Replacing a glow plug relay with a transistor

[Creative25]

I know this sounds a bit crazy but my friend has a diesel car.
The Glow plug relay does not work any more.
Currently he starts the car by shorting the wires directly.
He asked me to make something with a transistor to switch the power for the glow plugs on and off.
It has to be high side switching, because the earth is provided from the engine, which is earthed.
I was thinking of adding a small microchip so it could have a timer.
What is the easiest was of High side switching? Efficiency is not very important since it is only used a few seconds before starting the engine.
Important is that the Glow plugs can not turn on if something goes wrong with the circuit.
Any Ideas how to do the hardware side. I was thinking of a fet, but do not know how to drive it on a high side.
Best Regards:
Uli

[Enamul]

Hi ULI,
Can you please check the amount of current needed when he short the wire manually to switch the power for the glow plugs on and off? I guess that is done with the 12V battery of the CAR. So MOSFET might be a good choice but everything depends on current.

[Creative25]

Hi Enamul.
I measured on my car not sure if it is the same on his.
Mine draws quite a bit, between 30 and 40 A
So to be safe I would like to have a circuit that can handle at least 70A
And a low on resistance.

Best Regards:
Uli

[Enamul]

Any Ideas how to do the hardware side. I was thinking of a fet, but do not know how to drive it on a high side.

I think first of all please read the following post..to know about high-side and low-side switching..
http://answers.yahoo.com/question/index ... 427AAyMeHu

[Creative25]

The last to answers about the truck and accelerating make me really laugh.
Ok I see for an N channel you need a driver like a charge pump to get the gate voltage higher than the supply voltage to be able to switch the N channel fet.
I also read about P channel fets, but i get a bit confused what would be needed to drive a P channel fet.
I see the P channel is not always that easy to drive, but I do not know what would be needed in my case.
Could you help me to understand better?
If P channel is not much easier for high side switching then I would go for an N channel.

Best Regards:
Uli

[Enamul]

I also read about P channel fets, but i get a bit confused what would be needed to drive a P channel fet.

You can use this kind of driving circuit with NPN transistor base is pull down by 10K..

[Enamul]

Here is a 100A P-channel MOSFET..
http://www.onsemi.com/pub_link/Collateral/SMP3003-D.PDF

[Creative25]

Thanks,
This looks fairly simple.
Could I use any N channel transistor for this, like a bc337 or a bc 547?
Best Regards:
Uli

[Enamul]

Yes..you can.

[Creative25]

Thanks.
I get an idea.
Now how about the fet.
Digi Key does not have this one in stock.
What are the important caracteristics to look fore.
Trench fet
Hexfet,
Optimos
whatever you call them.

Would you add Addtional components to protect the circuit?
I see the pull up resistor is 10K could I go even higher so to save battery since this one has to pull power all the time.
Best Regards:
Uli

[Enamul]

Would you add Addtional components to protect the circuit?
I see the pull up resistor is 10K could I go even higher so to save battery since this one has to pull power all the time.

Actually NPN transistor will ON only when you want to turn P-channel MOSFET On, so NPN will be kept OFF most of the time..that's why 10K is pretty OK. But to have more protection you can go for something like this..

[Creative25]

Thanks this is explaining it nicely.
Just one question R3 forms a voltage divider.
Is it correct that this one can be left out if the supply voltage is 12 Volt?
Best Regards:
Uli

[Enamul]

Just one question R3 forms a voltage divider.
Is it correct that this one can be left out if the supply voltage is 12 Volt?

Actually R3 is gate current limit and D1 is to protect surge in VDG..