You mean a voltage regulator? That is possible to build but not with this circuit (Joule thief).
The Joule thief charges an inductor in one half of each cycles, and let’s it discharge into the load in the next half. Inductors like to emit a constant current and will kick up the voltage to maintain the current, so it’s more like a current source. Because it drops to 0 quickly before the transistor starts charging it again, it delivers its output as a series of pulses.
So it’s a pulsed current source. The only way to get a constant voltage out of it is to 1) feed the current into a fixed resistance load 2) filter the output via a capacitor.
I thought about how to make a single transistor voltage source but I gave up. Transistors are so cheap there’s not much point. The world has a bizarre preference for voltage sources (maybe due to the fact that early batteries were DC, or maybe it’s just Edison’s ghost). Most real-world loads have useful outputs proportional to current (LEDs, magnetic fields) and the voltage is just a secondary variable.
The Joule thief charges an inductor in one half of each cycles, and let’s it discharge into the load in the next half. Inductors like to emit a constant current and will kick up the voltage to maintain the current, so it’s more like a current source. Because it drops to 0 quickly before the transistor starts charging it again, it delivers its output as a series of pulses.
So it’s a pulsed current source. The only way to get a constant voltage out of it is to 1) feed the current into a fixed resistance load 2) filter the output via a capacitor.
I thought about how to make a single transistor voltage source but I gave up. Transistors are so cheap there’s not much point. The world has a bizarre preference for voltage sources (maybe due to the fact that early batteries were DC, or maybe it’s just Edison’s ghost). Most real-world loads have useful outputs proportional to current (LEDs, magnetic fields) and the voltage is just a secondary variable.