I received a Google Alert with a link to this Instructable. The author uses the two transistor, single winding voltage booster, often incorrectly called a Joule Thief, because he wanted to avoid having to wind the two winding coil.
The Joule Thief name was given to the single transistor, single resistor, two winding coil version. The two transistor voltage booster has been around a long time, and was never called a Joule Thief. Only recently have people been adding confusion by calling this and other V boosters Joule Thiefs just because they happen to light a LED similar to the conventional Joule thief.
It might be cheaper to use a single winding coil in a production environment – silicon is cheaper than copper. But the experimenter can wind his own two winding coil using a toroid core from a dead CFL light for nearly free and a few minutes of his/her time. I think it’s good practice for learning electronics assembly.
I’ve built many of both types and have found each has its advantages and disadvantages. The conventional one transistor Joule thief is so much simpler that the neophyte can make it work almost all of the time. The two transistor version has some gotchas when it comes to variations in the gain of the two transistors. The circuit the author posted shows R1 as 1k, I’ve often seen it lower, 470 or even less. And it is a good idea to put a resistor, maybe 1k, from Q2’s base to its emitter. This helps to keep leakage current from Q1 collector from being amplified by Q2. But it will work okay the way it is most of the time without any changes or improvements.
The conventional (single transistor) Joule Thief has a limitation that the supply voltage should never exceed 5 volts. This assumes that the total voltage across the LEDs is more than the supply voltage (two LEDs in series adding up to 6 or more volts). The voltage across the LEDs is also reflected in reverse across the feedback winding and hence across the base to emitter junction – it’s the same negative voltage, assuming the windings are the same number of turns. This voltage must never be more than negative 5V (for most transistors). If the voltage is higher, then the emitter to base junction goes into breakdown and the transistor is damaged.
In either circuit, you should use the DMM on ohms range to measure the resistance of the coil winding that connects to the collector. The resistance should not be more than a quarter of an ohm. But some DMMs use test leads that are so cheap that when you short the probe tips together, the meter says it’s a half ohm resistance, or maybe even more. So you have to guesstimate what 1/4 ohm is.
The efficiency of the conventional Joule Thief is about 40 to 70 percent, typically 50%. I’ve measured this on several examples. The efficiency of the two transistor “JT” is somewhat similar. I have done a few measurements and found nothing notable about this circuit. But then that is to be expected when either is compared to my SJT, which has an efficiency of over 70%.
Back to experimenting…