The article describes how an accurate transformerless over and under voltage cut out circuit can be made at home for protecting the domestic appliances from sudden dangerous high and low voltage influxes.
The AC mains high and low voltage cut off circuit explained in this article is very easy to build and yet very reliable and accurate. The circuit utilizes a single IC LM 324 for the necessary detections and instantly switches the relevant relays so that the connected loads are isolated from the dangerous inputs.
The circuit also provides visual indications of the respective voltage levels during any instant.
The following circuit utilizes a transformer for powering the circuit
Parts List for the proposed high, low mains voltage protector circuit.
R1, R2, R3, R4, R5, R6, R7, R8 = 4K7,
P1, P2, P3, P4 = 10 K Presets
C1 = 1000 uF/ 25 V,
OP1, OP2 = MCT 2E,opto coupler
Z1, Z2, Z3, Z4 = 6 Volts, 400 mW,
D1, D2, D3, D4 = 1N4007,
D5, D6 = 1N4148,
T1, T2 = BC547B,
LED = RED, GREEN as preferred,
Transformer = 0 – 12 V, 500 mA
Relay = SPDT, 12 Volt, 400 Ohm
In one of my previous posts, we saw a very simple yet effective design of a mains over voltage and low voltage cut off circuit, which is 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 5 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.
Let’s understand the circuit in details:
How the opamps Operate as Comparators
The op amps, A1, A2, A3, 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 four op amps are rigged as voltage comparators. The inverting inputs of all the op amps are clamped to a fixed reference value of 6 volts which is done through a resistance/zener network for ech of the op amps discretely.
The non-inverting input of A1 to A4 are connected to the power supply of the circuit through a voltage divider network formed by the presets P1, P2, P3 and P4 respectively.
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.
The outputs of A1 to A4 are integrated to LED indicators in a rather special way. Here instead of following th conventional method of connecting the LED cathodes to the ground, it is connected to the output of the preceding io amp’s output.
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.
How the optocouplers function
Two opt couplers are introduces in series with the uppermost and the bottom LEDs so that the optos also conduct with the relevant LEDs during high and low voltage levels, specified as dangerous thresholds.
The conduction of the opto couplers instantly switches the internal transistor which in turn toggles 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.
Why the opamps Comparators Arranged in Series
At normal levels opamp A1, A2 or even A3 may be conducting because all these are arranged in an incremental order and go on switching in sequence in response to gradually rising voltages and vice versa.
Suppose at certain normal levels A1, A2 and A3 are all conducting (outputs high), and A4 not conducting, at this point only the LED connected to R7 would illuminate, because its cathode receives the required negative from the output of A4, whereas the cathodes of the lower LEDs are all high because of the high potentials from the above op amps.
The LED connected to R8 also remains shut off because A4’s 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 A4 respectively.
Using Triac instead of Relays for the cut off
After some analysis, I realized that the above high, low mains voltage cut off protector circuit could be simplified into a much easier version using a single triac. Please refer to the diagram given below; it's self-explanatory and very simple to understand.
However if you have problems understanding it, shoot me a comment.
Modifying the Design into a Transformerless Version
The transformerless mains high low voltage cut off circuit version of the above explained design can be visualized in the following diagram:
If a single relay is intended to be used instead of a triac, the design could be modified as shown in the following figure:
Please use a 22uF/25V capacitor across the transistor base and ground, just to make sure the relay does not stutter during the changeover periods...