Practical electronic fuse circuit

Fuse, also known as fuse (fuse tube), is an essential component for household appliances, electronic instruments and industrial equipment, and plays a role in load overcurrent fuse protection. The electronic fuse circuit introduced in this article has a simple structure, and the fuse current can be adjusted arbitrarily within the range of 0.1 to 10A. It can be reused. It not only completely eliminates the trouble of repeatedly burning fuses when repairing power-related faults, but can also be widely used in charger circuits of various mobile devices. When designing and trial-producing new electronic circuits or devices, appropriate parameters can be selected to replace fuses.

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

The two lead-out terminals of the load are connected in series with a group of normally closed contacts of relay K, a 12A ordinary fuse Fu and the primary winding of the ring sampling transformer T1. The primary of T1 is connected to the current threshold range selection switch S1. R1~R4 form a resistor network, and the current threshold is divided into two gears, the low gear is 0.1~6A, and the high gear is 1~10A, and the high gear has a certain coverage. C1 and C2 are high-frequency noise elimination capacitors.

The control circuit is composed of a four-voltage comparator LM339 (only three of them are used) as the core component. 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 the in-phase input terminal of IC-3 a fixed level. The filtered DC signal is amplified by IC-3. The output from the collector of the driver tube VT is used as the trigger signal of the thyristor VS. When the input current exceeds the threshold set by RP, VS is turned on. K is attracted to open its normally closed contact, and the LED lights up, indicating that the circuit to be insured has been cut off from the power supply. If you need to re-power, just press the reset switch S2.

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

(1) Adjust RP to the middle scale (5.0) and power on the autotransformer. Slowly adjust the output of the autotransformer to 1A in the smallest step, and then turn RP until the electronic fuse is disconnected (LED lights up, K closes). Note 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 is powered on, then adjust the autotransformer so that the ammeter reads 2A (or 1.5A, etc.), turn RP until the electronic fuse is disconnected, and record the RP scale value.

(3) Repeat the above process until all the fuse current values ​​within the high and low current ranges are calibrated.

(4) Finally, list the potentiometer RP scale values ​​and corresponding current values ​​(which can be posted on the panel of this device) for future use.