Delayed electronic fuse

The delayed electronic fuse introduced in this article is designed based on the principle of delayed fuse. It can prevent the motor, color TV, etc. from malfunctioning when a large current impact is generated at the moment of power on.

How it works

The delayed electronic fuse circuit is shown in Figure 1. It consists of two parts: a fuse that can "fuse" and an audible and visual alarm. When the load RL works normally, the voltage drop across the sampling resistor RS is very small, which is not enough to make the optocoupler Ic work, and the AC relay K does not work. Its normally closed contact K-1 is in a closed state, and the 220V AC mains power is supplied to the load RL through RS and K-1. When the load current increases for some reason, the voltage drop on RS increases, and after rectification by VD1, the input-stage light-emitting diode of the optocoupler Ic emits light, and the photosensitive thyristor of the IC output stage is turned on after receiving light. After the relay K receives the 220V AC voltage, it is attracted, and its contact state is converted. K-1 cuts off the power supply to the load RL and starts the "fuse" sound and light alarm circuit; at the same time, K-2 is closed, making the relay K self-locking. After the load overcurrent fault is eliminated, it is necessary to press the normally closed button SB to instantly disconnect the power supply circuit to release the relay K, and the load can resume power supply.

When the electronic fuse is "blown", due to the disconnection of K1, the alarm circuit obtains 220V AC voltage through RS and RL. After rectification by VD2, current limiting by R3, and filtering by C2, a DC voltage of more than 300V is obtained. Since the piezoelectric ceramic buzzer HTD has a certain inter-electrode capacitance, the DC high voltage of more than 300V is charged to HTD through R4. When the charged voltage of HTD reaches the ignition voltage of the neon lamp Ne (usually 70-100V), Ne is turned on to discharge and emit light, quickly discharging the charge on HTD. Then HTD is charged and discharged again, thus forming an oscillation, causing the neon lamp Ne to flicker and emit light, and HTD to produce a chirping sound.

When a large instantaneous current appears in the power supply circuit (such as when a color TV or inductive load is turned on), due to the charging delay of R1 and C1, the voltage of the light-emitting tube in IC cannot reach the start-up voltage immediately and does not emit light. After a short impact of large current, C1 will discharge through R2 to reach a balanced voltage, so as not to cause the electronic fuse to "fuse". Only when C1 is charged with a continuous large current can the input voltage of the optocoupler IC be increased and turned on, so that the relay K can be attracted.

Component selection and production

RS can be made by connecting multiple strands of high-power electric furnace wire in parallel. Its resistance value is determined by the current limiting requirement. When RS is about 0.2Ω, the rated "fuse" current is about 10A. Ic uses MOC3041 and other high-power optocoupler thyristors to directly drive the relay. K uses a 220V AC relay with a coil rated voltage. The contact capacity can be selected according to the load, and the relay can also drive an AC contactor with a larger contact capacity. Ne can use the neon lamp in the test pen. HTD is best to use a variety with an auxiliary sound chamber. There are no special requirements for other components.

After all components are installed and checked, they can be debugged. The debugging is mainly about C1 capacity, which is to ensure that the relay K will operate when the load is overcurrent and that the instantaneous large current can pass normally.