High-Speed Electronic Fuse for Solid-state Circuits

May 112025

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High-Speed Electronic Fuse

Why a High-Speed Electronic Fuse Is Necessary?

The use of fuses is important for preventing damage to electronic circuits. However, in some cases, such as with solid-state circuits, ordinary fuses operate very slowly. Power transistors are sensitive to overheating when large currents pass through them.

The high-speed electronic fuse operates in approximately one-hundredth of a microsecond (μs), which is much faster than an ordinary fuse. It is also fast enough to protect a power transistor. This circuit can handle currents of up to 60 amperes.

How the High-Speed Electronic Fuse Works

When the current draw of the protected circuit increases, it triggers the SCR1 thyristor, which causes the base of the Q2 transistor to reach 0V. The Q2 and Q1 transistors, which are connected in a Darlington configuration, then go into the cutoff state. Without Q1 conducting, the load receives no current, and the protected circuit is safe.

When the SCR1 thyristor fires, the I1 lamp lights and the filament resistance increases to approximately 100 ohms, which decreases the current flowing through the thyristor. The lamp serves as an indicator, showing when the electronic fuse is protecting the circuit.

In its normal operating state, thyristor SCR1 does not conduct, and the base current of transistor Q2 flows through lamp I1. This current is sufficient to conduct transistors Q2 and Q1, allowing current to pass normally to the load.

The potentiometer R3 sets the maximum current allowed by the fuse. When current flows through R2 or R1 depending on the option selected by switch S1 and the set current limit is exceeded, transistor Q3 conducts. This creates a positive voltage across resistor R5, which activates the thyristor. Resistor R6 then limits the current at the thyristor’s gate to a safe value.

Diode D1 enables the electronic fuse to function with an inductive load, preventing damage to transistors Q1 and Q2.

Since the circuit can withstand up to 60 amperes, resistors R1 and R2 must each dissipate 45 watts. Therefore, they must be provided with a large heat sink. The same must be done with the transistors and thyristor.

Potentiometer R3 is calibrated using several resistive loads that allow the passage of currents of specific values. Resistor R3 is adjusted so that the lamp lights up when a specific current is reached. A plate-type dial is placed behind the knob that controls potentiometer R3 to help identify the calibration points.

Once activated, the thyristor will continue to conduct indefinitely. To deactivate it, turn off the circuit.

The position of switch S1 depends on the amount of current you want to allow through the electronic fuse.