Electro-Mechanical to Digital Relays

[MacCube]

After reading E46m3zcp's post yesterday about the SMG salmon relay, I started thinking if anyone has tried replacing these relays with a digital design? Is this relay simply a single pole design or does it control something else other than the SMG pump motor? I'm thinking that a digital replacement could solve at least one problem.

[sapote]

After reading E46m3zcp's post yesterday about the SMG salmon relay, I started thinking if anyone has tried replacing these relays with a digital design? Is this relay simply a single pole design or does it control something else other than the SMG pump motor? I'm thinking that a digital replacement could solve at least one problem.

It's a single output relay. Yes it can be replaced with a high-side-switch but has to handle the high current.

It's an active low control relay, so should be easily find a P channel FET to handle over 40A.
This one is $3 for P channel FET and handle 75A current:
https://www.digikey.com/en/products/...AGJBBAKiAugL5A
I think this is a better FET and easier to solder and cheaper at $1.64:
https://www.digikey.com/en/products/...axCdufIWMkSJQA

To protect the Gate from being damaged by the alternator Load Dump, should add a 10v Zener anode to the Gate and cathode to Source with 1K resistor connected to the Gate and the other end to relay control input wire (brown pin4)
Connect the FET Source to 12v (RT/GE) and Drain to the relay output (RT) which drives the motor. Also add a 40A Schottky diode cathode to Drain and anode to ground as a flywheel diode to conduct the motor current when the FET turned off.

60A Schottky diode:https://www.digikey.com/en/products/...0D60BG/1494799

10V Zener diode: https://www.digikey.com/en/products/...hoCLeYQAvD_BwE

Everything can fit inside the stock relay housing except the ground wire for the Schottky diode. I wish the factory had the diode added across the motor leads as it would help prolong the relay contacts from arcing. Maybe the motor already has the diode but it doesn't show on schematic.

So how hot the FET can be when driving the motor at 10A?
Internal FET resistance is 6mOhm, so heat = I^2 * R = 10^2 * 6mOhms = 0.6watts. To be safe I would solder a piece of copper as heat sink to the Drain tab (the large metal tab under side, which is the same signal as pin2 Drain).

[cobra]

It's a single output relay. Yes it can be replaced with a high-side-switch but has to handle the high current.

It's an active low control relay, so should be easily find a P channel FET to handle over 40A.
This one is $3 for P channel FET and handle 75A current:
https://www.digikey.com/en/products/...AGJBBAKiAugL5A

To protect the Gate from being damaged by the alternator Load Dump, should add a 10v Zener anode to the Gate and cathode to Source with 1K resistor connected to the Gate and the other end to relay control input wire (brown pin4)
Connect the FET Source to 12v (RT/GE) and Drain to the relay output (RT) which drives the motor. Also add a 40A Schottky diode cathode to Drain and anode to ground as a flywheel diode to conduct the motor current when the FET turned off.

60A Schottky diode:https://www.digikey.com/en/products/...0D60BG/1494799

10V Zener diode: https://www.digikey.com/en/products/...hoCLeYQAvD_BwE

What about a solid state relay?

Risk with solid state components is the failure mode can be on, whereas a mechanical relay failure mode is usually off. Not sure how that affects SMG.

[sapote]

What about a solid state relay?

Risk with solid state components is the failure mode can be on, whereas a mechanical relay failure mode is usually off. Not sure how that affects SMG.

Solid state == semiconductor
They can fail shorted (ON) or opened (off). Mechanical relay can fail shorted (ON) too when the contacts have grown long tits and welded together by arcing, in general and not specific to M3 cars.

Semiconductor relay is more reliable when designed with high current parts and good safety margin.

[MacCube]

Thank you both for your replies. But I was hoping that it would be more plug-in-play as I have very little electronics skills. Perhaps this could be a profitable business venture for someone.

[cobra]

Solid state == semiconductor
They can fail shorted (ON) or opened (off). Mechanical relay can fail shorted (ON) too when the contacts have grown long tits and welded together by arcing, in general and not specific to M3 cars.

Semiconductor relay is more reliable when designed with high current parts and good safety margin.

What I meant was what about using a pre made solid state relay instead of making one with a fet and other parts.

[sapote]

What I meant was what about using a pre made solid state relay instead of making one with a fet and other parts.

That would be good if we can find one that match the stock relay spec.