Joule Thief: Practical Application: Simple NPN Transistor Checker

The JT Basic circuit is used with a 1n914 (or 1n4148) diode in place of the Base Resistor. Cathode towards transistor Base. The circuit is the Basic 2n2222 or 2n3904 JT, available everywhere. the inductor is a two-coil inductor on a rectangular frame of good ferrite, pulled from a discarded TV set. The windings are the same, 1:1, and I estimate around 80-100 turns each or fewer. The coils measured about 21 mHy each on my inductance meter. The wiring is standard: the connection of one coil to the next coil is connected to the battery positive pole. Voltage drop across the one-ohm current viewing resistor is used to monitor the total current in the system. The DMM has significant resistance in the milliammeter mode but the voltmeter mode does not disturb the circuit and the 1 ohm CVR is also not much power drain. Sockets for the transistors make changes quick and easy. TO-3, TO-220, and other packages are tested, different transistors compared using subjective impression of the LED light and the total current at the CVR. Don’t fool yourself with jumpers that inadvertently bypass the CVR with an alternate current pathway.

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