Protected Solid State Relays | First Look

Electromechanical and solid-state relays are used in all kinds of applications, providing a simple way to switch a load on and off. In harsh and electrically noisy environments, faults can quickly lead to overcurrent conditions that damage the relay, the load, or both, leading to extended downtime and costly repairs. Littelfuse offers protected solid state relays with load current ratings up to 1A and with integrated active current limiting and thermal shutdown, with select parts also incorporating voltage-triggered shutdown. Combined with auto-restart functionality, these relays provide enhanced robustness and reduced downtime in telecom equipment, home automation, medical devices, industrial controls, and other AC and DC applications that can’t be down for long periods of time. The SSRs provide active current limiting through circuitry that continuously senses the load current and then regulates the gate drive to limit the current when a fault condition occurs. To prevent damage to the SSR due to excessive heating during current limiting, an integrated thermal sensor monitors the die temperature and will shutdown the relay if the temperature gets too high. Once the fault condition has been removed and the die temperature has dropped to an acceptable range, the SSR will automatically resume normal operation. Select parts include voltage triggered shutdown as an additional protection mechanism. While current limiting is active, if the voltage across the load terminals exceeds the over-voltage threshold, the load current will be reduced to 100µA until the load voltage drops below the over-voltage threshold. Littelfuse offers protected SSRs with blocking voltages from 60V to 600V. They have low input current control and provide 3,750Vrms isolation while providing the added robustness needed in harsh applications.