Mains AC Home Protector Circuit

This simple mains 220 V, 110 V AC voltage home protector circuit will protect all the mains operated appliances of your home from high and low voltage situations and also from sudden voltage surge.

The circuit can be used with 220 V AC inputs and also with 110 V AC inputs.

The circuit will basically provide the following two types of protection to all your home appliances:

• Protection against high and low mains voltages.
• Protection against sudden voltage switch ON surge whenever AC mains restores after a brief voltage failure.

This circuit performs incredibly well and is highly suitable for home appliances.

Circuit Description

With reference to the following diagram, the working of the proposed AC mains home protector circuit can be understood with the help of the following points:

Parts List

• All Resistors are 1/4 watt CFR 5%
• R1, R2, R4, R5 = 10k
• R3 = 4.7k
• R6 = 22k
• P1, P2 = 10k presets.
• Capacitors
• C1, C2 = 1000uF/25V Electrolytic Capacitors
• Semiconductors
• Diodes D1, D2, D3, D4, D7 = 1N4007
• Diodes D5, D6, D8 = 1N4148
• Zener Diodes Z1, Z2 = 4.7V, 400mW
• Transistors T1, T2 = BC547
• IC op amps A1, A2 = 1/2 IC LM324, or LM358 (pinouts will change accordingly)
• Transformer TR1 = 0-12V, 500mA
• Relay = 12V SPDT 30 amp

The two op amp comparators form the heart of the circuit which are configured as a window comparator for sensing the over and under voltage thresholds.

Window Comparator

A window comparator circuit is usually built using two op amps. One op amp is configured to sense the upper voltage limit threshold and the other op amp is configured to sense the lower voltage limit threshold.

Whenever the upper or the lower voltage limit thresholds are exceeded, the relevant op amp output goes high.

On the other hand as long as the voltage level stays within the safe limits, both the op amp outputs remains logic low or turned OFF.

In our home protector circuit, the upper op amp A1 is configured to sense the low voltage threshold, while the lower op amp is wired to sense the high voltage threshold.

For op amp A1, its non-inverting pin is clamped to a fixed voltage reference using a zener diode, while the inverting input is configured with a preset to sense the lower voltage threshold.

As long as the transformer DC (which is proportionate to the mains AC level) at the inverting input of A1 stays above the non-inverting pin's reference voltage, the output of A1 remains at 0V.

However, in an event when the input voltage at the inverting pin goes lower than the reference voltage at the non-inverting input, causes the output of A1 to go high.

Identically, but with an opposite function, op amp A2 utilizes a zener diode to establish a constant voltage reference at its inverting input pin, while the non-inverting input is configured with a preset to detect the high voltage threshold.

The output of A2 remains at 0V as long as the transformer DC, proportional to the mains AC level, at the non-inverting input stays below the reference voltage at the inverting input pin.

However, if the voltage at the non-inverting input rises higher than the reference voltage at the inverting input, it prompts A2's output to go high.

Thus, the two op amps makes sure that as long as the input voltage level stays within the preset "window" level, the outputs of both the op amps remains at 0V or logic low.

These voltage levels inside the comparator "window" signifies the safe limits at which our home appliances can work normally without any danger.

This safe limit, or conversely the upper and the lower unsafe cut-off limits can be appropriately adjusted and set using the P1 and P2 presets.

Transistor/Relay Switching

Now, while the mains AC voltage is within the safe limits, the outputs of the op amp remain at 0V, which keeps the transistor T1 switched OFF.

While T1 remains switched OFF, T2 remains switched ON causing the relay also to be in the switched ON position.

In this position the relay contacts are at the N/O position which allows the home AC supply to the appliances to be switched ON and working.

However, in an event when the input mains AC voltage tends to go higher or lower than the set threshold, the relevant op amp's output goes high. In either situation, T1 is switched ON.

As soon as T1 is switched ON, T2 base is grounded by T1.

This causes T2 to be switched OFF. When T2 switches OFF, it switches OFF the relay as well, causing its contacts to move towards its N/C position.

With the relay contacts shifted at N/C, the voltage supply is cut off for the home appliances, which safeguards them from the hazardous voltage situation.

Switch ON Surge Protection

The above functioning takes care of the high and low voltage correction and cut off, but what about the sudden voltage surge, during power failures and restorations?

The switch ON voltage surge is handled by the little delay ON timer configuration built using D8, C2, R6.

Whenever there is an input AC mains supply failure or interruption, C2 is fully discharged via R6.

Now, when the AC mains voltage returns, T2 and the relay are inhibited from switching ON instantly.

During this period, C2 slowly charges and keeps the base voltage of T2 below 0.6V causing it to be switched OFF, so that the relay and the home appliances also remain switched OFF.

As C2 charges slowly via R5, after some delay the voltage across C2 reaches above 0.6V which is enough to switch ON T2, relay and the appliances very softly.

This slight delayed switch ON after the mains has restored safeguards the home appliances completely from a possible hazardous switch ON voltage surge.

Calculating the Tripping Points

To setup the presets correctly we first need to confirm what levels of the DC voltages correspond to the 220V 120V AC side high and the low levels.

This can be actually quickly implemented using a variac, however since a variac may not be accessible to most of the users, we can try an alternative method through some calculations and practical testing.

Initially, keep the entire control circuit detached from the bridge rectifier.

Assuming your input AC supply is normal, switch ON the 220V or 120V to the transformer from the primary side and measure the corresponding DC output across the C1 or the bridge rectifier +/- ends.

