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Motor Speed Regulator
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This circuits regulates the speed of a small drill motor I use for PC board work.  It is much handier for delicate work that a large AD powered tool.  The tool that I have came with a useless 9 volt AC adapter.  The motor would whine at high RMPs unloaded, but would slow to a crawl when loaded.  This circuits solved the problem.

Q1, Q2 and U2 form a variable-frequency pulse width modulator, controlled by U3.  Q4 forms part of a step0down power converter.  Speed regulation is accomplished by sensing the motor current with R17 and using it as positive feedback to compensate for motor resistance loss.  The gain pot should be set to a point just below the point where the motor speed oscillates.  After finding this point, you may want to change the value of R11 to get better speed control range.

Q3 limits motor current.  D4 and C8 capture some of L1s inductive kick to produce a loosely regulated -12 volts for U3.  SCR1 acts like the usual flyback diode one c8 has stored the needed energy, preventing significant power losses.

 

Figure 1 Exterior view

Figure 2 Interior view
 

More:  Control CircuitsMotor Control Circuits
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