High-Low Voltage Block /Over-Under Voltage Block Circuit
Operational Amplifier LM741
Components:
12V step down transformerBridge rectifier diode (DF02M)
Diodes D1, D2, D3= 1N4001
Red LED
6.2V zener diode
1000µF, 35V capacitor
12V DC relay
Transistor MPS2907A
2 Op-Amp LM741
Resistors: R1, R5 = 180ohms
R2 = 200K
R3 = 50K
R4 = 47K
R6 = 1K
Variable resistor VR1 & VR2 = 200K
Plug
Socket
The name I gave this circuit was simply base on what it does. Of course, you can call it High-Low Voltage Cut Out or Electronic Fuse (may not be too correct to describe it as the circuit does not blow off) as it protects your expensive electrical appliance or electronics from abnormal(over or under) voltage. The thought for designing this simple circuit came when I lost my CRT television to over voltage. Components used in this circuit are cheap and readily available in market. So, you can build one for to protect your expensive equipment.
The circuit makes use of two comparators, IC1 and IC2. IC1 detects high voltage (say 245V as in this circuit) while IC2 detects low voltage (say 180 volt). Diode D1 and D2 are connected to act like OR gate (they can actually be replaced with logic OR). Zener diode 6.2 volt set a reference for bolt comparators. A variable transformer is needed to set your high and low voltage limit. Plug in your circuit to the transformer and turn your transformer to produce 245 volt. Then turn the potentiometer VR1 such that its value increases and stop at the instance the LED light. In this way, high voltage limit has been set. To set the low voltage limit, turn the transformer to produce 180 volt and vary potentiometer VR2. Stop at the instance LED light.
At voltage above 245 volt, non inverting input of IC1 is at higher potential to inverting input, hence output is high. At voltage below 180 volt, non inverting input of IC2 is at higher potential. The output is also high. In both cases, transistor T (PNP) is not conducting (i.e, block). Reverse is the case in both comparators when supply voltage falls between range 180 – 245 volt.
There is a short coming in this over-under voltage Block circuit, anyway it depends on the application. If the supply quickly jump out of the range and return in few seconds, it means your appliance will respond the same way (off and on) which is not good for most appliances. In other versions of this circuit, I incorporate reset circuit in one and delay circuit in the other.
One with reset circuit allows you to turn up your appliance at will after which it has cut off due to abnormal voltage by simply press the reset switch, SW. Other with delay circuit is good for appliances with compressor e.g fridge, AC etc. it delays the time (predetermine with selected value of capacitor and resistor) it supplies voltage to the appliance after interruption resulting from abnormal voltage.
The circuit makes use of two comparators, IC1 and IC2. IC1 detects high voltage (say 245V as in this circuit) while IC2 detects low voltage (say 180 volt). Diode D1 and D2 are connected to act like OR gate (they can actually be replaced with logic OR). Zener diode 6.2 volt set a reference for bolt comparators. A variable transformer is needed to set your high and low voltage limit. Plug in your circuit to the transformer and turn your transformer to produce 245 volt. Then turn the potentiometer VR1 such that its value increases and stop at the instance the LED light. In this way, high voltage limit has been set. To set the low voltage limit, turn the transformer to produce 180 volt and vary potentiometer VR2. Stop at the instance LED light.
At voltage above 245 volt, non inverting input of IC1 is at higher potential to inverting input, hence output is high. At voltage below 180 volt, non inverting input of IC2 is at higher potential. The output is also high. In both cases, transistor T (PNP) is not conducting (i.e, block). Reverse is the case in both comparators when supply voltage falls between range 180 – 245 volt.
There is a short coming in this over-under voltage Block circuit, anyway it depends on the application. If the supply quickly jump out of the range and return in few seconds, it means your appliance will respond the same way (off and on) which is not good for most appliances. In other versions of this circuit, I incorporate reset circuit in one and delay circuit in the other.
One with reset circuit allows you to turn up your appliance at will after which it has cut off due to abnormal voltage by simply press the reset switch, SW. Other with delay circuit is good for appliances with compressor e.g fridge, AC etc. it delays the time (predetermine with selected value of capacitor and resistor) it supplies voltage to the appliance after interruption resulting from abnormal voltage.
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