[sebhc] Motor control

Carroll Waddell carrollwaddell at sc.rr.com
Tue Jun 27 20:12:18 CDT 2006

Lee Hart wrote:

>Carroll Waddell wrote:
>>Does anyone know how a diskette motor speed control works? It
>>appears to have some kind of feed back tach in the motor to
>>indicate the motor speed. I'm working on a Siemens drive motor
>>control card.
>Heath provided a manual for the Siemens FDD-100 drive which includes a
>schematic. I've got one around here somewhere. As I recall, it is an
>analog circuit that senses speed with a tachometer built into the motor.
>A pot sets the speed. I'll try to find it and take a look at the
Thanks Lee. I have the schematic, but I was just wondering how the 
circuit works. It seems that the 555 timer functions as a monostable 
multiv. It appears that the pulse width is about 1.67 msec. At 300 RPM, 
that would be about 1/2 of a revolution.

The original problem was that the motor control card had a shorted cap 
which smoked L1, a 10uh choke. I removed all the parts from the board, 
and checked all the parts. I replaced the bad parts, the IC's and the 
transistors. Now the motor runs, but I can't adjust the speed. Of 
course, I could have created a problem when I reassembled the board.

I was just curious about what the motor control card tick.
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