Here is an electronic fuse using op-amp 741 which features slow-blow properties, a variable current limit, and ‘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.
We all know that fuse protection of expensive and sensitive gadget is old fashioned. Fuse is an electrical device which 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 gadget like a refrigerator draw much heavier current when switched on, as compared to the steady nominal current they draw later. In such case, it is quite difficult to use 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 a number of advantage over the conventional device. There is no tripping caused by normal starting surge. ‘Reset’ is as easy as pushing a button, and on fuses have to be replaced. In fact, 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 figure 1 and 2. Figure 1 is the power supply circuit for electronic fuse. X1 is a stepdown transformer. D1 through D4 is a bridge rectifier. R1 and ZD1 from 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 emitter-follower stage of transistor T1. The regulated DC output is available at the emitter of Tl.
Figure 2 shows the actual control circuit and mains 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 “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 energísed. 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 pushbutton can be added, either in series with 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 still 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 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 heavy current that can burn the winding if 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.
In fact, the novelty of this electronic fuse using op-amp 741 is that it actually 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 do not draw more current initially, this delay feature may be useless and even be harmful. Therefore, in such cases, disable this feature by simply disconnecting capacitor C3.
Useful hints for Electronic Fuse using op-amp 741
- 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 series path, as shown in Figure 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.
- 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
|Resistors (all ¼-watt, ± 5% Carbon)|
|R1 = 500 Ω, ½ W
R2 = 1 Ω, 5W
R3, R8 = 10 kΩ
R4 = 100 Ω
R5 = 22 kΩ
R6, R7 = 1 kΩ
R9 = 1 MΩ
VR1 = 10 kΩ, Wire-wound potentiometer
|C1 = 500 µF, 25V
C2, C3 = 500 µF, 12V
C4 = 0.33 µF, 400V (optional)
|IC1 = LM741C (8-PIN Metal Pack)
T1 = SK100
T2 = BC177
T3 = BC108C
D1 – D7 = 1N4001
D8 = BY127 (optional)
ZD1 = 12V Zener Diode, FZ12A
|X1 = 230V to 15V step-down transformer, 250mA Sec.
N1 = Neon bulb
RL1 = 9V, 20mA relay with two changeover contacts with 3A or higher current rating.