Convenient and easy-to-use electronic fuse circuit

The principle circuit is shown in the attached figure. It not only has the same over-current protection function as ordinary fuses, but also has the characteristics of fast action and easy use (no need to replace the fuse) that ordinary fuses do not have.

Rl is a load current sampling resistor, which is connected in series to the mains live wire. Its resistance is estimated as R1=0.8/1, where 1 is the fuse circuit action current (the design value in the attached figure is I=4A). When the load electrical appliance uses less than 4A, the voltage drop generated on R1, after rectification by VD1, current limiting by R2 and filtering by Cl, forms a DC voltage less than 0.6V, which is lower than the triggering conduction voltage of the thyristor vs. The fuse circuit does not operate, and the load electrical appliance can work normally. When the load current I≥4A, the peak voltage Um across R1 is ≥1.1V, and the voltage on C1 reaches above 0.6V. The thyristor (vs) obtains a suitable triggering voltage and conducts, and the relay (K) is energized and attracted. After its normally open contact K-1 is closed, the AC contactor (AK) power supply is connected, and the AC power supply circuit is cut off by the large-capacity normally closed contact AK-1 of AK, realizing the overcurrent "fuse" protection function.

The current limiting resistor (R2) is used to protect the vs control electrode. When a load short circuit occurs, the 220V AC voltage is instantly added to R1. Due to the existence of R2, the average current flowing through the vs control electrode will not exceed 9.9mA. Without R2, vs will be damaged instantly. R2 also forms a circuit with C1 and R3 to prevent circuit malfunction, which can effectively eliminate the malfunction of the electronic fuse circuit caused by various instantaneous strong currents of the power grid (such as the startup current of refrigerators, color TVs, etc.). The value of Cl directly affects the sensitivity of the circuit action.

LED and R4 form an electronic fuse "blown" indication circuit. SB is a reset button. After the power supply load or line short circuit fault is eliminated, just press this button to restore normal power supply. The sampling resistor Rl can be replaced by constantan wire or a section of electric furnace wire. The value should be as accurate as possible, otherwise the operating current will change. vs uses BT169 unidirectional thyristor. T uses AC220V/12V, 3W small high-quality power transformer. K can use 12V JZX-17F and other small electromagnetic relays. AK uses a single-phase AC contactor (its contact capacity is generally 5A-10A).