FAULT_TOLERANT_RELAY_DRIVER

Published:2009/7/17 4:09:00 Author:Jessie | From:SeekIC

Mechanical relays are useful in remote-switching applications that require electrical isolation between control and switched circuits. The traditional approach for driving the relay coils uses a single-transistor common-emitter switch. The figure shows a circuit that can reduce the likelihood of having an unrecoverable failure in the relay driver circuit. Additional transistor switches, inserted in series with the original controlling transistors, maintain proper operation if a single transistor fails. The diodes connected to the upper transistors prevent reverse base-current flow if the collector-base junctions break down. The diodes connected to the lower transistor provide proper biasing for the circuit. The upper and lower transistors will have a similar Vbe because they are the same type and have nearly identical currents. Therefore, the Vbe of the lower transistor will be about the same as one diode voltage drop, and the device will be operating in the active region. Power to the relays can be provided by one of two voltage sources connected together through 'diodes in a wired-OR configuration. The additional diode clamps the coil inductance voltage spike in the event that one of the suppression diodes fails shorted.



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