Electronic Design

Build Solid-State Circuit Breaker

An optically coupled solid-state relay (SSR) can be used to form a programmable dc circuit breaker (see the figure). In addition to its breaking capacity, the circuit obviously has the accompanying advantages of solid-state: fast response, small size, and high reliability.

Looking at the details of the setup, the AT&T LH1523AB is a dual form-B (normally closed contacts) SSR with 10-Ω (typical) contacts that can handle 0.2 A when operated independently. Channel 1 of the SSR and R1 form a 5-mA current source to provide LED current to the breaker switch on Channel 2.

As the output current increases, the voltage across the circuit breaker increases proportionately. When this voltage reaches ≈3 V, sufficient voltage is available to trip the SCR, which is formed by Q1 and Q2, into its low resistance state. This allows Channel 2 LED current to flow, which rapidly turns off the SSR.

After the breaker trips, a small quiescent current of 5 mA continues to flow to maintain the SSR in the off state. R2 adjusts the trip current value, which is simply 3 V/(10 Ω + R2). The circuit breaker remains latched in the off state until the voltage across it approaches 3 V, at which point the SCR turns back off and the SSR turns on. This, in effect, resets the circuit breaker.

The circuit arrangement shown will trip at a current of 0.2 A. The voltage across the circuit breaker must be limited to less than 120 V to maintain the SSR power dissipation below the absolute maximum rating of 0.6 W. The SSRs also can be configured easily in series or in parallel for increased voltage or current ratings.

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