Let's say you get a corresponding DC output of 16V. So this will be your DC equivalent of a normal 220V AC input.

Let's denote the high voltage DC tripping point as HV and the low voltage DC tripping point as LV.

We know that 220V AC corresponds to 16V DC from the above description. Let's assume the high voltage input at which the circuit is supposed to trip as 280V.

Therefore, the DC equivalent for this 280V AC high voltage can be calculated using the following cross-multiplication.

220/280 = 16/HV

HV = 20V

This 20V DC becomes the high voltage cut-off equivalent for the 280V AC side high voltage input.

Now, let's assume the low AC voltage cut off point to be 190V.

Similarly, as above, we can calculate the DC low voltage equivalent using the following cross-multiplication:

220/190 = 16/LV = 13.81V

LV = 14V

This 14V becomes the DC low voltage equivalent for the 190V AC side low voltage.

Now, since we have the high voltage and low voltage DC tripping points in hand, we can now quickly setup the presets P1, P2 through the following steps.

How to Setup

Please refer to the following modified diagram while setting up the circuit. I have improved the circuit design with a few modifications as indicated below:

I have added LED1, LED2, LED3 for indicating the various operating levels of the circuit.

LED1 (red) indicates a low voltage situation. LED2 (red) indicates a high voltage situation, LED3 (green) indicates a normal voltage situation.

I have changed the position of R3 (4.7k) to the base of T1, for improved T1 conduction.

I have added D9, D10 diodes (both 1N4148) at the emitter of T2 to enable improved Delay-ON switch ON feature of the circuit.

Take a variable DC power supply with a maximum range of 24V DC.

Disconnect the transformer stage from the circuit and hook up the variable power supply to the op amp circuit.

Keep R5 end disconnected from the positive supply.

In the circuit, P1 determines the high voltage cut-off point, and the P2 preset determines the low voltage cut-off point.

Initially, keep the P1 wiper arm of the preset fully towards ground. Keep the wiper arm of the P2 preset fully towards the positive supply side.

Adjust the power supply to around 16V and switch it ON, you might notice the following things.

You will find LED1, LED2, LED3 all turned OFF.

Now, adjust the variable power supply output to 20V, and carefully adjust the P1 preset until the LED2 just illuminates. To confirm the result try reducing the 20V to 19V, you should find the LED2 shutting off, this would confirm your high voltage cut-off point is fixed.

Now, reduce the variable power supply voltage to 14V, and carefully adjust the P2 preset until LED1 just illuminates. To confirm the results, try increasing the 14V to 15V, you should find the LED1 immediately shutting off. This would confirm your low voltage cut-off is fixed.

The above procedures complete your high voltage and low voltage settings of the presets.

Turn off the variable power supply and proceed to the following step.

Setting up the Switch ON Delay Function

Now, let's see how the delay-ON surge protection can be setup.

Recall we had disconnected the R5 upper end with the positive supply, restore this connection back with the positive supply.

Keep the R4 end disconnected from the LED junction.

Now, adjust the variable power supply to 16V and switch it ON.

You will find the green LED quickly coming ON, but the relay should not switch ON immediately. After a few seconds the relay should also switch ON.

If you are able to witness the above operation, will prove that your delay-ON surge protection is working perfectly, as intended.

To further confirm, you can turn off the power supply, wait for a few seconds and turn it ON again. You should be able to notice an identical delay ON function of the relay happening.

This confirms the delay ON function of the home protector circuit, and this setup is complete.

Next, join the R4 end back with the LED1/LED2 junction, and turn ON the power supply (adjusted with 16V DC output).

You should find LED1, LED2 turned OFF and LED3 turned ON immediately, and the relay turning ON after a few seconds.

So far so good.

Now, try increasing the power supply voltage to 20V, which should instantly turn ON LED2, and turn OFF LED3 and the relay (at N/C position).

Next, start decreasing the voltage, as you go below 19V, the LED2 must instantly shut off, turning ON LED3 and the relay (at N/O position). The relay should turn ON with some delay, as per the previous setting.

As you lower the voltage, keep going until it reaches around 14V. At this point the LED1 should instantly turn ON, turning off LED3 and the relay (back to N/C point).

Repeat the process up/down until the relevant outcomes are thoroughly confirmed as explained in the above setting up procedures.

Once you are convinced, remove the variable power supply and configure the transformer DC power supply with the circuit and switch it ON.

Assuming the input AC is normal during this period, the red LEDs should remain turned OFF, and the green LED must turn ON. The relay should turn ON after some delay.

That's it, the setting up procedure of the home protector circuit is complete and is ready for the final integration with your AC mains, for implementing the intended high/low AC voltage cut-offs and the switch ON surge protections.

Home Protector Circuit with Extended Delay ON Surge Protection

In case the AC mains power outages are frequent and inconsistent in your area, you may want to have the delay ON surge protection time to be longer.

As suggested and confirmed by Mr. Ali who is a dedicated reader of this blog, the following home protector design can be used for implementing a much longer delay ON time surge protection.

Video Test Results

The following video shows the basic testing procedure of the above explained home protector circuit. The video was contributed by Mr. Ali.

A heartfelt thanks to Mr. Ali for his hard work. His testing and video contribution to homemade-circuits.com are much appreciated.

Conclusion

The above explained mains AC home protector circuit can be built and installed in homes for getting a full fledged protection from fluctuating high and low voltage situations and from sudden voltage in rush during power outages. If you have any further questions and doubts regarding the above concept, feel absolutely free to contact me through the below given comment box.