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mosfet reverse battery protection

MrDEB

Well-Known Member
Presently using a diode for reverse battery protection on two different projects but want to improve my circuit design by using a P mosfet instead of the diode.
input is 4 AA batteries so 6v max
circuits have a pic mictocontroller that drive an 8 segment led matrix (have two circuits, the second drives an LCD display)
Have noticed that the LCD starts displaying little squares when battery voltage drops so adjusting the 10K pot gets to be an issue.
Selecting the right SMT P_mosfet is my challenge but am sure members have a favorite P-mos they use.
any suggestions?
 

ChrisP58

Well-Known Member
Most Helpful Member
What is the max current that your circuit draws from the battery? Need to know that to choose the mosfet.

Yes, a P mosfet is a better way of doing reverse polarity protection. The diode will always have 0.3V to 0.7V (depending of type) of loss, but the voltage loss of a properly sized mosfet can be near zero.
 

MrDEB

Well-Known Member
I recall measuring the current draw last year. Less than 2amps.
will remeasure again after I redraw my schematic
 

MrDEB

Well-Known Member
I finally got back to this project and using a digital multi meter set on MA it says .06
so 60ma is with some flashing leds.
 

MrDEB

Well-Known Member
I need to add the mosfet etc to my schematic. I have it included on another project but havn't measured the current as it has an LED matrix instead of an LCD.
 

MrDEB

Well-Known Member
here are both schematics
 

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AnalogKid

Well-Known Member
Most Helpful Member
P-channel MOSFET in series between the battery anode and the circuit power input. Source to the battery, Drain to the circuit, Gate to GND. Add a resistor from the Gate to the Source to guarantee rapid turn-off when the battery is disconnected. 10 K - ish.

ak
 

MrDEB

Well-Known Member
not sure about adding the 10k+ resistor. All the circuits I found use on resistor from Gate to Source
Pictured circuits are using an N channel mosfet
 

AnalogKid

Well-Known Member
Most Helpful Member
not sure about adding the 10k+ resistor. All the circuits I found use on resistor from Gate to Source
Not sure what "on resistor" means. It looks to me like there is a condition when the gate might be floating, not a good thing. Also, those look like p-channel devices.

With a max battery voltage of 6 V, what is the purpose of the 100 ohm gate resistor?

ak
 

crutschow

Well-Known Member
Most Helpful Member
Add a resistor from the Gate to the Source to guarantee rapid turn-off when the battery is disconnected. 10 K - ish.
Why do you need rapid turn-off at the input (source)?
 

tumbleweed

Active Member
Here's a good explanation of how this is supposed to work and the purpose of various components.


If you're impatient then around 1:50 into the video.

If you're working with a max of 6VIN you probably don't need the zener or gate resistor, but check the P-chan FET datasheet for max VGS.
 

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