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Solid-State Circuit Breaker
A circuit breaker with no moving parts protects direct-current (DC) loads. The current at which the circuit breaker opens (the trip current) is adjustable and so is the time delay before the breaker trips. The forward voltage drop rises to 1.2 V as the current rises to the trip point. The breaker is reset by turning the source power off and then back on again.
The breaker uses solid-state electronic elements as can be seen in the figure. It can be placed on either the positive or negative side of the load, but must be connected so that current flows through it in the direction shown.
The circuit inside the electronic breaker is divided into two parts: the current-carrying circuit and the trip circuit. At low voltage across the breaker, Q1 conducts little current. Consequently, the current through constant-current diode D1 saturates Q2 and thereby turns on Q3 and Q4 to saturation. The total voltage drop across the breaker increases with increasing load current through the parallel resistance of R5 and R7. When the total drop across the breaker circuit reaches about 1.2 V, Q1 turns on to saturation, thus diverting the current through D1 (about 0.47 mA), so that Q2, Q3, and Q4 turn off. The breaker is thus open. The breaker then conducts only 0.47 mA plus (Vsupply-VBE)/(R2 + R3), which can be only a few milliamperes.
Zener diode D2 protects the breaker electronics by limiting the forward voltage to 56 V and the reverse voltage to 0.7 V. Coarse adjustment of the trip level is obtained by changing R5 and R7; the values shown are for about 1.6 A. Fine adjustment is accomplished by changing R3. Thermistor provides temperature compensation, and capacitor C1 produces a delay before the breaker trips. The capacitance of C1 is chosen for the desired trip delay.
The two-terminal configuration of the solid-state circuit breaker makes it resemble a fuse. Therefore it is adaptable to existing DC power supplies that contain fuses. Since nearly the full supply voltage is applied to the breaker terminals in the "open" condition, Q1 continues to conduct, preventing Q2, Q3, and Q4, from accidentally turning on again. Therefore, the power supply must be turned off or the load disconnected to reset the circuit breaker.
Useful link: 1N5290 datasheet (pdf)
