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Fail Safe Relay Actuator/Driver

IP.com Disclosure Number: IPCOM000087298D
Original Publication Date: 1977-Jan-01
Included in the Prior Art Database: 2005-Mar-03
Document File: 2 page(s) / 32K

Publishing Venue

IBM

Related People

Buhler, OR: AUTHOR

Abstract

The illustrated fail-safe relay driver is activated by a positive input signal. If either one transistor shorts (collector-to-emitter short), relay K1 will still turn off.

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Fail Safe Relay Actuator/Driver

The illustrated fail-safe relay driver is activated by a positive input signal. If either one transistor shorts (collector-to-emitter short), relay K1 will still turn off.

With a positive input signal, Q1 and Q3 are driven into conduction. With Q1 in conduction, Q2 will also be in conduction, thus, completing a current path for K1 through Q2 and Q3. Both Q2 and Q3 must conduct to pick K1. If Q1 or Q2 shorts, Q3 will still be capable of turning K1 off. Likewise, if Q3 shorts, K1 is capable of being turned off by Q2. The probability of two devices shorting at the same time is extremely low.

To cover the possibility of Q2 shorting now and Q3 shorting at some later time, an indicator system indicates a maintenance requirement. When both Q2 and Q3 are conducting, the drops across indicators IND1 and IND2 will be very small compared to that required for indicator actuation. With both Q2 and Q3 off, the supply voltage across series-connected CR1 and CR2 Zener diodes will be insufficient for Zener breakdown. Thus, the drops across IND1 and IND2 will again be too small for indicator actuation.

If, however, Q2 should be off and Q3 on, current will flow from Vcc through IND1, CR1, K1 and Q3 to signal a maintenance requirement. With Q2 on and Q3 off, current flows from Vcc through Q2, K1, IND2 and CR2, again signaling a maintenance requirement. The total Zener voltage (Vz) of CR1 + CR2 should be greater than Vcc. Also, Vz of CR1 or CR2 should...