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Using Mosfets to switch large amounts of current?

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jdraughn

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I am looking to replace the ignition switch in my car with a Microcontroller. I am still experimenting, but right now I am using a small button, and a quick press of the button will turn the ignition on or off, while holding it down for 1 second will turn on the ignition and engage the starter until the car starts, or 2 seconds, whichever happens first.

Anyway, normally the ignition switch sends 12v to the starter solenoid, and the ignition wire sends +12v to the distributor, ECU and dash. These are the only two wires that I need to power to start my car, and my circuit works by using two relays, one relay to the starter solenoid, and a separate relay powers the ignition system. I have been driving around for the last week or so with the circuit on a bread board.

I would like to replace the relays with a more reliable solution and I am thinking multiple mosfets in parallel would do the trick. I don't know how much current is actually pulled when I start the car, but the two wires that go to the starter solenoid and ignition switch are pretty heavy gauge, around 12 gauge or so. It does not seem to me that the starter solenoid would require wire that heavy, so I am concerned about the mosfets not being able to carry enough current or overheating on me.

In order to drive the car I still need to insert the key and turn it on because there are two additional heavy gauge wires that are used to power the rest of the car, like lights, AC, power windows/seats, ect..., and I would like to add relays or mosfets for these two wires too, to eliminate needing to insert the key at all (I plan on defeating the steering wheel lock and adding an RFID security system).

All I really need to do is connect together all 3 wires that run the car (leaving out the starter wire), and use one really large bank of mosfets or multiple relays. When the key is on, all 3 wires are powered at the same time from the same fusible link from the battery.

Does anyone have any thoughts on the best way to swtich that amount of current? I don't have to worry about switching losses, they will either be on or off. I am having a little trouble figuring out how much heat a mosfet will handle with/without a heatsink if I know the specs like RDSon.

I have a few surface mount mosfets that have an RDSon of .065 ohms and was wondering how much power they could continously handle at 15 volts.

Every time I double the amount of mosfets the RDSon effectively drops in half correct? Which means the amount of power they could normally carry not only doubles from the doubling of the amount of mosfets, but doubles yet again since the resistance has dropped in half? So 4 of these mosfets in parallel would have an RDSon of under .017 ohms and 16 of them would be barely over .004 ohms? So how much power would they be able to dissipate if mounted on a circuit board?

The mosfets I have are IRLR024N, but I just happen to have a few of those on hand, I may go with T0220 mosfets that handle much more current and have a larger packet to pass the heat once I figure out what I need.

---------- Post added at 12:48 ---------- Previous post was at 12:36 ----------

I just wanted to quickly add, that any parts of the car that require a large amount of current already have relays, so I don't think it would have to carry all that much current, maybe if I had all the lights on, had the blower for the heater going full blast, and was rolling up both windows while adjusting both seats all at the same time would I pull much current. over 99% of the time I don't see the car pulling much more then 5 to 10 amps.
 

prateek_k_chd

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Power MOSFETs can handle pretty large values of current...The IRF540 for example is ubiquitous and handles ~30A with a heatsink

Now,
(1) Ignition would only need momentary power transfer (correct?) So A single power MOS would suffice (probably without even a heatsink)

(2) " double the amount of mosfets the RDSon effectively drops in half......................amount of power they could normally carry....... doubles yet again............ "
-- No ,amount of power they can carry remains same. Think of it in terms of current...if one MOS is carrying 10A , two of them in parallel carry 5A each, reducing the individual heat dissipation but total heat produced remains same.

(3) "if I had all the lights on, had the blower for the heater going full blast, and was ....."
-- Does the current for all that flows through the ignition switch ? I mean,when you turn the ignition key to "on" doesn't the power supply connect to these functions directly ?. If so then you would need a MOS only for starting the car.
If not then you're going to need a few MOS in parallel....
 

keith1200rs

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Bear in mind that a common failure mode for MOSFETs is to fail short circuit.

Keith
 

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