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. It's 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.

1 comment:

  1. Your discussion is entirely wrong. The "primary winding" is the feedback winding and the feedback winding is the primary winding.

    In the beginning the voltage of the battery is high enough to turn the transistor ON. This allows current to flow through the collector-emitter junction and the magnetic field in the transformer increases and creates a voltage in the base winding that turns the transistor ON more. This continues until the transistor is fully saturated OR the magnetic flux in the core creates SATURATION.
    At this point the magnetic flux is no longer EXPANDING and the extra voltage and current being passed to the base of the transistor suddenly STOPS and only the originating current into the base is present. This is not enough to keep the transistor turned on FULLY and the magnetic field starts to COLLAPSE. The collapsing magnetic field cuts all the turns on the transformer and produces a voltage in the base that has an opposite polarity to the originating voltage. This turns the transistor FULLY OFF.
    The transistor effectively COMES OUT OF CIRCUIT and the lead connected to the collector produces a voltage that is opposite in polarity to previously and this means the voltage is HIGHER than the voltage of the battery by a value that can 10v to 20v.
    This voltage is high enough to illuminate the LED and even 2 or 3 LEDs can be connected to the circuit.
    But a LED has a characteristic voltage depending on its colour and for a white LED this is 3.2v. The voltage across the LED can never rise higher than this and the output voltage of the transformer is limited to this value and all the energy from the collapsing magnetic field is converted to light by the LED.
    At the same time the magnetism is cutting the turns of the base winding and producing an output voltage that is negative and keeping the transistor fully turned OFF.
    When the magnetic field has fully collapsed, the negative base voltage reduces and the battery voltage starts to take over, via the resistor and when it is above 0.6v, the cycle starts again. This process occurs at the rate of about 100,000 cycles per second.
    Persistence of Vision of your eye means you cannot see the flashing of the LED and it appears to be ON all the time.
    My website describes the operation in more detail.
    http://www.talkingelectronics.com
    Colin Mitchell
    talkingelectronics.com

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