>If it's an ac motor or a universal motor running on AC, you
>can't control it via a single transistor.

Hi Phil,

Don't tell that to Ampex then, because I've worked on 24-track Ampex MM1200
tape recorders that did just that.

They used a bridge rectifier with the DC side hooked to a transistor. The
AC side of the bridge was in series with the motor and the 110 VAC. The
transistor was driven by a simple circuit that used an optoisolator, so the
whole thing was DC controlled and isolated.

It did require large heatsinks but was smooth and quiet and could move tape
pretty well.

That stuff is ancient history and I barely remember it, but thought it was
cool for its simplicity. The motors I believe were AC capacitor-run,
similar to ones Ampex used in the relay-operated MM1100(which had big
resistors for different torque levels).

Cheerful regards,

Bob Blick

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