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Fail Safe Power Amplifier

IP.com Disclosure Number: IPCOM000081812D
Original Publication Date: 1974-Aug-01
Included in the Prior Art Database: 2005-Feb-28
Document File: 4 page(s) / 83K

Publishing Venue

IBM

Related People

Buhler, OR: AUTHOR

Abstract

In the conventional power amplifier of Fig. 1 motor 10 is bidirectionally energized from DC power supply 11, in accordance with the polarity and magnitude of the DC control signal applied to input terminal 12. If, for example, the positive side of the power supply fails, the voltage on conductor 13 is reduced to that of ground 14. This, in turn, causes the base electrode of transistor 15 to assure the approximate negative potential of conductor 16. As a result, the motor is energized with the full negative voltage of the power supply, and the input signal at terminal 12 has lost control.

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Fail Safe Power Amplifier

In the conventional power amplifier of Fig. 1 motor 10 is bidirectionally energized from DC power supply 11, in accordance with the polarity and magnitude of the DC control signal applied to input terminal 12. If, for example, the positive side of the power supply fails, the voltage on conductor 13 is reduced to that of ground 14. This, in turn, causes the base electrode of transistor 15 to assure the approximate negative potential of conductor 16. As a result, the motor is energized with the full negative voltage of the power supply, and the input signal at terminal 12 has lost control.

In the event that the negative side of the power supply fails, the full positive voltage of the power supply is applied to the motor.

Fig. 2 is a fail-safe modification which prevents this uncontrolled energization of motor 10. The input portion of Fig. 2 is identical to the input portion of Fig. 1. Specifically, the signal at terminal 20 of Fig. 1 is applied to terminal 20 of Fig. 2.

When power supply 11 is operating properly, voltage sensing network 21 maintains switches 22 and 23 closed. With this switch condition transistors 24 and 25 are both conductive and saturated. With transistor 24 saturated, the base of transistor 26 is at approximately the potential of conductor 13. This voltage is higher than the collector voltage of transistor 27 and transistor 26 is, therefore, in a state of nonconduction. Diode 28 is in series with transistor 26 to prevent emitter-base voltage breakdown.

Transistors 25 and 29 operate in a similar manner, and transistor 29 is nonconductive. Thus, in normal operation circuitry 21, 24, 25, 26 and 29 has no effect on the energization of motor 10.

If power supply 11 should fail in that either of its conductors 13 or 16 assumes a low potential, such as ground potential, network 21 operates to open both of the switches 22 and 23. With these switches open, transistors 24 and 25 are both rendered non-conductive. This results in the base electrodes of both of transistors 26 and 29 assuming a near-ground potential. If transistor 27 tends to drive the voltage, across motor 10 in a positive direction, as it would if the negative side of the power supply failed, the output current of transistor 27 is shunted to ground via diode 28 and transistor 26. Thus, the positive voltage across motor 10 is limited to one diode voltag...