Replace 12 volts DC – 30 amperes mechanic relay with a solid state device, or mosfet device

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igeorge

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I have a relay for starter solenoid on my car. Due to increasing number of thefts in my area, I want to put a kind of “kill switch”. Something to stop the starter to turn on.
I want to cut the wire from my starter relay to starter solenoid (30 amperes current) and put my own circuit, which I can control from a small Chinese remote. The 30 amperes relays are big, like here :
https://www.amazon.ca/gp/product/B07G2RCLNJ/ref=ppx_yo_dt_b_asin_title_o03_s00?ie=UTF8&psc=1
I would like to use a solid state version based on mosfets.
I found some schematics, but all of them are or to put the load on the high side or low side.
I want to find a suggestion what to use, and be free to put the load any side I want. Something like having 2 mosfet in series with the sources tied together and the drain free for load.
I am not good at all on analog, so I need a suggestion and eventually a hand drawing of a schematic with part numbers.
Something similar, like here ? Where to put load, and where to put plus and minus ? Can I put them anywhere I want ?

Because I want to put it hidden on my car, not easy accessible, I need a RELIABLE schematic, because if something is going wrong in the middle of the night I cannot just get the starter wires and twist them together to start the car.
Thank you, and please help.
 

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I understand, but in the original electrical diagram on the starter circuit it is a 30 amperes fuse. Maybe 30 amperes mosfet or even 40 can resist.
 

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igeorge said:
I understand, but in the original electrical diagram on the starter circuit it is a 30 amperes fuse. Maybe 30 amperes mosfet or even 40 can resist.
Click to expand...
I don't the case but automotive fuses are time delayed so the fuse may stay in alive for few milliseconds but this is hard to say for MOS. If I were you Id' use a reliable and robust relay. Because if the MOS fails, you will have to repair it.
 
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wwfeldman said:
you might want to look at these, or some other solid state relay

turn on inrush current is an issue - motors tend to use about 10x nominal current for turn on
Click to expand...
The load is not a motor. It is a solenoid which provide power to the motor.
So the load is only the coil of the solenoid, like another relay but with strong contacts to support 600 to 700 amperes for the starter motor to turn.
 

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igeorge said:
The load is not a motor. It is a solenoid which provide power to the motor.
So the load is only the coil of the solenoid, like another relay but with strong contacts to support 600 to 700 amperes for the starter motor to turn.
Click to expand...
A solenoid is still an inductive load and, thus, will still have an inrush current. There is also the inductive flyback that a relay can handle but a MOSFET might not.
 
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or this:
--- Updated ---

igeorge said:
The load is not a motor. It is a solenoid which provide power to the motor.
So the load is only the coil of the solenoid, like another relay but with strong contacts to support 600 to 700 amperes for the starter motor to turn.
Click to expand...
this is not consistent with my understanding of starter motors and solenoids
to start a car, the starter motor has to turn, and the solenoid has to advance the gear that engages the flywheel to turn the car's engine.

i suppose it depends on how things are wired in your car
i assume that the same relay that causes the starter motor to turn also causes the solenoid to extend the gear

Starter motor, starting system: how it works, problems, testing

How a car starting system works: system diagram, starter motor, solenoid, starter relay, neutral safety switch. Common starting system problems, testing
www.samarins.com

referring to the diagram in the above link:
maybe you want to put your gadget in the low power red line from the starter relay to the starter control
circuit on the starter motor, instead of in a high power line, red or black.
 
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Unfortunately, I cannot put it there as the connection is on a PCB inside the fuse box. To buy a separate fuse box, modified and then replace it is out of question as the fuse box assembly costs 700 dollars for my 2016 Acura MDX.
 
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A old-time trick is to install a hidden switch in the wiring of the relay circuit. Only you know its location (under the dash, seat, etc). It's robust enough to handle current to the relay. You press it as you turn the key to start the engine.

Does 30 Amperes really go through your ignition switch? Sounds like a lot.
Instead doesn't the ignition switch send maybe 1A to the relay...
which sends 10-20A to the solenoid...
which sends 400A to the starter?
 

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If you car is equipped with a Central Control unit such as Main Vehicle Computer, you can also block this unit so the engine will never start. It's easier than shutting down high current engine starting motor.
The theft can start the engine but it will never supply neither sparking nor gasoil.
 

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barry said:
A solenoid is still an inductive load and, thus, will still have an inrush current. There is also the inductive flyback that a relay can handle but a MOSFET might not.
Click to expand...
An inductive load like a solenoid doesn't have an inrush current. The current ramps up from zero to the final value (decided by the wire resistance). A capacitive load has an inrush current.
The "inrush current" of an electric motor has combined mechanical/electrical reasons.
 
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igeorge said:
Unfortunately, I cannot put it there as the connection is on a PCB inside the fuse box. To buy a separate fuse box, modified and then replace it is out of question as the fuse box assembly costs 700 dollars for my 2016 Acura MDX.
Click to expand...
okay
but there's a wire from the fuse box to the starter motor.
you can put it at the starter motor, or anywhere in between where you can mount it
 

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The on response of a solenoid -




Regards, Dana.
 
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You’re right. I forgot to turn my brain on.
 

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