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| How a Joule thief works |
Joule thief (1)
1. Before battery is connected the transistor is off. When the battery is connected, currentflows through base coil and base resistor and turns on transistor.
Joule thief (2)
2. When transistor turns on a large current will flow through the collector coil larger than base current by gain of transistor. The increasing collector currentflowing through collector coil will induce a current in the base coil that opposes the currentflowing into base of transistor from the battery. This induced current will flow back into the battery or into a capacitor in parallel with the battery if there is one and cancel out the current flowing into base of transistor from the battery. This results in the transistor turning off.
Joule thief (3)
3. When the transistor turns off there will no longer be current flowing through the collector coil and therefore its magnetic field will collapse. The collapsing magnetic field of the collector coil will create a positive voltage spike at the end of the collector coil which is attached to the transistor collector. This voltage is large enough to turn on an LED connected between the collector and emitter of the transistor. Since there is now a larger voltage at the transistor end of the collector coil than at the battery end, current will fiow through the collector coil back into the battery tor a capacitor if there is one in parallel with the battery.
Joule thief (3)
If there is no LED or other device providing an alternative path for the current, all off the current from the collapsing magnetic field will fiow back into the battery at a high current due to the high voltage that exists across the coil. This fiow of high current back into the battery will induce a current in the base coil that will fiow into the base of the transistor and turn the transistor back on. If the energy of the collapsing magnetic field is somehow diverted to a path other than back through the collector coil, then the transistor will remain turned off. If the transistor remains off, then there is no increasing currentflowing in the collector coil to cancel the current from the battery in the base coil and thus the current from battery will again fiow into base oi transistor and turn it on.
Joule thief (4)
4. Steps 2 and 3 are repeated until the battery voltage becomes too low to turn on the transistor.
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| How a Joule thief works |
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