Joule Thief Circuit
The Joule Thief Circuit is a voltage booster circuit which converts a constant low voltage input into a periodic output of a higher voltage. This circuit can be most often seen lighting an LED with an almost dead AA battery. The peaks in voltage occur rapidly, causing the LED to flash at a very fast rate. However, the LED appears to be constantly lit to the human eye due to the persistence effect.
How does the Joule Thief circuit work?
The circuit is an arrangement of a power source, a resistor, a transistor and a ferrite toroid core wrapped with two wires coming from the positive terminal of the power source, one through a resistor.
A magnetic field is created around the ferrite toroid because of the current that passes through the wires. The extra current causes the transistor to switch off and power to the ferrite toroid is cut off. As a result, the magnetic field is converted into electrical energy which is given as output. Once the magnetic field no longer exists(the pulse ends), the transistor is switched on again and conducts electricity to create the magnetic field again. This process occurs rapidly enough to provide a somewhat constant power output. The frequency of voltage spikes generated by the Joule Thief circuit is over 5KHz . The video explains the working of the Joule Thief circuit very well.
How can a Joule Thief circuit be made?
The components required are a NPN transistor, a 1kΩ resistor, a ferrite toroid (that can be salvaged from an old CFL bulb), wiring, a 3Volt LED and an AA battery which doesn't light up a LED by itself. Use the AA battery and connect two wires from the positive terminal and take one through the 1kΩ resistor and wrap the wires around the ferrite toroid. The wire that goes through the resistor, after it comes out from the ferrite toroid, is connected to the base terminal of the transistor. The other wire from the ferrite toroid goes to the collector terminal of the transistor which is also connected to the positive terminal of the LED. The negative terminal of the LED is connected to the emitter terminal of the transistor.
More information is available at an Instructables tutorial.
How can the Joule Thief circuit be optimized or modified?
The inductance in the Joule Thief circuit is determined by the loops around the ferrite toroid. The more the number of loops, the greater the inductance. An increase in inductance generally decreases the current flowing through the circuit and an increase in the duty cycle(on time%). This suggests that an increase in inductance increases the efficiency of the circuit. However, too much inductance is not good either. Trial and error can help find the sweet spot of the circuit where the least current is drawn. Increasing or decreasing one loop at a time will help find the number of loops a certain circuit requires for maximum efficiency.
What are the applications of this circuit?
Lighting an LED with a dead battery is not the only application of the Joule Thief circuit. The circuit helps utilize almost all the energy that is stored in a battery. For example, a battery that has come out from a toy can easily light up a torch that makes use of the Joule thief circuit for hours or even days. The circuit can also be used in battery chargers, wall clocks, solar cell chargers where a low voltage input has to be increased for its intended application.
The principle behind the joule thief can be used on any low voltage source, even if it's not a "dead" battery. For example, aluminium cans filled with water and wood ash, and simple electrodes can be used with a joule thief circuit for a battery charging application.
What are the disadvantages of using a Joule Thief circuit?
Another disadvantage is Without significant improvements to the circuit, it is hard for the circuit to power more than simple led because heavy demand is put on the transistor for high current at very low voltage
What are some alternatives to the Joule Thief circuit?
- Supercharged Joule Thief circuit: Has an efficiency of above 80% while conventional Joule Thief circuits have efficiencies between 40% and 60%
- Buck-boost converters: Can be used for applications which require more power. The output voltage is always reversed in polarity with respect to the input
- Voltage multiplier: Converts AC electrical power of a lower voltage to DC electrical power of a higher voltage
- Split-pi topology: DC-DC converter that uses MOSFETs making it bidirectional and good for applications revolving around regenerative braking
How does the supercharged Joule Thief circuit have such a high efficiency?
All that is required in addition to the components for a conventional Joule Thief circuit is a 680 pF capacitor. Apart from this, the feedback wire which is connected between ground and the wire from the ferris toroid which is also connected to a 1.5kΩ resistor and a diode, according to the circuit diagram which shows a circuit that can be used to switch between the conventional Joule Thief circuit and the Supercharged Joule Thief circuit.
The name Joule Thief is coined by Clive Mitchell and a tutorial is published on his website. Clive's variation includes a smaller resistance than the original circuit published in the EPE magazine.
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- Split-Pi Technology
- Buck-Boost Converter
- Voltage Multiplier