I have a lot of these 600 ohm telephone hybrid transformers that came out of a PBX (see here for much more info). They’re sturdy and well built, and make great audio transformers. Last week someone posted a video to Youtube of some Joule Thiefs that used an audio transformer and the frequency of oscillation was so low that it could be seen with the eye: just a few Hertz. So I decided I should try to make a similar one that uses an audio transformer and runs at a low frequency.
Each winding starts on one side and ends on the other side of the transformer, directly across from the start. The 5th pin is not connected and is for alignment purposes. For the primary winding (white wires in the pic) I connected two of the 600 ohm windings in series for an impedance of 2400 ohms. I used a single third winding (red wires) for the feedback winding. Each winding has a resistance that is about 50 ohms, which is too high for an efficient Joule Thief, but it works. I can see the detrimental result; the battery current is only 1/3 of a milliamp at 1.5 volts and the LED is dim. The transistor is a 2N4401 and the resistor is 39k. I tried other lower values of resistance, but the oscillations were intermittent and unstable.
Well, the frequency was very low, but not as low as the ones in the video. At 1.5V the frequency was 144 Hz, too high to see visually, but very low for hearing on a small speaker. The thought came fleeting through my mind that it might be possible to get the frequency lower by adding the fourth winding in series with the other two primary windings. But I didn’t dwell on it because I could see that the two windings already had far too high a resistance to allow enough current to light the LED properly. I think the better way to go would be to replace the transformer with one similar to the ones seen in the video.
Back to experimenting…
