In previous projects, a capacitor was used to smooth the signal converted from AC to DC. The purpose of using the filtering capacitor was to protect the charging circuit from any fluctuations and ripples in the input voltage. Fluctuations or ripples are quite common when circuits are turned ON and OFF suddenly. In this project, a more robust circuit is designed to handle input fluctuations. The circuit is built using OPAM and SCR. It is used to operate a relay that controls power supply to the load. A rectified supply of 12V is assumed at the input of the circuit and the same voltage without ripples will be obtained at the output.
Figure 1: Voltage Guard prototype based on OPAMP and SCR
The circuit disconnects the load from the supply when receiving excessive fluctuation. Supply can be resumed by pressing a push-to-on button on the circuit. It is assumed that the power supply is already converted to 12 Vdc, passing the main supply voltage to a 12-0-12 V transformer and a full bridge rectifier.
Required components –
Fig. 2: List of components required for OPAMP and SCR based Voltage Guard
Circuit Connections –
The circuit is built around the 741 OPAM IC. The 12V DC input is applied to the OPAM non-inverting input via an RC network. The same voltage is applied to the inverting input of the OPAM through a resistive voltage divider (the voltage divider is constructed using resistors R2 and R3 in the schematics). A negative voltage at pin 4 of the IC is defined as ground and a positive voltage at pin 7 is defined as 12V input. The output is taken from pin 6 of the IC and passed to the gate pin of the SCR via a push-to-on switch. A red LED with a pull-up resistor in series is connected to the OPAM output to obtain a visual cue of power continuity. The cathode of SCR is connected to ground while its anode is connected to the anode of diode D3. The coil terminals of a 24V relay are connected across diode D3 and 12V input is provided at the COM point of the relay. The load is connected between the NO point of the relay and the common ground.
How the circuit works –
The OPAM is connected as a voltage comparator in the circuit. The potential divider circuit formed by resistors R2 and R3 connects to the OPAM inverting input. The values of R2 and R3 are the same, so they provide half the input voltage to the inverting pin. So inverting the OPAM input gets 6V at the pin because the input voltage is 12V.
When 12 VDC input voltage is supplied to the circuit, initially the output of OPAM pin 6 is LOW as capacitor C1 at the non-inverting input of OPAM takes some time to charge. When C1 is fully charged, the voltage at the non-inverting input of OPAM, i.e. pin 3 of the IC, becomes 12V, which is greater than the voltage at pin 2 of the inverting input, which is 6V. As the OPAM acts as a voltage comparator, it provides a positive voltage, i.e. 12V at the output corresponding to the positive voltage at pin 7 (12V) of the OPAM. Capacitor C1 is used to maintain 12V on pin 3 (Non-Inverting Input) of the OPAM. Thus, at the output 12V is supplied by OPAM. These 12V are then supplied to the LED (D1) which lights up upon reception of the HIGH output. The red LED is used to indicate that the power is on.
When the push-to-on button is pressed, the SCR port begins to receive a positive voltage that activates the SCR. The SCR requires little voltage at the gate terminal to be activated. It acts as a short circuit and current starts flowing from anode to cathode through resistance R6 and the green LED lights up. Therefore, at point 'a' (shown in the schematic), the voltage drop is zero as all the current passes to ground through the SCR. So point 'b' is at 12V and the relay gets the required 12V at one end of its coil energizing it. The normally open (NO) point of the relay is connected to the common pin (C) and power is supplied to the load. The green LED on the circuit indicates that the load is being powered.
When there is a power failure and power resumes later, there is a chance of high spikes in the input supply voltage. These spikes can damage the charging circuit. In the circuit, the SCR anode does not receive any voltage, therefore it remains off. Therefore, the relay also does not receive supply voltage and remains de-energized. Therefore the load remains disconnected from the power supply. Until the push-to-on button is pressed, the SCR gate pin is not driven and the relay remains de-energized, keeping the load off the power supply. Diode D3 connected in parallel with the relay is used to prevent circuit countercurrent by the relay inductor when power fails. Thus, the inductor is discharged through this diode in case of power failure.
When power resumes, the push-to-on switch must be pressed to activate the SCR again, which in turn operates the relay and resumes power to the load. The circuit helps reduce the risk of a voltage spike at the load input whenever power is resumed after a fault and thus prevents damage to the load.
Tests and precautions –
The following precautions should be taken while assembling this circuit –
The current and voltage rating of the relay must be greater than or equal to the current that will be driven at the output. Otherwise, it will not be able to provide the required current and voltage at the output.
The SCR must be chosen according to the required current and voltage rating.
Diode D3 should be connected to the relay input in parallel to reduce the risk of counter current of the relay coils during power failure.
A voltage divider circuit must be used that maintains lower voltage at the inverting pin of the OPAM compared to the noninert pin.
A capacitor C1 must be used to maintain a constant voltage in the OPAM non-inversion.
Once the circuit has been assembled, it must be tested with a 12V DC source. The input power must be cycled on and off and the status of the red and green LEDs must be monitored to verify circuit operation.
This circuit can be used as a voltage protector to protect against fluctuations in case of a power failure.
Circuit diagrams
| Circuit Diagram-OPAMP-SCR-Based-Voltage-Guard |  |
