| 摘要 |
A linear servo circuit for A.C. motors is disclosed. The servo circuitry is used in a magnetic tape transport to control the motor which operates a tape reel on which the magnetic tape is stored. The servo circuitry develops two complimentary saw-tooth voltage waveforms synchronized to the sinusoidal A.C. line voltage. One saw-tooth waveform is used to control the motor in the forward direction and the other saw-tooth waveform is used to control the motor in the reverse direction. Comparator circuitry compares the saw-tooth waveforms to a D.C. control voltage which is proportional to the angular position of an arm that controls the tension of the magnetic tape. The comparator circuitry provides a control pulse to a photodiode when the saw-tooth waveform exceeds the D.C. control voltage. Light pulses from the photodiode impinge on a light sensitive SCR which in turn triggers a triac to apply power in the forward or reverse direction to the A.C. tape reel motor.
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