Joule Thief

Every once in a while, I get the itch to do something analog but doesn't require a major investment in time. This eventually lead me to the following experiment:

What do you do when your supply voltage is lower than the target voltage needed? For example, when your solar lamp receives less than the voltage necessary to light the LED? You use a Joule Thief circuit!

The Joule thief circuit is small and simple enough that it can be assembled in about 15 minutes. You need:

• Variable power supply (or 1.5V battery)
• A toroid core
• Solid wire
• 1 Kohm resistor
• 2N3904 transistor or substitute
• An LED

For demonstration purposes, I used a blue LED because these require higher voltages than say a red one. Figure 1 shows a hastily drawn schematic.

Figure 1. Joule Thief circuit.

Pay attention to the dots on the toroid windings. To create the transformer, twist a pair of solid wires together (cordless drills work great for this). Then wind as many turns as you can around the core (about 20).

Figure 2 illustrates my test setup, though you need not use special equipment. A 1.5V battery can substitute for a power supply.

Figure 2. Test setup.

If you have the polarity of the windings correct, your circuit should work the first time. Figure 3 illustrates the wave forms when 0.55 volts were supplied:

Figure 3. Input supply is 0.55 volts.

With 0.55V supplied, the frequency of oscillation was about 21.3 kHz. The upper trace in yellow shows the voltage appearing across the LED, which was about 2.65V max. The lower trace shows the base voltage varying between +1.08V and -2.04V. The pulse width is narrow at this minimum startup level and the average current flow was about 8 mA.

Figure 4 illustrates the wave forms when the input supply is 1.5V.

Figure 4. Input supply voltage is 1.55V.

The voltage across the LED increases slightly to 3.47V in this trace. The output frequency drops to about 14.7 kHz at this level. The average current was about 39.9 mA at this operating point. 

Conclusion

The circuit presents a very simple way to boost the voltage up to a level suitable for current consumption by an LED. It's only disadvantage is the requirement of an inductor or transformer.  Pull a toroid out of your junk box and give it a try. Perhaps even an junk audio transformer can be used.

Find the lowest input voltage for your circuit. Mine came to life at about 0.55V. Yours may be higher or lower, depending upon the parts used.