Circuit Project Electronic

Reverse engineered circuit diagram of a motor speed regulator out of a line powered Cassette dictation machine containing a single heafty motor for all functions. This circuit works amazingly well, keeping the motor speed constant regardless of shaft load.
Unmarked didoe are silicon so use 1N4148. This circuit works just as well at only 12VDC. Resisror are 1/4W CF 5% except as noted. It’s ok to use 2.0ohm resistor in place of choke. Motor power draw under different loading is:
-52mA at 4.2V under zero load freerunning.
-120mA at 4.5V under normal regulated load.
-250mA at 5.6V under normal maxiumun load.
-1.4A at 11V with shaft frozen & motor stalled.

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