Joule thief circuit is famous among electronics enthusiasts. It has many implementations but probably most common is very minimalist voltage booster based on NPN transistor, coil, resistor and LED. Its primary intent is to squeeze the remaining juice of dying batteries while lighting a LED.
Since a pile of dying batteries keep growing I decided to give it a try. But instead of building circuit blindly, why not have some fun and look whats going on inside the circuit. So I build this circuit on LTSpice simulator. If you don’t know how to add transformer to circuit, there is a great video how to do this (LTSpice Transformer).
In simulation file we attached a 1.5V batter. This is the voltage of new Alkaline battery. But it is not enough to light a LED. Depending on different LED technologies forward voltage starts at about 1.85V and it gets higher. So Joule Thief circuit simply boosts the voltage to level enough to light LED.
Lets see at simulated waveforms and try to explain whats going on here.
In waveform you can see three signals: Blue is transistor collector voltage, green is between resistor and L1 coil, red is base voltage. So from the beginning when battery is attached to circuit base current starts to flow through resistor. It starts to open transistor and so current flows through second coil L2.
When transistor is open the voltage drop on it is about 0.1V. And so coil (and so LEDs) voltage is near zero. The current through L2 coil and transistor rises and it induces even bigger current through base (see high voltage at L1).
Eventually ferrite coil magnetic field current saturates and current in L1 stops increasing. At this moment L2 stops inducing coil L1. The base current suddenly drops closing transistor which also cuts the current through L2 coil. But coil is inertial and holds energy and it starts building voltage due collapsing magnetic field in the coil. It rises until reaches LED break-out voltage and so it lights up releasing coil energy. As you can see L2 coil voltage doesn’t rise higher than diode voltage with some overshoot. This is because LED absorbs energy once it reaches the 3.2V level. You can try adding different diode like 8V Zener and you will see that L2 voltage reaches 8V and then drops because current starts flowing through diode. The voltage can go much higher and if you would like to power more energy hungry devices, you should add more windings to L2 coil.
When L2 coil energy is depleted, base voltage and so current again starts rising and process repeats. Depending on component selection and transformer, joule thief oscillates at tens and even hundreds kHz. Due to persistence of vision it seems that LED is ON constantly.
In practice such circuit works from like 0.3V
Download LTSpice schematic here: Joule_thief_circuit
Lets build Joule thief prototype and see it working in reality.
In the circuit I used about 1cm diameter ferrite ring and put 12 turns of each wire. Transistor is standard NPN BC549. Resistor is 320Ω. Probably the trickiest part is making transformer and connecting transformer right. In schematic you can see that transformer dots are in opposite sides. This means that wires are connected from different ends. To do it eight turn two wires together until you fill the whole core with one layer. Then take two wires from opposite ends and solder together. One remaining wire goes to transistor collector and another to base through resistor. You can use different sizes of ferrite rings.
Here it is working with two series LEDs – white (~4V) and green (~2V):
As you can see no problem to drive two LEDs in series. Also tried to attach three LEDs – they worked, but intensity dropped drastically. Probably adding wire turns to secondary (L2) coil would be able to deal with this.
Real measured signals with scope when only green LED is lit:
Green wave is base voltage; yellow – collector voltage. As you can see collector voltage rises up to 3.2V and then LED drains the power from inductor. In this time base voltage and so current increases slowly and then turns on transistor.
Hope you enjoyed this little experiment as I did. Please leave comment on what can be improved here to make joule thief more efficient.