I have an application where I need to switch a contactor on and off using a spare output of an off the shelf Battery Management System (BMS)

The input is the signal coming from the BMS, while the output is the signal going to the contactor


  • When the input is FLOATING, the output should be OFF
  • When the input is GROUNDED, the output should be ON

The manufacturer of the BMS provided the following circuit diagram to follow: manufacturer example


  • BMS output must not sink more than 175mA
  • BMS output must not have more than 24V across it

The main supply of this circuit is a battery which will have a maximum operating voltage of 35V. Looking through the P MOSFETS available in my country, I can only find a max Vgs of -20V. I have looked at gate voltage protection circuits and tried to adapt the following circuit to work: Adapted Circuit

The circuit I have derived is the following:

Zener diode: BZX85C22V PMOS: Vishay IRF9540PB

My Circuit

My logic is that the 10K resistor acts as the pull-up when the BMS input is floating. When the input is grounded, then the Zener starts conducting and clamps the gate to 22V. Therefore Vgs is -13V (well under the -20V maximum). Iz looks to be about 22mA for the zener to conduct, so I assume let 25mA flow. so then

R2 = (35-22)/0.025 = 520R. Choose 470R, which gives 28mA. Resistor power dissipation is P = I^2 * R = 0.36W (Choosing 0.6W resistors)

Zener dissipation = V * I = 22 * 0.025 = 0.55W (Under the maximum of 1.3W).

My question is that will this circuit work reliably? I am not experienced with circuit design, so your feedback would be very appreciated.

  • \$\begingroup\$ Why the 470Ω pullup resistor? \$\endgroup\$ – Hearth May 13 '18 at 23:57
  • \$\begingroup\$ @FelthryThe zenor needs 22mA to breakdown. I calculated that 520R is the minimum that will provide this. \$\endgroup\$ – Russell May 14 '18 at 0:03
  • \$\begingroup\$ You realize this is directly in parallel with your 10k pullup, and thus effectively your pullup resistor is just 470Ω? \$\endgroup\$ – Hearth May 14 '18 at 0:11
  • \$\begingroup\$ Typically one would use a common emitter with a collector R to limit current with R pullup to act as a Voltage divider to say 15V across source gate so no Zener needed or emitter resistor. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 May 14 '18 at 0:16
  • \$\begingroup\$ @TonyStewartolderthandirt Yes I saw that from the circuit I adapted, but I was not sure how to make it work with the input signals for my application. \$\endgroup\$ – Russell May 14 '18 at 0:18


simulate this circuit – Schematic created using CircuitLab

  • \$\begingroup\$ How does Q1 behave when a floating input is present? \$\endgroup\$ – Russell May 14 '18 at 1:40
  • \$\begingroup\$ If the wire is like an induction wire it can be sensitive to turning on with 10uA , but any CMOS driver is low impedance \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 May 14 '18 at 1:46

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