I stumbled on this "circuit" online last night and tried it. It actually works. If you have a box full of old steppers and wish you could use them as ordinary motors for whatever, you can. It runs the stepper as a synchronous motor by using a capacitor that produces a roughly 90 degree phase angle change in the sine wave to one of the windings. Calculating the synchronous rpm is easy. For a 7.5 degree per step motor it's 300 rpm.

360 degrees / 7.5 degrees per step=48 steps per rev

48/4 steps per cycle =12 cycles per rev

3600 cycles per minute/12 =300 rpm

I have tried it with three very different motors and they all worked. The capacitor must be an electrolytic non-polarized type. If you don't have one you can connect two polarized capacitors of twice the value back to back so that both positives are connected together and the negatives become the leads of the non-polar capacitor.

I used a 12vac centre tapped xformer for the test and found the smaller two motors ran best on six volts while the large motor ran only on 12vac. The torque isn't great but it is certainly enough to be useful. By experimenting with the capacitor value you can change the phase angle to produce a preferred starting direction. If the phase angle is very close to 90 degrees it will randomly start either direction.

360 degrees / 7.5 degrees per step=48 steps per rev

48/4 steps per cycle =12 cycles per rev

3600 cycles per minute/12 =300 rpm

I have tried it with three very different motors and they all worked. The capacitor must be an electrolytic non-polarized type. If you don't have one you can connect two polarized capacitors of twice the value back to back so that both positives are connected together and the negatives become the leads of the non-polar capacitor.

I used a 12vac centre tapped xformer for the test and found the smaller two motors ran best on six volts while the large motor ran only on 12vac. The torque isn't great but it is certainly enough to be useful. By experimenting with the capacitor value you can change the phase angle to produce a preferred starting direction. If the phase angle is very close to 90 degrees it will randomly start either direction.

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