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12v 2A

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Scarr

Member
I want to switch 12v 2a for incar use, LCV screens, video switch etc.

I think I can use a N channel MOSFET for this or am I wrong? the MOSFET will be controlled by a Attiny12 MCU. still reading up on MOSFETS but thought I'd ask you guys as you always a great help when my head gets full of "webwool" from looking at the internet.

Thanks

Steve
 

Sceadwian

Banned
You need to look at the PDF for some logic level FETs to find out what FET will be low ohmic with a drain/source current of 2 amps at 5 volts gate drive. Just because it's a logic level FET doesn't mean it will switch it's full rated amperage at 5 volts. Link the PDF or share the model number/maker of a few fets you're thinking of using.
 

bountyhunter

Well-Known Member
I want to switch 12v 2a for incar use, LCV screens, video switch etc.

I think I can use a N channel MOSFET for this or am I wrong? the MOSFET will be controlled by a Attiny12 MCU. still reading up on MOSFETS but thought I'd ask you guys as you always a great help when my head gets full of "webwool" from looking at the internet.

Thanks

Steve
The problem with using a N FET is you will need to generate a voltage of about 18V - 20V to be able to pull the gate up to for turn on. A P-FET would be a lot easier to use, since you turn it on by pulling the gate down. There are tons of P-FETs that turn on very strong at 2A and have very low ON resistance. BTW: the P-FET will have 12V of gate drive when pulled down (since it is in the 12V line) and they all turn on very strong at that level.
 
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Boncuk

New Member
The problem with using a N FET is you will need to generate a voltage of about 18V - 20V to be able to pull the gate up to for turn on. A P-FET would be a lot easier to use, since you turn it on by pulling the gate down. There are tons of P-FETs that turn on very strong at 2A and have very low ON resistance. BTW: the P-FET will have 12V of gate drive when pulled down (since it is in the 12V line) and they all turn on very strong at that level.

Hi bountyhunter,

which datasheet did you refer to telling it needs 18 - 20V to pull the gate of an N-channel MosFet high enough to turn on?

Here is an example for an N-channel of 0.022Ω RDS(ON): VGS(th)=0.7V, RDS(ON) = 0.022Ω at VGS 4.5V and ID = 4.1A, RDS(ON) = 0.030Ω at VGS 2.7V and ID = 3.5A. ID @ TA25deg/C = 8.7A, ID @ TA70deg/C = 7.0A, package SO-8.

The babe's name: IRF7401 (International Rectifier)

Go and find a P-channel MosFet with a lower RDS(ON) and the same current rating!

Last not least, being controlled (gate) by the output of an MCU they use positive logic in 99% of all cases. Controlling a P-channel would mean an extra inverter (or rewriting the software).

Boncuk
 

MikeMl

Well-Known Member
Most Helpful Member
Most car appliances are grounded to the car frame (battery negative) which means that the switching must be in the "high-side" positive lead. If you try to use an N-Fet for this, then you need a source of gate voltage that is 18V or more.

The complexity of generating the gate voltage far exceeds the ease of just using a PFet, whose gate just needs to be grounded to turn the appliance on.
 

greggles

New Member
May help

Basic is sometimes easier than overthinking.
Whats wrong with your MPU driving a relay? 10k from the output to the base of a NPN transistor, emmitier to ground and relay between supply and collector, dont forget back EMF diode protection.
 

bountyhunter

Well-Known Member
Hi bountyhunter,

which datasheet did you refer to telling it needs 18 - 20V to pull the gate of an N-channel MosFet high enough to turn on?
This is an automotive app using an N-FET to switch 12V, so it's in the "high side" line. That means you have to pull the gate up AT LEAST 5V above 12V to get good turn on. Since auto "12V" actually runs 14 - 14.5V under operation, I recommend 18V - 20V.

That's why I recommended using a P-FET in the high side so you can pull the gate down to ground and get 12V of gate drive.
 
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bountyhunter

Well-Known Member
Here is an example for an N-channel of 0.022Ω RDS(ON): VGS(th)=0.7V, RDS(ON) = 0.022Ω at VGS 4.5V and ID = 4.1A, RDS(ON) = 0.030Ω at VGS 2.7V and ID = 3.5A. ID @ TA25deg/C = 8.7A, ID @ TA70deg/C = 7.0A, package SO-8.

The babe's name: IRF7401 (International Rectifier)

Go and find a P-channel MosFet with a lower RDS(ON) and the same current rating!
I am keenly aware that P-FETs usually have higher ON resistances than their N-FET counterparts for a given voltage/current/price point. That's a fact of life for any power supply designer, but sometimes the advantages of the P-FET outweigh the cost increase. Like in this case where you would have to build a charge pump to create a 20V rail to run the N-FET gate from.
 

Boncuk

New Member
I couldn't detect any automotive device to be switched on. The OP appearently wants to operate video stuff in the car.

That kind of equipment has normally no connection to the car chassis and he could use low side switching.
 
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