I was watching a Youtube video of Xee2vid’s no toroid Joule Thief 4, which is a conventional JT that uses a Radio Shack 273-1380 audio output transformer (Xee2 doesn’t allow comments). The ad doesn’t say what the specs are but IIRC, it’s a 1000 ohm center tapped primary and an 8 ohm secondary. The small size means it can handle only a few hundred milliwatts. Also, since it’s an audio transformer, it’s most likely that the frequency is going to be much lower than a normal JT, possibly in the audio range.
The schematic shows that the secondary is connected to the battery positive and the collector of the transistor. This 8 ohm winding most likely has a DC resistance of several ohms, but I don’t know absolutely since I’ve never measured one; I’m just going by my previous experiences with this kind of transformer. The center tapped winding is higher DC resistance but it’s of no concern since the current is minimal. One concern is the ratio of the two windings. The voltage across the center tapped winding is substantially higher than the 8 ohm winding, I think it’s the ratio of the turns, which is the square root of the impedance ratios. So 1k / 8 is 125 so about 11 to 1. But it’s half that because only one half of the winding is connected. That would be about 5 or 6 to 1. For every volt on the 8 ohm winding, there would be 5 or 6 volts on the half of the center tapped winding, which may be high enough to exceed the maximum emitter to base breakdown voltage for the transistor. As I’ve shown, this can damage the transistor.
One way to get around this is to use only the secondary and not use the 8 ohm winding. The battery positive is connected to the center tap and one end goes to the collector, the other end goes to the resistor and capacitor. The DC resistance of the winding is going to reduce the collector current, but it will always light up the LED because you can’t get the connections wrong.
Another concern is can the 8 ohm winding with several ohms DC resistance conduct enough current to fully light the LED? Probably not. There may be nothing wrong with this if the builder is not concerned with getting the most out of the circuit. But it would be an obstacle for those wanting to get the JT to its maximum performance.
All in all, using this transformer for the coil in a Joule Thief is an easy way to get started. But it’s definitely not the best way to make a JT.
