Electronic Fuse Using Op-amp 741

Here is an electronic fuse using op-amp 741 which features slow-blow properties, a variable current limit, and a ‘live’ indication of its operation. Moreover, it is simple to restore. The electronic fuse using op-amp 741 allows the gadget to draw a higher current initially but keeps a watch on it. if the gadget continues to draw more current beyond the set time limit, it disconnects the supply. The fuse, therefore, acts as a cut-out and indicates this by a neon glow. It is replaced by simply resetting the circuit once.

Introduction to Fuse | Electronic Fuse Using Op-amp 741

We all know that fuse protection of expensive and sensitive gadgets is old-fashioned. A fuse is an electrical device that is supposed to blow and protect the gadget from damage or burnout. But replacing the fuse after it has done its job (burn out) is an inconvenience. Also, certain gadgets like a refrigerator draw a much heavier current when switched on, as compared to the steady nominal current they draw later. In such a case, it is quite difficult to use an ordinary fuse for protection.

The fuse rating can be varied by a potentiometer control continuously to any value.

This electronic fuse using op-amp 741 offers several advantages over the conventional device. There is no tripping caused by a normal starting surge. ‘Reset’ is as easy as pushing a button, and fuses have to be replaced. The device is a resettable electronic circuit-breaker.

Circuit Diagram and Working of Electronic Fuse Op-Amp 741

The circuit diagrams of Electronic Fuse using op-amp 741 are shown in Figures 1 and 2. Figure 1 is the power supply circuit for the electronic fuse. X₁ is a stepdown transformer. D₁ through D₄ is a bridge rectifier. R₁ and ZD₁ form a potential divider across the unregulated voltage which is smoothed by capacitor C₁. The reference voltage across the zener diode is fed to the base of the emitter-follower stage of transistor T₁. The regulated DC output is available at the emitter of T_l.

Figure 2 shows the actual control circuit and main supply connections, along with a neon indicator. Current monitoring resistor R₂ is connected in series with the neutral line. The voltage drop across R₂ is rectified by diode D₅ and smoothed by capacitor C₂. The potentiometer VR₁ allows the monitoring voltage to be set to a suitable level, depending on the load current limit. The op-amp is connected as a simple comparator.

Resistors R₄ and R₃ form a potential divider across the DC supply and provide a small reference voltage at the IC’s inverting input. The monitoring voltage is fed directly to the non-inverting input. Initially, when the load (gadget) is not connected, the monitoring voltage is zero and IC gives a “low” output. As the load is connected, the monitoring voltage appears. If it exceeds the reference voltage, the IC output goes to “high’.

The output is passed through a starting-time delay circuit. Whenever the output is high, and also if it persists beyond the delay provided, the relay gets energized. The relay contacts remove the AC supply to the gadget by opening the R/1 contacts, and also self-latch it by closing the R/2 contacts. Thus, thereafter, the relay continues to get its driving voltage independent of load conditions.

R/1 contacts also close the neon bulb circuit and make it glow to indicate the cut-out (electronic fuse) operation. The circuit can be reset manually by switching off the AC supply through SW₁ for a moment. If one desires, a normally closed push button can be added, either in series with a relay or in -V supply path, for use as a reset switch.

If the current drawn exceeds the set limit, the circuit again repeats its performance. The delay feature is set for about 15-20 seconds, after which the circuit is finally cut out for, safety if the high current persists.

How to adjust Electronic Fuse using op-amp 741

Initially set the potentiometer VR₁ towards the top end. Connect the actual gadget or any load to draw the equivalent current. Slowly adjust VR₁, until the relay operates. Now slightly reduce the VR₁ setting so that it is just on the point of operation. Provide some margin for the load current variations and AC supply variations.

While making the above adjustments, the delay feature and self-latching arrangement will complicate and confuse the settings. Therefore, it would be better to remove C₃ and R/2 contacts, while adjusting. The potentiometer position can be calibrated for the current limit if one desires.

Applications of Electronic Fuse using op-amp 741

This electronic fuse using op-amp 741 can be of tremendous use in saving motors or compressors of refrigerators. When the applied AC voltage is too low, the motor may not be able to start. Therefore, the back emf developed is zero and the motor draws a heavy current that can burn the winding if the supply to the refrigerator is not switched off promptly. In such cases, this electronic fuse comes to the rescue, acting as a safe low-voltage cut-out too.

The novelty of this electronic fuse using op-amp 741 is that it monitors the dynamic performance of the motor, irrespective of the AC input fed. The best example is that of a jammed motor. Even if the input voltage is within limits, the motor will not move because it is jammed. Neither a high nor a ‘low’ voltage cut-out can save it from damage if the motor is not switched off in time. This electronic fuse removes the supply without fail, warns that something is wrong with the motor, and does not allow it to burn out in any case.

BEP Lab Note:- The delay feature is very useful with refrigerators and motors that draw more current at switch-on. For some sensitive equipment that does not draw more current initially, this delay feature may be useless and even harmful. Therefore, in such cases, disable this feature by simply disconnecting capacitor C₃.

Useful hints for Electronic Fuse using op-amp 741

1. If one desires, a neon blinker circuit may be added to indicate the fuse operation. A blinking neon can draw attention better than a simple glowing neon. For this, shunt neon N₁ with a capacitor C₄, and also add D₈ in the series path, as shown in Figure 2.
2. For gadgets consuming more than 200W power, R2’s wattage should be increased. It may be preferable to parallel two or more 1-ohm, 5W resistors, but one can also use one or more 0.5-ohm, 5W resistors in parallel.
3. Care should be taken to connect the Live, Neutral, and Earth. pins correctly. Check the AC output socket, before plugging in this unit, for proper connections.

PARTS LIST Of ELECTRONIC FUSE USING 741 IC