Practical electronic fuse (fuse) circuit

Fuses, also called fuses (fuses), are necessary components for home appliances, electronic instruments, and industrial equipment, and serve as protection against overload and overcurrent fuse. The electronic fuse circuit structure introduced in this article is simple, and the fusing current can be adjusted arbitrarily within the range of 0.1 ~ 10A. It can be used repeatedly. It not only completely relieves the trouble of repeated burning insurance when repairing power-related faults, but also can be widely used in charger circuits of various mobile devices. When designing and trial-producing new electronic circuits or devices, you can choose appropriate parameters instead fuse.

The electronic fuse circuit is shown in the figure, which is essentially a circuit opener with adjustable breaking current, high sensitivity and fast response, that is, a circuit breaker. Its leading end is connected to the fuse holder of the circuit under test through an alligator clip To replace the original fuse.

The two terminals connected to the load are connected in series with a set of normally closed contacts of the relay K, a 12A ordinary fuse Fu and the primary winding of the ring sampling transformer T1. T1 primary connects the current threshold range selection switch S1. R1 ~ R4 to form a resistance network, the current threshold is divided into two blocks, the low block is 0.1 ~ 6A, the high block is 1 ~ 10A, the high block is covered. C1 and C2 are high-frequency noise canceling capacitors.

The control circuit consists of a four-voltage comparator LM339 (only three of which are used) as the core components. IC-1 amplifies the AC sampling signal, rectifies it by VD1 and feeds it to IC-2. Potentiometer RP is used to adjust the threshold current. VD3 is a clamping diode to make IC-3 non-inverting input terminal a fixed level. The filtered DC signal is amplified by IC-3. The output from the VT collector of the drive tube is used for the thyristor VS trigger signal. When the input current exceeds the threshold set by RP, VS is turned on. K pulls in to make the normally closed contact open, and the LED lights up, indicating that the required safety circuit has been cut off. If you need to re-power, just press the reset switch S2.

RP is a current setting potentiometer. It uses a 0.5w, 25kΩ multi-turn precision potentiometer with a scale knob, so that calibration and use are very convenient. The calibration device is also very simple: connect the output end of an autotransformer with a heating wire of about 1200w. After the input end of the transformer is connected in series with a 10-20A AC ammeter and this electronic fuse circuit, connect to 220V AC mains. The high and low current ranges are calibrated separately. Take low-end calibration as an example. The calibration method is as follows:

(1) Adjust RP to the middle scale (5.0), and energize the autotransformer. Slowly adjust the output of the autotransformer to 1A with the smallest step, then turn RP until the electronic fuse is disconnected (LED is on, K pulls in) . Write down the precise scale of RP at this time (such as 6.2, etc.), the current value corresponding to this scale is 1A.

(2) Adjust RP to the middle scale, press the reset switch S1 after the autotransformer returns to zero, and then adjust the autotransformer to make the ammeter read 2A (or 1.5A, etc.), turn RP until the electronic fuse is disconnected Write down the RP scale value.

(3) Repeat the above process according to this method. Until each insurance current value in the high and low gear current ranges is calibrated.

(4) Finally, the potentiometer RP scale value and the corresponding current value list (which can be pasted on the panel of this device) are ready for use. (Organization of China Educational Equipment Purchasing Network)

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