Imagine taking your dog for a stroll at night when your flashlight, gradually dimming, suddenly goes out. Instead of navigating your way home in the dark, wouldn’t it be advantageous to extract a little more energy from the batteries inside?
This concept, while seemingly outlandish, is quite feasible. When a device ceases to function due to depleted batteries, we often describe it as “dead.” In reality, the voltage remains above zero; it simply isn’t sufficient to allow current to flow through the bulb or LED, or any other load you may have.
However, with a simple physics trick, you can extend the life of that light. Allow me to guide you in constructing a straightforward electrical circuit that combines a transformer and a transistor, enabling you to harness that residual energy. In jest, we refer to this circuit as a ‘joule thief.’
Not only is it enjoyable to build, but it also serves as an excellent illustration of Faraday’s law of induction, the same principle employed in electric generators and induction cooktops.
Understanding Basic Circuits
Let’s start with a very simple circuit: a standard AA battery rated at 1.5 volts connected to a small incandescent bulb via a single copper wire.
This forms a closed circuit. Electric current flows from one end of the battery, passes through a bulb containing a tungsten filament, and then returns to the other end of the battery. Given that the filament is extremely thin, the current heats it to approximately 2,482°C, causing it to glow a bright red.
Fortunately, tungsten possesses the highest melting point of all pure metals.
As long as the circuit remains closed, current will continue to flow, gradually depleting the chemical potential energy stored in the battery. This scenario is akin to a flashlight being left on. As the voltage diminishes, the current decreases, eventually reaching a point where it cannot produce any light. In a sense, this circuit also acts as a joule thief. If you leave the switch on, current will persist even after the light goes out, consuming any remaining energy in the battery.
LEDs vs. Incandescent Bulbs
Nowadays, most devices utilise LEDs instead of incandescent bulbs. An LED generates light not by heating objects but through a solid-state process involving a forbidden band. When electrons in the current drop to a lower energy level, they release excess energy in the form of light.
This method is far more efficient, as it avoids the waste of thermal energy. The only drawback is that a white LED requires 3 volts, so now we need two AA batteries.
As the batteries deplete, they may fall below the 3-volt threshold. You might still measure 2.8 volts, but the light won’t activate. This is where the magic of the joule thief comes into play. In fact, we can illuminate a 3-volt LED with just a single 1.5-volt battery. However, we need two components: a transformer and a transistor.
The Role of the Transformer
There are various methods to generate electricity. A battery is the most apparent choice, but a magnetic field can also achieve this. This is where Faraday’s law comes into effect: when the magnetic field around a loop of wire changes, it induces a voltage across that wire.
This is precisely how a transformer operates. It comprises two coils of wire wound around a common core. The wires are insulated, preventing contact; thus, they form two separate circuits. However, when a current flows through one coil, it produces a magnetic field, and if that field changes in any way, a current is induced in the second coil.
This is the homemade transformer I will use. As you can see, there are two sets of red wires; the iron ring enhances the magnetic field’s intensity, making the transformer more effective.
To clarify, it is not the magnetic field itself that induces voltage in the second coil, but the change in that magnetic field. When the current is turned on or off in one coil, a voltage spike occurs in the other coil. If the current is left on continuously, that voltage falls to zero.
In fact, the magnitude of the induced voltage partially depends on the rate of change. Slowing down the magnetic field yields a lower voltage, whereas shutting it off quickly produces a higher voltage. You can also achieve a higher voltage by increasing the number of turns in the secondary coil.
The Functionality of the Transistor
This is the essence of this clever trick. The transformer allows us to boost the voltage reaching the secondary circuit, where the LED is connected, even if the battery provides much less. The next step is to make the primary circuit turn on and off rapidly, time and time again. This is where the transistor comes in.
Simply put, a transistor acts as a valve that regulates the flow of electricity: it can block current or allow it to pass. This “valve” opens or closes by means of a small electrical signal. It is akin to using a small stream of water to control a much larger flow.
This is essentially all you need to know about how the ‘joule thief’ operates. The transistor switches the current on and off thousands of times per second, effectively creating an oscillating current that enables the transformer to produce a higher voltage.
Assembling the Complete Circuit
Now we are prepared for the complete circuit. There are numerous tutorials available online to guide you in constructing one of these, but I will use a very straightforward version. Here is a basic diagram:
Thus, the battery drives a current through the primary coil (blue) of the transformer, generating a magnetic field and, consequently, a current spike in the secondary coil (red). This induced current then flows to the transistor, cutting off the primary current, which in turn causes another jump in the magnetic field. Each cycle generates a 3-volt spike in the second coil, allowing the LED to illuminate. However, the battery cannot sustain this, as it only provides 1.5 volts, prompting the transistor to revert to the initial loop.
Here is a photo of the joule thief I constructed. As you can see, it operates with just a 1.5-volt battery, which is half the voltage required by the LED. At the time of the photograph, the light had been continuously illuminated for several days, and it will remain lit until the battery is entirely depleted.
Applications Beyond the Laboratory
This invention may not be something you carry everywhere. However, many high-end pocket flashlights incorporate a circuit similar to a joule thief, capable of elevating the voltage to function with a single battery. All of this fits into a small electronic board known as a boost converter.
The same principle of creating oscillations with a direct current power supply is employed to elevate voltage in other devices as well. For example, solar panels are often connected to home batteries for energy storage that can be used at night. Yet, on cloudy days, the panels may only generate a potential of 10 volts, insufficient for charging a 12-volt battery. Consequently, these systems frequently include a boost converter.
As you can see, this is not merely an ingenious laboratory trick. Joule thieves are ubiquitous in various applications, and that is certainly a positive development!