My initial try at a current limiter is a foldback circuit I used back in the '80s for charging a small battery. See below. The charging current flows from Vin through Q2 collector to emitter, then through current sensing resistor R2. When the voltage across R2 reaches about 0.65 volts, then Q1 turns on and robs the base of Q2 of current, limiting the current to 0.65V/R2, in this case the current limit is 0.79A.
As built the circuit is shown below. It functioned as predicted, limiting the battery charging current to about 0.78A. However my lab power supply only goes to 18V and this circuit required 3 or 4 volts to operate properly. But the real problem was that it was difficult to predict the final charge voltage once the battery current fell off. This gives a risk of undercharging or overcharging the battery. I will try a different approach with a LM317 regulator IC.
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