hi current car starter b.o.m. help

Hey guys,

I want to create a push button car starter that is arduino (3v) based. I want to try to keep it compact and solid state. I was wondering if i am on the right track with the following components: there are 3 states to the starter. Acc, Run and Crank/start

i was thinking about getting the following: Optoisolator - 4 Channel https://www.sparkfun.com/products/784

N-Channel MOSFET 60V 30A https://www.sparkfun.com/products/10213

i'd heat sink the crank mosfet as this will probably be a high current draw. One question, can i double up on the mosfet to increase the current handling (being able to switch 60 amps?)

if that wouldn't work, would you suggest using a Darlington Driver (8-Channel ULN2803) instead of the n-channel mosfets and having that switch 3 automotive relays?

Darlington Driver 8-Channel ULN2803: https://www.sparkfun.com/products/312

thanks in advance

As far as i know the starter motor solenoid needs a positve voltage because its other terminal is grounded. So you will need a high side switch of some kind. I think this will do: http://www.irf.com/product-info/datasheets/data/auips6031.pdf

I do not recommend bypassing the startmotor solenoid or replacing it with a mosfet. We are talking hundreds of amperes here...

I'm not planning in replacing the starter solenoid. Just piggybacking the key mechanism like they do in remote car starters. The ones I've seen use automotive relays

http://www.remotestartexperts.com/how-dei-valet-remote-car-starters-work.html

BTW those sparkfun MOSFETs are not low on-resistance by todays standards, they are 35 and 47 milliohms, whereas 5 milliohm or lower devices are readily available (n-channel).

p-channel MOSFETs have somewhat higher on-resistances, but it sounds like you may need one (and therefore a level-shifting circuit to drive the gate).

But what current does the solenoid require? How is inductive kickback handled?

When MOSFETs are paralleled separate gate resistors are needed for each device I believe.

I dont think you'll need mosfets.. the same thing could be carried out with using a a few transistors. 2n3906 or (2n3904??Cant remember) driving a pair of TIP3055 as a darlington pair would be more than enough to drive a solenoid and you could still use the 5v logic.

Why not keep it simple, and use a relay ? The ignition key switch is a mechanical one, after all.

Just add some snubber components on the contacts of the relay, and choose one that is adequately rated for switching that kind of power (use an Automotive relay)

Also, add a Snubber network, just for extra safety. http://www.electronics-tutorials.ws/io/io_5.html

Use a 100nF Capactor and a 100 Ohm resistor for the snubber.

// Per.

Chris1448: I dont think you'll need mosfets.. the same thing could be carried out with using a a few transistors. 2n3906 or (2n3904??Cant remember) driving a pair of TIP3055 as a darlington pair would be more than enough to drive a solenoid and you could still use the 5v logic.

How can you make an high side switch with these components? Have you ever looked att Hfe for 2N3055, can 2N3906 support enough base current to the 2N3055 Nope

Pelle

Chris1448: I dont think you'll need mosfets.. the same thing could be carried out with using a a few transistors. 2n3906 or (2n3904??Cant remember) driving a pair of TIP3055 as a darlington pair would be more than enough to drive a solenoid and you could still use the 5v logic.

How can you make an high side switch with these components? Have you ever looked att Hfe for 2N3055, can 2N3906 support enough base current to the 2N3055 Nope

Pelle

Hey..I said I couldn't remember ....