Circuit of the week

JOULE THIEF CIRCUIT DESCRIPTION

A “Joule Thief” is a simple voltage booster circuit. It can increase the voltage of a power source by changing the constant low voltage signal into a series of rapid pulses at a higher
voltage. You most commonly see this kind of circuit used to power LEDs with a “dead” battery, but there are many more potential applications for a circuit like this.
In this project, I am going to show you how you can use a Joule Thief to charge batteries with low-voltage power sources. Because the Joule Thief is able to boost the voltage of a signal, you are able to charge a battery with a power source whose output voltage is actually lower than the battery itself. This lets you take advantage of low voltage power sources such as thermoelectric generators, small turbines, and individual solar cells.

This circuit used in this project is a

modified "Joule Thief."

 A Joule Thief is a self-oscillating voltage booster. It takes a steady low voltage signal and converts it into a series of high frequency pulses at a higher voltage.

 Here is how a basic Joule Thief works, step by step:

1. Initially the transistor is off.

2. A small amount of electricity goes

through the resistor and the first coil to the base of the transistor. This partially opens up the collector-emitter channel. Electricity is now able to travel through the second coil and through the collector-emitter channel of the transistor.

3. The increasing amount of electricity

through the second coil generates a

magnetic field that induces a greater

amount of electricity in the first coil.

4. The induced electricity in the first coil

goes into the base of the transistor and

opens up the collector-emitter channel even more. This lets even more electricity travel through the second coil and through the collector-emitter channel of the transistor.

5. Steps 3 and 4 repeat in a feedback loop until the base of the transistor is saturated and the collector-emitter channel is fully open. The electricity traveling through the second coil and through the transistor are now at a maximum. There is a lot of energy

built up in the magnetic field of the second coil.

6. Since the electricity in the second coil is no longer increasing, it stops inducing

electricity in the first coil. This causes less electricity to go into the base of the

transistor.

7. With less electricity going into the base of the transistor, the collector-emitter channel begins to close. This allows less electricity to travel through the second coil.

8. A drop in the amount of electricity in the second coil induces a negative amount of electricity in the first coil. This causes even less electricity to go into the base of the transistor.

9. Steps 7 and 8 repeat in a feedback loop until there is almost no electricity going through the transistor.

10. Part of the energy that was stored in

the magnetic field of the second coil has

drained out. However there is still a lot of

energy stored up. This energy needs to go somewhere. This causes the voltage at the output of the coil to spike.

11. The built up electricity can't go through the transistor, so it has to go through the load (usually an LED). The voltage at the output of the coil builds up until it reaches a voltage where is can go through the load and be dissipated.

12. The built up energy goes through the

load in a big spike. Once the energy is

dissipated, the circuit is effectively reset

and starts the whole process all over again.

It should also be noted that: 

In a typical Joule Thief circuit this process happens 50,000 times per second.