If Murphy lurks in your workshop and shows up periodically to destroy your special semiconductor devices in the safest of experiments, then the circuit in Fig. 5 might just send Murphy a message and save the day. That single transistor circuit places an electronic lid on the maximum current that can pass through it. By placing the current limiting circuit in series with your delicate
project and its power source (DC only), you can keep the smoke from getting in your eyes, and Murphy at bay.
The circuitís current-limiting range is adjustable from about 0.5 to 8 milliamps and can be used with circuits having supplies ranging up to 30-volts. If a more bullish circuit is required, replace the 2N2222 with a 2N3055 and R1 with a 10-ohm, 1-watt resistor and R2 with a 500-ohm, 2-watt potentiometer. That allows a maximum current of about 200 milliamps, and a minimum of 5 milliamps.
When setting up the circuit, connect the current limiter in series with a current meter and connect that combination across the power source thatís to be used. Adjust R1 for the maximum current that you want to pass through your circuit. OK get in there and give Murphy a kick for me.
Copyright by Bill Bytheway, K7TTY February 2012