

1 – Ensure the voltage accross the capacitor is at least 3V higher than the highest output voltage required, and no higher than 37 volts accross the IC. A tx pack on charge can reach 14 volts.

2 – You can play with the 27-ohm resistor to change the current for other applications, however the 56 Ohm should also be changed so that the voltage across the LED is 1.5 volts. (So you don’t get a SED (smoke emitting diode))!

3 – Mount the IC on an adequate heatsink for the power it will be dissipating.

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