Disposable batteries, one could say, are a necessary evil in modern society. They’re used once, then discarded when their stored energy becomes too low. Consider that if a battery reads, for example, .9V instead of its nominal 1.5V rating, there is still some power left, though it’s usually discarded with the cell’s physical husk. With this simple joule thief circuit, however, now you can take advantage of the power in a “dead” battery.
Building a Simple Joule Thief
To build this, you’ll need the following components:
· Battery, rated at (or discharged to) ~ .5 – 1.5V
· 1k ohm resistor
· 2N2222 NPN transistor or similar
· Coupled (2 separate windings, 4 connections) toroid inductor—2mH (millihenry) and 10mH tested, many others will work
· LED
Connect components per the schematic below:
Once connected, you can power an LED with a voltage drop well beyond the cell’s rating, even when it’s mostly discharged. In the image below, you can see this type of circuit lighting a red LED at a mere .5V, while drawing 10mA of current. The coupled toroid shown was made by cutting a single winding inductor into two and scratching off the insulation with sandpaper. You can also hand-wrap your own ferrite core, which can be useful if you find one to recycle. If you’re buying a device specifically for this use, however, you’ll want to pick up a coupled toroid.
Low voltage required to keep this joule thief LED running. Coil cut in two here forms a coupled inductor.
The 10mH coupled inductor pictured here works, but makes an audible noise.
How Does a Joule Thief Work?
When the circuit starts up, voltage is applied to the transistor's base through the inductor. This creates a magnetic field and provides voltage on the base to open the collector-to-emitter path of the transistor. This, in turn, allows electrons to flow through the other coil, increasing the magnetic field and inducing even more voltage to the base. When the core becomes saturated, not able to further increase its magnetic field, the induced voltage goes away, closing the emitter-collector gate. The field then collapses, inducing current that travels through and lights the LED, instead of traveling through the collector-emitter path, at a voltage higher than the battery can provide by itself.
After the field collapse, and the LED’s momentary lighting, the process starts over again. This cycle happens so fast that the human eye resolves these pulses as a constant light source.
Oscilloscope 2mH readings (1st image), 10mH readings (2nd image)
The resulting output of a 2mH, and larger 10mH, inductor circuit are shown on an oscilloscope in the above images. Note the higher frequency of the 2mH model. While both worked, the 10mH gave off an audible sound, which may be related to its ~220µs period. This works out to a 4500 Hz frequency, well within the normal human hearing range. The smaller device comes in at 10,000 Hz, which would theoretically be audible; however, our ears are less sensitive to that pitch, and I did not notice it.
Joule Thief: Free Energy
If you’re stuck in the dark with a bunch of half-discharged batteries, the Joule thief could potentially come to the rescue! When light isn’t a problem, it’s an excellent way to learn about magnetic fields and inductors, and is a fun circuit to wire up in a few minutes.
