Overvoltage Protection Circuit Diagram

Circuit Description:

  • Transistor VT1 functions as a series pass switch, controlling the power to the output.
  • DA2 is a comparator with an open-collector output.
  • DA1 is a precision voltage reference, set to 1.235 V.
  • DA3, an Adjustable Current Source, provides a stable bias for DA1. While a simple resistor could be used here, the current source ensures more stable operation of the voltage reference.
  • The input voltage is scaled down by the resistive divider R1 and R2.
  • The voltage at the midpoint of the divider (across R2) is fed to the comparator DA2 and compared against the 1.235 V reference.
  • Operation:
    • If the voltage across R2 is below 1.235 V, the comparator's output is in a state that keeps VT1 turned ON.
    • When the voltage across R2 exceeds 1.235 V, the comparator switches and VT1 is turned OFF, disconnecting the load.

For DA2: The maximum allowable supply voltage is 36 V.

For DA1: The maximum allowable supply voltage is 40 V.

For the transistor ( in this case, the AP2305GN-HF-3 ), the maximum voltage is -20 V.

Selection Challenge:

The transistor is the most difficult component to find. It needs to meet two key requirements simultaneously:

  1. Support a higher voltage.
  2. Have a very low threshold (turn-on) voltage.

For the AP2305GN-HF-3 ,  the gate threshold voltage  ranges from -0.5 V to -1.6 V.

For a transistor like the DMG3401LSNQ :

          The maximum allowable Drain-Source Voltage  is -30 Volts.

         The Gate Threshold Voltage  is between -0.5 V and -1.3 V.

Perhaps some hysteresis should be added to the comparator. I leave this to the readers.

Some pictures :