Simple DC Over Voltage Protector Circuit

A DC Overvoltage Protector circuit is designed to safeguard electronic devices or circuits from damage caused by excessive voltage levels in a DC power supply.

Such circuits are crucial in various applications where stable and controlled voltage levels are essential.

Circuit Working:

Parts List:

In conjunction with the 12V supply lines a silicon controlled rectifier SCR is positioned and connected to a normally closed 12V relay.

The voltage applied is monitored through the SCRs gate circuit.

As long as the applied voltage remains below a specific threshold SCR1 remains inactive and the contacts of relay remain closed, allowing current to flow to the load.

When the source voltage exceeds 12V, sufficient current is directed to SCR1s gate prompting it to conduct and activate the relay thereby interrupting the current flow to the load.

This mechanism serves as a safeguard against overvoltage scenarios shielding the load from potential harm.

The trigger point of SCR1 is determined by adjusting R1.

Upon activation of SCR1 triggering the relay, the contacts of relay open, preventing further current flow to the load.

Formulas:

The following formulas and calculations are necessary for creating the DC over voltage protection circuit design:

Calculation of Voltage Divider:

You should use a voltage divider formula to set the reference voltage for activating the SCR using the preset.

Assuming that you are constructing a voltage divider with a preset resistor, Vp and a fixed resistor Rf:

V_ref​ = V_in​ × R_p​ / R_p + ​​R_f​

where,

• The reference voltage that you wish to use to activate the SCR is Vref.
• The input voltage is represented by Vin 12V in the circuit diagram).
• The preset resistance is represented by Rp.
• The fixed resistor, Rf is wired in series with the preset.

Compute SCR Gate Resistor(Preset):

You must select a resistor Rg between the SCRs gate and setting in order to guarantee that the gate receives enough current to activate.

The SCR datasheet specifically will determine the gate current Ig, which is normally between 10 and 50 mA.

I_g ​= ​V_gate​−V_ref​​ / R_g

where,

• The SCRs gate voltage Vgate, is typically between one and two volts, for precise figures, consult the datasheet.
• The reference voltage established by the voltage divider is denoted by Vref.
• The gate resistor is represented by Rg.

To obtain Rg, rearrange the equations as follows:

R_g​ = V_gate​−V_ref​​ / I_g​

Coil Current Calculation for Relays:

For the relay coil to activate, there must be enough current.

Make that the SCR has the capacity to supply the relay coils necessary current.

The current flowing through the relay coil of Irelay is:

I_relay​ = V_supply​​ / R_coil​

where,

• The voltage supply to the relay is denoted by Vsupply 12V.
• The relay coils resistance is denoted by the letter Rcoil.

Dissipation of Power in the SCR:

Make that the SCR has enough power dissipation capacity.

An estimate of the power dissipation PSCR in the SCR is as follows:

P_SCR ​= V_T​ × I_load​

where,

• The voltage drop across the conducting SCR is VT for a typical value, see the SCR datasheet; it is usually between 1-2V.
• Iload is the current that flows through the relay and load.

Using these formulas you can create a accurate design, be sure to verify using real component values and datasheets.

Please feel free to change the settings according to your own needs and component ratings.

How to Build:

Building a circuit based on the described circuit need to keep in mind that electronic projects require precision and knowledge of components.

Circuit Connections:

• Connect the positive terminal of the 12V power supply to one side of the load.
• Connect the other side of the load to one of the terminals on the normally closed NC contacts of the relay.
• Connect the common terminal of the relay to the positive terminal of the 12V power supply.
• Connect the SCR1 anode to the point where the load and the relay contacts are connected.
• Connect the SCR1 cathode to the negative terminal of the 12V power supply.
• Connect a preset R1 between the SCR1 gate and the point where the load and relay contacts are connected.
• Connect the SCR1 gate to the point where the load and relay contacts are connected.

Notes:

• Adjust the preset R1 to set the trigger point according to your desired overvoltage protection level.
• Make sure the components used can handle the voltage and current requirements of your specific application.
• Double check the polarity of the components and connections to avoid damaging the circuit.
• Consider using a suitable enclosure to protect the circuit and ensure safety.
• If you are unsure or inexperienced with electronics consult with someone knowledgeable or seek professional assistance.

Conclusion:

In conclusion, a DC Overvoltage Protector circuit is a crucial component in electronic systems providing a robust defense mechanism against potential damage caused by excessive DC voltage levels.

The careful selection of component values ensures a tailored response to specific voltage thresholds, making the DC overvoltage protector an essential safeguard in applications where maintaining stable and controlled voltage levels is paramount.

References

Overvoltage Protector for High-Voltage Loads