What is the Joule Thief Circuit and how does it work

A joule thief is a compact, low-cost, and easy-to-build self-oscillating voltage booster that is often used to drive small loads(3.3v 5mm LED). Other names for this circuit include blocking oscillator, joule ringer, and vampire torch. The circuit is a blocking oscillator variation that functions as an unregulated voltage boost converter. The output voltage is increased at the price of a larger input current draw, but the output integrated (average) current is reduced and the luminescence brightness is reduced. This joule thief Circuit uses a 1.2 V or 1.5 V single-cell electric battery to power LEDs. However, as the supply voltage reaches 3V, the LED begins to light.

The theory of operation of a Joule thief circuit

Joule Thief Circuit

The joule thief circuit operates on a fairly basic basis. It operates by quickly switching the transistor.

When the transistor is first switched off, a tiny amount of current flows via the resistor, primary winding, and base-emitter junction, assisting in the opening of the collector-emitter channel.

The current can now pass via the feedback winding and the transistor's collector-emitter channel. The growing current via the feedback winding creates a magnetic field, which induces more current in the main winding. The generated current in the main winding flows into the transistor's base, widening the collector-emitter channel even further. This allows greater current to flow via the feedback winding, which is the transistor's collector-emitter channel.

As a result, these operations are repeated in a feedback loop until the transistor's base is saturated and the collector-emitter channel is completely open. The current flowing between the feedback coil and the transistor is now at its peak. The magnetic field of the feedback winding generates a lot of energy.

When the transistor is switched ON, the current in the feedback winding no longer grows, and it no longer induces a current in the primary winding. As a result, less current flows through the transistor's base, and the transistor's collector-emitter channel begins to shut. As a result, less current flows through the feedback winding.

A decrease in current in the feedback winding causes a decrease in current in the primary winding. This causes considerably less current to pass through the transistor's base. Thus, in a feedback loop, these operations are repeated until nearly no current flows through the transistor. The voltage at the coil's output is caused by a portion of the energy stored in the magnetic field of the feedback winding.

Because the produced current cannot pass through the transistor, it must pass through the load (often an LED) and be dissipated. When the energy is dissipated by the load, the circuit is essentially reset and the process begins again. This process occurs 50,000 times per second in a Joule Thief circuit.

Working Video