An AC mains high/low cut-off device will cut off or disconnect the mains supply from home electrical whenever a high voltage or low voltage situation is detected. In this way it ensures total safety to the home wiring and appliances from fire electrical due to abnormal over voltages or brown out low voltages.
The article describes 3 accurate automatic over and under voltage cut out circuits can be made at home for protecting the domestic appliances from sudden dangerous high and low voltage influxes. The first designs explains a LM324 transformer based circuit, the second circuit uses a transformerless version, that is it works without a transformer, while the third concept explains a transistor based cut off circuit, all of which can installed at home for controlling over and under voltage cut off protection.
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 detection 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 opamps 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 opamps, A1, A2, A3, A4 are obtained from a single IC 324, which is a quad opamp IC, means consists of four opamp 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 opamps are rigged as voltage comparators. The inverting inputs of all the opamps are clamped to a fixed reference value of 6 volts which is done through a resistance/zener network for ech of the opamps 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 opamps when the relevant input level crosses the reference level set over the inverting inputs of the respective opamps.
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 opamp’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 opamps.
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 opamps.
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:
Warning: The below shown circuit is not isolated from mains AC. Handle with extreme caution to avoid a fatal mishap.
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...
Using PNP Relay Driver
As shown in the given mains AC high, low voltage protector circuit, we can see two opamps from the IC LM 324 are used for the required detection.
The upper opamp has its non inverting input rigged to a preset and is terminated to the supply DC voltage, pin #2 here is provided with a reference level, so that as soon as the potential at pin #3 goes above the set threshold (by P1), the output of the opamp goes high.
Quite similarly the lower opamp is also configured for some voltage threshold detection, however here the pins are just reversed, making the opamp output go high with low voltage input detection.
Therefore, the upper opamp responds to high voltage threshold and lower opamp to low voltage threshold. For both the detections, the output of the respective opamp becomes high.
Diodes D5 and D7 make sure that their junction produces a common output from the opamp output pin outs. Thus whenever any one of the opamp output goes high, it is produced at the junction of D5, D7 cathodes.
Transistor T1’s base is connected to the above diode junction, and as long as the opamps output remain low, T1 is allowed to conduct by getting the biasing voltage through R3.
However the moment any of the opamp output goes high (which may happen during abnormal voltage conditions) the diode junction also becomes high, restricting T1 from conducting.
Relay R1 instantly switches OFF itself and the connected load. Thus the connected load remains ON as long as the opamp outputs are low, which in turn can only happen when the input mains is within the safe window level, as adjusted by P1 and P2. P1 is set for detecting high voltage levels while P2 for the lower unsafe voltage level.
Pin Details of IC LM 324
Parts List for the above mains high, low voltage protector circuit
R1, R2, R3 = 2K2,
P1and P2 = 10K preset,
C1 = 220uF/25V
All diodes are = 1N4007,
T1 = BC557,
Relay = 12 V, 400Ohms, SPDT,
opamps = 2 opamps from IC LM 324
Zeners = 4.7 volts, 400mW,
Transformer = 12V, 500mA
So far we learned an IC version of the circuit, now let's see how a mains 220V or 120V operated over voltage and under voltage protection circuit can be built using just a couple of transistors.
A very simple circuit presented when installed in the house electrical may help in restricting the issue to a great extent.
Here we will learn two designs of over and under voltage circuits, the first based on transistors and the other one using an opamp.
Over/Under Voltage Cut Off Circuit Using Transistors
You will be surprised to know that a nice little circuit for the said protections can be built using just a couple of transistors and a few other passive components.
Looking at the figure we can see a very simple arrangement where T1 and T2 are fixed as an inverter configuration, meaning T2 responds oppositely to T1. Please refer to the circuit diagram.
In simple words when T1 conducts, T2 switches OFF and vice versa. The sensing voltage which is derived from the DC supply voltage itself is fed to the base of T1 via preset P1.
The preset is used so that the tripping thresholds can be determined precisely and the circuit understands when to execute the control actions.
How to Set the Preset for Automatic Cut off
P1 is set for detecting high voltage limits. Initially when the voltage is within the safe window, T1 remains switched OFF and this allows the required biasing voltage to pass through P2 and reach T2, keeping it switched ON.
Therefore the relay is also kept activated and the connected load receives the required AC voltage.
However in case suppose, the mains voltage exceeds the safe limit, the sensing sample voltage at the base of T1 also rises above the set threshold, T1 immediately conducts and grounds the base of T2. This results in switching OFF of T2 and also the relay and the corresponding load.
The system thus restricts the dangerous voltage from reaching the load and safeguards it as expected from it.
Now suppose the mains voltage goes too low, T1 is already switched OFF and at this situation T2 also stops conducting due to the settings of P2, which is set so that T2 stops conducting when the Mains input goes below a certain unsafe level.
Thus the relay is once again tripped OFF, cutting of power to the load and prompting the required safety measures.
Though the circuit is reasonably accurate, the window threshold is too wide, meaning the circuit triggers only for voltage levels above 260V and below 200V, or above 130V and below 100 V for 120 V normal supply inputs.
Therefore, the circuit may not be very useful for folks who might be looking for absolutely accurate tripping points and controls which can be optimized as per ones personal preference.
To make this possible a couple of opamps may be required to be included instead of transistors.
Parts List for the above AC mains over voltage, under voltage protection circuit.
- R1, R2 = 1K,
- P1, P2 = 10K,
- T1, T2 = BC547B,
- C1 = 220uF/25V
- RELAY = 12V, 400 OHMS, SPDT,
- D1 = 1N4007
- TR1 = 0-12V, 500mA