220V Over Under Voltage Cut Off Circuit Using Transistors

You might be surprised to learn that an effective circuit for the mentioned protections can be constructed using just a pair of transistors and a few other passive components.

What is a 220V Over Under Voltage Cut Off Circuit:

A 220V over under voltage cutoff circuit is a protective electronic circuit designed to monitor the voltage level of a 220V power supply and take appropriate actions to disconnect the load in case the voltage goes beyond specified limits, either above overvoltage or below undervoltage the safe operating range.

This type of circuit is commonly used to protect electrical appliances and devices from potential damage due to fluctuations in the mains voltage.

Circuit Working:

Parts List:

Examining the diagram reveals a basic setup where T1 and T2 are configured as an inverter meaning T2 responds inversely to T1.

In simpler terms when T1 is conducting T2 switches OFF, and vice versa.

The sensing voltage derived from the DC supply voltage itself, is directed to the base of T1 through the preset P1.

The preset is employed to precisely set the tripping thresholds allowing the circuit to discern when to initiate control actions.

How to adjust the preset for automatic cut off P1 is adjusted to detect high voltage limits.

Initially, when the voltage is within the safe range T1 remains OFF allowing the required biasing voltage to pass through P2 and reach T2, keeping it ON.

Consequently, the relay stays activated and the connected load receives the necessary AC voltage.

However, if the mains voltage surpasses the safe limit, the sensing sample voltage at the base of T1 also exceeds the set threshold.

T1 immediately conducts grounding the base of T2.

This leads to the deactivation of T2 the relay, and the connected load.

This system prevents the dangerous voltage from reaching the load safeguarding it as intended.

Now, if the mains voltage drops too low T1 is already OFF.

In this situation, T2 also stops conducting due to the settings of P2 which is configured to make T2 cease conducting when the mains input falls below a certain unsafe level.

Consequently, the relay is tripped OFF again cutting off power to the load and activating the necessary safety measures.

While the circuit is reasonably accurate the threshold window is relatively broad.

This implies that the circuit triggers only for voltage levels above 260V and below 200V or above 130V and below 100V for 120V normal supply inputs.

Therefore, the circuit may not be ideal for individuals seeking highly precise tripping points and controls that can be customized to personal preferences.

To achieve this incorporating a couple of op amps may be necessary instead of transistors.

Formulas:

The output voltage Vout from the voltage divider may be found using the following formula:

1. Voltage Divider formula for T1 transistor:

V_out = (P1 / (R1 + P1)) * V_in

where:

• V_out is the output voltage
• R1 is the resistor values in the divider.
• P1 is the preset value in the divider
• V_in is the input voltage (mains voltage in this case).

2. Voltage Divider formula for T2 transistor:

V_out = (P2 / (R2 + P2)) * V_in

where,

• V_out is the output voltage
• R2 is the resistor values in the divider.
• P2 is the preset value in the divider
• V_in is the input voltage.

How to Build:

To build the circuit described for voltage protection, follow these construction process:

Gather Components:

• Collect all the required components mentioned above.

Identify Transistor Pinouts:

• Identify the pinouts of the NPN transistors T1 and T2.
• Typically, transistors have three legs: emitter, base and collector.

Connect Transistors T1 and T2:

• Connect the transistors in an inverter configuration, meaning the collector of T1 is connected to the emitter of T2 and vice versa.
• Use the circuit diagram as a reference.

Connect Presets P1 and P2:

• Connect P1 to the base of T1 and adjust it for detecting high voltage limits.
• Connect P2 to allow the required biasing voltage to reach T2 keeping it ON when the voltage is within the safe range.

Connect Relay:

• Connect the relay to the collector of T2.
• This ensures that the relay is activated when T2 conducts.

Set Preset Thresholds:

• Adjust P1 and P2 to set precise tripping thresholds for high and low voltage limits.

Add Safety Measures:

• Integrate diodes, capacitors, and resistors as needed to ensure stable operation and to protect the components.

Power Supply:

• Connect the circuit to a DC power supply.

Testing:

• Test the circuit with varying input voltages to ensure that it responds correctly by activating or deactivating the relay.

Adjustments:

• Fine tune the circuit if needed by adjusting preset resistors for optimal performance.

Important Notes:

• Carefully follow the circuit diagram provided to ensure correct connections.
• Use appropriate values for resistors, capacitors, and diodes based on the circuit requirements.
• Exercise caution while working with electrical components and follow safety guidelines.
• If you are not familiar with electronic circuit building it might be helpful to consult with someone experienced or seek professional advice to ensure safety and accuracy in constructing the circuit.

Conclusion:

Such circuits are commonly employed in areas where voltage fluctuations are prevalent helping to safeguard electronic devices and appliances from potential damage.

They find applications in various sectors including residential, industrial, and commercial environments.

References:

Overvoltage