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Auto Reset Over/ Under Mains Voltage Cut- Out is an over/under voltage cut-out that will save your costly electrical and electronic appliances from the adverse effects of very high and very low mains voltages. This circuit able to switch and cut off the mains power from reaching the connected appliances once the input voltage crossed over or below the dangerous thresholds. However due to the over simplicity of the design, involving just a couple of transistors, the circuit has its own limitations, the major limitation being less accuracy and considerable hysteresis, resulting high threshold gap of more than 60 volts between the high and the low limits.
The present design of a high voltage and low voltage cut off circuit is not only highly accurate but also provides visual indications regarding the relevant voltages insteps. The accuracy is so high that virtually the thresholds can be separated and sensed within 30 volts range. The incorporation of op amps in the circuit equips it with the above feature and therefore the whole idea become very much reliable.
About the circuit :The op amps, A1, A2, A3, and A4 are obtained from a single IC 324, which is a quad op amp IC, means consists of four op amp blocks in one package. The IC is outstandingly reliable and easy to configure and hardly poses a problem with its functioning, in short it has robust specs and is too flexible with most of the configurations. The two op amps are rigged as voltage comparators. The inverting inputs of the op amps are clamped to a fixed reference value of 12 volts which is done through a resistance/zener network for each of the op amps discretely. The non-inverting input of A1 are connected to the power supply of the circuit through a voltage divider network formed by the presets VR2 .The presets can be adjusted as desired to flip the outputs of the respective op amps when the relevant input level crosses the reference level set over the inverting inputs of the respective op amps. This special arrangement ensures that only one relevant LED is switched ON in response to the rising or falling voltage levels from the op amps. Two Transistor are connect to the output of op-amp and the LEDs in series so that the transistor conduct with the relevant LEDs during high and low voltage levels, specified as dangerous thresholds. The conduction of the transistor instantly switches the respective relay. The poles of the two relays and the poles of the relays are connected in series before supplying the output through them to the load. The series connection of the contacts ensures that if any one of the relay conducts, cuts of the mains supply to the load or the connected appliance. Suppose at certain normal levels A1, A2 are all conducting (outputs high), at this point only the LED connected to Resistor would illuminate, because its cathode receives the required negative from the output transistor, whereas the cathodes of the lower LEDs are all high because of the high potentials from the above op amps. The LED connected to Resistor also remains shut off because op-amp output is low. The above results appropriately influence the respective opt couplers and the relays such that the relays conduct only during dangerous low or dangerous high voltage levels detected by only A1 and A2 respectively.
Dilip Bhoyar (Elektrokit)
Keshav Imperial,Shop Nop B-101,B Wing First Floor,Nagpur, , Shani Mandir, Near Shani Mandir, Sitabuldi, Nagpur-440012, Maharashtra, India, Shankar Nagar
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