mosfet reverse battery protection

MrDEB

Well-Known Member

• Dec 17, 2021

• #1

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

• Dec 17, 2021

• #2

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

• Dec 17, 2021

• #3

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

 

P

Pommie

Well-Known Member

Most Helpful Member

• Dec 17, 2021

• #4

MrDEB said:

Less than 2amps.

Click to expand...

That's a lot of current and will flatten AA batteries in a little over an hour.

Mike.

 

MrDEB

Well-Known Member

• Dec 20, 2021

• #5

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

• Dec 20, 2021

• #6

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

• Dec 20, 2021

• #7

here are both schematics

 

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AnalogKid

Well-Known Member

Most Helpful Member

• Dec 20, 2021

• #8

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

• Jan 1, 2022

• #9

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

• Jan 1, 2022

• #10

MrDEB said:

not sure about adding the 10k+ resistor. All the circuits I found use on resistor from Gate to Source

Click to expand...

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

• Jan 1, 2022

• #11

AnalogKid said:

Add a resistor from the Gate to the Source to guarantee rapid turn-off when the battery is disconnected. 10 K - ish.

Click to expand...

Why do you need rapid turn-off at the input (source)?

 

T

tumbleweed

Well-Known Member

Most Helpful Member

• Jan 1, 2022

• #12

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

Reverse Voltage Protection With A P-FET

[Afroman’s] latest video shows you how to add reverse voltage protection with minimal power loss. At some point, one of your electronic concoctions will turn out to be very useful. You want t…

hackaday.com

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.