My first Joule Thief had an LED as the load. When I replaced the LED with several different capacitors, they all charged up to the same low voltage: 0.4V. This surprised me at first because the LEDs ran at 2 to 3 Volts, but the capacitors did not get charged that high. 0.4V is also about the lowest voltage I have seen the Joule Thief work and light up an LED. Probably not a coincidence.
Capacitors do not get a full charge without a diode in place to rectify the current. You can use an LED as the diode. When I used an LED, I got a capacitor to charge up in series with the LED, as shown in the sketch above. Adding the LED increased the max voltage across the capacitor from 0.4V to 20.5V.
I now assume I could intermittently power a load with this Joule Thief up to about 20V.
I have successfully powered a couple of 12V LED strips with a single AA battery and the Joule Thief. The light output was dim, though. But I guess we could go as high as 20V with this circuit. However, I would not choose LEDs at that higher voltage output, since the light output gets too dim at the high voltages. Maybe the resistor could be lowered to allow more current to flow. But then we have to make sure the transistor has a high enough rating for IC.
Replace Battery with Capacitor?
Can a Joule Thief work with a capacitor as the power supply? When I replaced the battery with a charged capacitor, the LEDs did light up, but not for long… just about one second. A higher capacity capacitor (or super cap) would probably increase the time the LEDs stay on. But then you have to be careful about the voltage level of the capacitor. The voltage of the power supply should be less than the LED forward voltage to protect the LEDs from burning up. But if you are not using LEDs as your load, I think you could use a higher supply voltage. How high depends on how much current you will generate and what current your components are capable of handling, including your transistor, gauge wire, and the power rating of your resistors.
There are so many possibilities for the Joule Thief and I’m just getting started. If you want to keep up-to-date, you are welcomed to subscribe to the RSS feed. And I always welcome your comments and feedback.