Objective:
To design, develop and testing of joule thief circuit
Analysis/Discussion:
A "Joule Thief" is a simple voltage booster circuit. It can increase the voltage of 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 this kind of circuit.
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.
Figure 10: Circuit diagram for joule thief charging circuit
Here are the materials and tools that you will need to complete this project.
- Ferrite toroid core
- Insulated Wire
- NPN Transistor (2N2222, 2N3904, or similar)
- 0.01 microfarad Capacitor (capacitor code:103)
- 330 microfarad Capacitor
- 1 kohm resistor
- 6V Zener Diode
- Diode
In order to make a battery charger, I made a few changes to the standard Joule Thief Design.
First I added a capacitor to the node between the resistor and the first coil. This helps to stabilize the output a little.
Then I added a zener diode to the base of the transistor. This helps to protect the transistor from being damaged by voltage spikes. The Emitter-Base junction is the weakest point of the transistor. Most small NPN transistors will have a maximum allowable Emitter-Base voltage of 6 volts or less. So I added a zener diode between the base and the collector of the transistor. The diode prevents the Emitter base junction from becoming reversed biased.
At the output of the second coil, I added a diode. This allows the output voltage to pass through but it prevents electricity from the battery draining back through the transistor.
The capacitors and the zener diode also help protect the transistor from high voltage spikes that can occur if the circuit is turned on without a load. The voltage of the second coil will jump up as much as it needs to in order to be discharged. If there is no load attached, the coil voltage can reach over 60 volts. This could quickly damage the transistor. The zener diode and the capacitors help to limit these voltage spike.
Figure 11: Pretesting the circuit
Figure 12: Finalize the circuit
Conclusion:
The input voltage will affect how high the output voltage can get. With the components that I used you will get the best performance from power sources that are between 0.9 volts and 2.0 volts (with a maximum at 1.50 volts). Below 0.9 volts, the circuit will have difficulty boosting the voltage to a high enough to effectively charge a battery. Above 2.0 volts the output voltage will can start to get to high and it will be limited by the zener diode that is protecting the transistor.
