To understand how a Joule Thief can revive a battery, it is important to first understand how batteries work. Batteries are energy sources that rely on a chemical reaction to produce voltage. In theory, this voltage should remain consistent over time, depending solely on the chemical elements inside the battery.
However, as the battery discharges, the chemical reactions within the battery become less efficient, causing the voltage to gradually decay, although it typically remains stable. When the battery voltage drops below the minimum required for a device to operate, the device may either fail to turn on or function with reduced power. This leads many to conclude that the battery is dead or discharged. In reality, there are often still unreacted chemical elements inside the battery, suggesting that some energy remains unused.
Incidentally, the unit of energy measurement in the International System of Units is the joule, which is why this circuit is named the “Joule Thief.”
Components of a Joule Thief Circuit
The Joule Thief circuit can take many forms, but we will focus on one of the simplest configurations, which consists of just five components: a 1.5-volt battery, a resistor, a bi-filar toroidal coil, an NPN bipolar junction transistor, and an LED that requires at least 1.8 volts to light up.
Let’s quickly review how these components function individually before exploring how they work together in the circuit.
Resistor
A resistor is an electronic component designed to provide resistance to the flow of electric current. This resistance serves various purposes, such as guiding current through a circuit. In practical terms, if there are two possible paths for current, the current will prefer the path with less resistance, as energy tends to follow the path of least opposition.
Toroidal Coil
A toroidal coil is a passive component that stores energy in the form of a magnetic field when electric current flows through it. When current passes through the coil, a magnetic field is generated. According to Faraday and Lenz’s laws, this field induces a voltage opposite to the direction of the current that created it. Over time, the magnetic field will stabilize, and the inductor will behave like a simple wire. However, when current flow stops, the magnetic field begins to collapse, inducing a voltage in the opposite direction to the one generated by the growing magnetic field.
For this circuit, the toroidal coil is bi-filar, meaning it consists of two insulated wires that share the same magnetic field. This design allows the voltage induced in both wires to be opposite in direction when the magnetic field changes.
NPN Bipolar Junction Transistor
The NPN transistor is a key component with three terminals: the collector, the emitter, and the base. The transistor acts as a switch for the current, with the base controlling the current flow between the collector and emitter. When no current flows into the base, the connection between the collector and emitter behaves like an open switch. Conversely, when current flows into the base, the transistor acts like a closed switch, allowing current to flow between the collector and emitter.
How the Joule Thief Circuit Works
Once the battery is connected, the circuit has two possible paths for current flow:
- Path A: The current flows through the resistor, inductor, and base of the transistor.
- Path B: The current flows through the inductor, which then splits into two possible routes: the collector of the transistor or the LED.
Initially, most of the current will flow through path B, as the resistance of the copper wire is nearly zero. However, the voltage from the battery may not be enough to turn on the LED, and without current at the base, the transistor behaves like an open switch, preventing the current from flowing through the collector-emitter junction.
Thus, the current will follow path A, where it flows through the resistor and the base-emitter junction, turning the transistor on slightly. This allows current to start flowing between the collector and emitter, creating a small current through both paths. However, this is still not sufficient to light the LED.
The Role of the Inductor
The key to the Joule Thief’s operation lies in the inductor. As current passes through path B, it generates a magnetic field in the toroidal coil. This changing magnetic field induces a voltage in both wires, increasing the current through path A and further activating the transistor. As a result, current flow increases through path B, which causes the magnetic field to grow even stronger.
This creates a feedback loop, where the increasing voltage in path A causes more current to flow through the transistor’s base, and this feedback accelerates the process. Eventually, the transistor switches fully on, allowing current to flow through path B like a closed switch.
Turning on the LED
Despite the positive feedback loop, the voltage is still not sufficient to light the LED. The crucial moment occurs when the magnetic field in the inductor stops growing and becomes stable. At this point, no more voltage is induced in the coil, causing the current to stagnate and the transistor to return to its initial state.
As the magnetic field collapses, it induces a voltage in the opposite direction, which boosts the voltage in path B. This combined voltage from the battery and the collapsing magnetic field is enough to turn on the LED.
However, this state cannot last indefinitely. Once the magnetic field has fully collapsed, the inductor returns to its initial state, causing the LED to turn off. A small current flows back into the base of the transistor, and the cycle begins again.
The Effect of Continuous Switching
While the LED alternates between on and off states rapidly, this happens so quickly that to the human eye, it appears as though the LED is constantly on. The speed of the switching can be controlled by adjusting the inductance of the toroidal coil, which determines the duration of the on/off cycle.
In summary, the Joule Thief circuit relies on a positive feedback loop and the collapsing magnetic field of the inductor to generate enough voltage to light an LED, even when the battery’s voltage is insufficient on its own. The entire process happens very quickly, creating the illusion of continuous operation.
