2

I've been looking at the reverse polarity protection with a P MOSFET as part of a new PCB I am designing. My system is 12v. Most of the videos and diagrams I have seen, namely Afrotechmods YouTube & Components 101 use a P MOSFET alongside a zener diode and resistor. I believe the Zener diode is used to protect the P MOSFET from over voltage incase of reverse polarity.

I have found two sources which say I could connect a single MOSFET without a Zener diode/ resistor provided the maximum gate-source voltage exceeds the system voltage - Stack - 'throw away the zener' & Reverse Voltage Defender.

I was planning of using one of these Mouser IRF9540SPB which has a gate source voltage of - 20 V, + 20 V.

If I use a IRF9540SPB P MOSFET in a 12v circuit do I also need to connect a resistor and Zener diode or are these superfluous?

RussellH
  • 15,016
  • 2
  • 10
  • 38
tenniscelebrationparty
  • 23
  • 3

1 Answers1

2

If I use a IRF9540SPB P MOSFET in a 12v circuit do I also need to connect a resistor and Zener diode or are these superfluous?

Adding the resistor and Zener is about protecting the MOSFET from voltage surges (should they occur). A surge of over 20 volts would cause the absolute maximum gate-source voltage to be exceeded and might damage your MOSFET.

So, how do you know if your supply gets the occasional surge? Answer - it will so, add the resistor and Zener. After all they don't take up much space.

If you know that the 12 volt supply can never create a surge greater than 20 volts then don't bother but, is it really worth it?

I believe the zenner diode is used to protect the P MOSFET from over voltage incase of reverse polarity

No, it protects against over-voltage of either polarity. In the conventionally connected scenario, it limits the gate-source to around 12 to 15 volts (Zener diode dependent). In the reverse polarity situation it acts like a diode and limits gate-source to about 0.7 volts.

Andy aka
  • 456,226
  • 28
  • 367
  • 807
  • Great thank you, that clears things up. The Zenner diode restricts the voltage, would a 1N4744A be suitable, it should restrict the voltage to <15v?

    What specs am I looking for in the other resistor? The component 101 website says to choose 100R-330R if there might be a sudden reverse voltage in the circuit. If that isn't the case anything from the 1k-50k. I am connecting my circuit to a car battery and this is where the reverse polarity can occur, it won't be an immediate so I assume a 1k or 50k resistor should be appropriate?

    Thanks again for the help, Sam

     – tenniscelebrationparty Sep 24 '23 at 20:28
  • 1
    A 10 kohm resistor and a 250 mW to 500 mW Zener will be fine @tenniscelebrationparty --> if we are done here, please take note of this: What should I do when someone answers my question. If you are still confused about something then leave a comment to request further clarification. – Andy aka Sep 24 '23 at 20:33
  • Thanks Andy, that’s really helpful. What drives the decision on the wattage on the Zener diode? – tenniscelebrationparty Sep 25 '23 at 05:52
  • It only needs to be a low power Zener so, basically whatever you can get your hands on. – Andy aka Sep 25 '23 at 07:41
  • 1
    For instance, if the maximum excursion above 12 volts was (say) 18 volts for a longish period and, the Zener voltage was 12 volts and, the series resistor was (say) 2 kohm, there would be 6 volts dropped across the resistor resulting in 3 mA flow into the Zener which, in turn, results in a Zener power dissipation of 12 x 0.03 = 360 mW. The resistor would dissipate 180 mW. – Andy aka Sep 25 '23 at 08:32