# Reverse Polarity protection for 18650 battery charger (tp4056)

Thread Starter

#### cyberdyne

Joined Jan 29, 2019
3
I need advise on best value components for this Reverse Polarity protection schematic (pictured).

source.

The original author just used what he had laying around,
>2N2907A
>IRL2203N
as i will be buying everything new, that gives me a lot of options.
Requirements:
• 18650's charge up to 4.2 volts
• Max continuous 1000mA (1A)
i need to make 20 of these for my charging board,
so i dont want to waste money over engineering it.
Cheapest reliable parts will be fine.

im new to electronics & totally confused on how to select the right transistors?
Also do i need to be worried about the volt drop over the mosfet?

#### crutschow

Joined Mar 14, 2008
30,803
The BJT can be just about any PNP such as the common 2N3906.
The N-MOSFET can be any logic level type (with a Vgs threshold of <2V) that has an ON resistance of ≤10mΩ.

Yes, you need to be concerned about the voltage drop across the MOSFET.
10mΩ On resistance will give a maximum voltage drop of 10mV @ 1A.

Thread Starter

#### cyberdyne

Joined Jan 29, 2019
3
The BJT can be just about any PNP such as the common 2N3906.
The N-MOSFET can be any logic level type (with a Vgs threshold of <2V) that has an ON resistance of ≤10mΩ.

Yes, you need to be concerned about the voltage drop across the MOSFET.
10mΩ On resistance will give a maximum voltage drop of 10mV @ 1A.
Hey Thanks crutschow.
I just placed an order for the 50x 2N3906 fro $1. After searching on mouser.com to find a MOSFET options that met the specs you listed, i checked eBay for price. I cant seem to find anything cheaper than what the original guy used "IRL2203N" @$4.28 for 10pcs.
Good price?

Do you have a formula or rule i can use to work out for myself in the future? or a video guide or something?
I've also been wondering the best way to join them without a pcb...?

#### crutschow

Joined Mar 14, 2008
30,803
Do you have a formula or rule i can use to work out for myself in the future? or a video guide or something?
Don't know of anything offhand, other than experience.
I've also been wondering the best way to join them without a pcb...?
Probably just solder them to a small perfboard, such as this.

Thread Starter

#### cyberdyne

Joined Jan 29, 2019
3
small perfboard,
Nice, didnt know about perfboards, thanks. Just ordered a 5 pack of 2cm x 8cm boards for \$1.80

im suppressed MOSFETs are so much more expensive than BJTs
perhaps i just couldn't find the super common cheap model.

#### crutschow

Joined Mar 14, 2008
30,803
im suppressed MOSFETs are so much more expensive than BJTs
Part of it is that you are comparing a small BJT with a power MOSFET.
For example a 2N7000 small MOSFET is more comparable in price to the small BJT's.

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