You are walking your dog at night when the flashlight begins to fade. The beam grows dimmer and dimmer until it finally goes out. Wouldn’t it be useful to pull a little more energy from that nearly drained battery instead of stumbling home in the dark?
It may sound impossible, but a “dead” battery is not completely empty. Even after a battery-powered device stops working, the battery still contains chemical energy and produces some voltage. The problem is that its voltage is no longer high enough to push enough current through a bulb, LED, or another electrical load.
With a clever electronic circuit, however, you can use some of that remaining energy to keep a light glowing. The circuit combines a transformer and transistor to extract residual battery power. It is called a Joule thief—a fitting name for a circuit that steals the last useful joules from a battery.
Building a Joule thief is both fun and educational. The circuit demonstrates Faraday’s law of electromagnetic induction, the same principle behind electrical generators, transformers, and induction cooktops. Let’s see how it works.
How Batteries Power Light Bulbs
We’ll begin with a basic electrical circuit: a 1.5-volt AA battery connected to a small incandescent light bulb with a copper wire.
Photo: Rhett Alleyne
This forms a complete circuit. Current flows from one terminal of the battery through the bulb’s tungsten filament and then returns to the other terminal. The filament is extremely thin, so the electrical current heats it to approximately 4,500 degrees Fahrenheit. The hot tungsten glows, producing incandescent light. Tungsten is well suited to this job because it has the highest melting point of any pure metal.
As long as the circuit remains closed—such as when a flashlight switch is turned on—current continues to flow. The battery gradually loses its stored chemical energy, and its voltage begins to fall. As the voltage drops, the current also decreases. Eventually, the battery can no longer deliver enough power to produce visible light.
In one sense, this simple circuit is also a Joule thief. If you leave the switch on after the bulb stops glowing, current may continue to flow and drain the battery’s remaining energy. But a circuit that consumes the last of the battery’s power without producing useful light is not particularly helpful. The real Joule thief uses that energy more effectively.
LEDs Versus Incandescent Light Bulbs
Most modern flashlights use light-emitting diodes, or LEDs, instead of incandescent bulbs. LEDs create light without heating a filament. They are solid-state devices in which electrons release energy as they move across a semiconductor energy gap. Because far less energy is lost as heat, LEDs are much more efficient than incandescent bulbs.
A typical white LED has a forward voltage of roughly 3 volts, while a single AA battery supplies only 1.5 volts when fully charged. That is why a basic circuit usually needs two AA batteries in series to power a white LED. A Joule thief gets around this limitation by using a transformer-like coil and a transistor to briefly boost the battery’s voltage.
Photo: Rhett Alleyne
Source: www.wired.com