Joule Thief Circuit Working Explanation

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A joule thief circuit is small, low-cost, easy to build self-oscillating voltage booster.

Basically, this circuit used for driving small loads such as LEDs.

The joule thief circuit is also called blocking oscillator, joule ringer, vampire torch.

This inverter (driver) provides power for LEDs from a 1.2 V or 1.5 V single-cell electric battery.

But, the LED starts to glow when the supply voltage is only 0.4 V.


Joule thief circuit is finely working even far below the voltage where other circuits consider the battery fully discharged or "dead".

The name of the circuit Joule thief suggests the nation that the circuit is stelling energy or "joules" from the source.

The concept of this circuit is not new, that was patented many decades ago.


Joule thief Circuit diagram

Schematic of joule thief circuit shown below.


Working principle of Joule thief circuit

The working principle of the joule thief circuit is very simple.

Joule thief works by rapidly switching the transistor.

Initially, when the transistor is turned OFF condition, a small amount of current goes through the resistor, primary winding, and base-emitter junction of the transistor which helps to open the collector-emitter channel.

The current is now able to travel through the feedback winding and the collector-emitter channel of the transistor.

The increasing amount of current through the feedback winding generates a magnetic field that induces a greater amount of current in the primary winding.

The induced current in the primary winding goes into the base of the transistor and open the collector-emitter channel even more.

This lets even more current flow through the feedback winding, the collector-emitter channel of the transistor.

Thus, these steps are repeated in a feedback loop until the base of the transistor is saturated and the collector-emitter channel is fully open.

The current flow through the feedback winding and through the transistor are now at a maximum.

There is a lot of energy generated in the magnetic field of the feedback winding.

After the transistor is turned ON condition, the current in the feedback winding is no longer increasing, it stops inducing the current in the primary winding.

This causes less current flow through the base of the transistor and the collector-emitter channel of the transistor begins to close.

This allows less current to flow through the feedback winding.

A drop in the amount of current in the feedback winding induces a negative amount of current in the primary winding.

This causes even less current flow through the base of the transistor.

Thus, these steps are repeated in a feedback loop until there is almost no current flow through the transistor.

Part of the energy that was stored in the magnetic field of the feedback winding, this causes the voltage at the output of the coil.

The generated current can't go through the transistor, so it has to go through the load (usually an LED) and be dissipated.

Once the energy is dissipated by the load, the circuit is effectively reset and starts the whole process all over again.

Usually, in a Joule Thief circuit, this process happens 50,000 times per second.

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