Joule Thief, Stirling Engine

published 2026-04-15

by Christopher Howard

I experiment with Joule Thief circuits, as described on the Wikipedia page:

The basic circuit with the LED worked great. I was able to light up the LED with as little as 0.5 volts from the desktop power supply, although you need to bring it up to around 0.9 volts to get full brightness. Interestingly, once the LED is lit, you can bring the voltage down almost to 0.3 volts before the light will go out.

Then I did the "regulated output" circuit. This circuit is similar, except you remove the LED from the circuit, instead having positive and negative output terminals. You have a regular diode going to the positive output terminal, to prevent reverse current coming in from the circuit attached to the output. And you have a zener diode across the output terminals, to regulate to the desired voltage output. There is also a capacitor to smooth out the output, but I left that out because my aim was to charge up a large capacitor in the output circuit. I do not have any zener diodes, so I replaced that with 7 regular diodes giving around 5V regulation. When I initially tested this using just the variable resistor and the oscilloscope, I was seeing 5V pulses with a frequency around 50-100 Mhz, with the frequency being increased as I decreased the variable resistance.

When I hooked this circuit up to my desktop power supply, set to 1.8 V, and the 8F capacitor, with the variable resistor kept at about 500 Ω, I was able to see the capacitor charging up to over 3.6V, before the charging got slow enough that I lost interest. The PS screen was showing 20 milliamps current at that point.

I tried to do the same thing, replacing the desktop power supply with the stirling engine, but without clear positive results. The capacitor, initially still charged to 3.6V, did not seem to be increasing in voltage, and actually lost a few millivolts. Perhaps the stirling engine is not producing enough power to overcome losses from leakage. I might try this again with a lower capacitance capacitor.

I found it, I must confess, rather difficult to understand the various explanations I read about how the Joule Thief circuit works, or more generally the blocking oscillator. But the short, simple version, is that the circuit oscillation, and the reinforcement between the primary and secondary coils, causes the transistor to rapidly and abruptly switch between the fully on and fully off states. This causes a large voltage overshoot in the inductor, as inductors by nature resist a change in current by producing a voltage. I.e., the sudden changes in current cause the coil to generate large voltages.

[Update: Using a 2200 µF electrolytic capacitor, the capacitor charged up to 4.22 volts in 43 seconds, after which point the voltage seemed to be barely rising. By my quick calculations, that should be about 20 mJ of energy in the capacitor, with an average power of 0.5 milliwatts, not including power dissipated outside the capacitor.]

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