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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

Input/Output

  • 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

Constraints

  • 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.

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  • \$\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
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schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\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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