MOSFET or RELAY for High Current DC Switching

Thread Starter

DBTech7

Joined Apr 21, 2022
30
I'm working on a remote start system for an older gm diesel vehicle and so far everything has worked great, but I've hit a stump that I just cant figure out.

The engineers who designed the ignition switch of my truck though it would be a good idea to run starter solenoid power though the ignition switch itself, and they didnt bother to include a Starter Relay until 96, my system is designed to work on pre 96 vehicles so I dont get the luxury of tapping into that relay for starter engagement. The solenoid on the starter itself that engages the starter to the flywheel is what I need to control and based on the fuse ratings and wiring diagrams from the vehicle I believe the load wont exceed 40A for the starter engagement, and keep in mind this will only be engaged for maybe 20 seconds max per cycle, and being a remote start it will have irregular cycling.

Now for the ignition portion, the power flows from the ignition switch in the RUN position to a few different systems, ignition, fuel injection, lights, gauges, and transmission. Using the wiring diagrams, I calculated 85A max that will flow through the Ignition portion of the circuit. Obviously this is an absolute max and it will certainly never reach that load but all of the listed systems are run from a singular wire that comes off the ignition switch, the PINK wire in the diagram.

I've designed my circuit to handle the starter load and ignition loads separately, but my problem is finding relays capable of that current that are actually in stock through JLCPCB. I plan on making these in small batches IE 20-30 at a time based on demand, and I'm trying to not only ensure long term reliability but also future proofing as well. There are suitable options that I've found but they only have maybe 200ish in stock max and I don't want to find something that works just to have to change it next time I go to order more.

I've considered using mosfets as they are much cheaper than the relays im looking at and are more available, but im concerned about heat management. safety, and long term reliability of using mosfets over relays.

TLDR-
-I need to reliably and safely switch power to the PINK and PURPLE wires on the diagram
-PURPLE wire load is 40A abs max
-PINK wire load is 85A abs max

My question to anyone reading this is how would you design this circuit to safely and reliably switch the high current (40A for starter and 85A for ignition) with interest of keeping cost down, would you use MOSFETS or RELAYS, and why would you go that route?
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MaxHeadRoom

Joined Jul 18, 2013
25,212
The biggest problem is finding DC rated relays for that current, normally some kind of arc quench is required to prevent Plasma melting of contacts.
Have you considered a IGBT with suitable rating
 

Thread Starter

DBTech7

Joined Apr 21, 2022
30
The biggest problem is finding DC rated relays for that current, normally some kind of arc quench is required to prevent Plasma melting of contacts.
Have you considered a IGBT with suitable rating
I almost forgot those existed haha, but that's a great suggestion I appreciate it. I'm gonna spend my lunch today looking for one that has the proper rating I need
 

shortbus

Joined Sep 30, 2009
9,372
My question to anyone reading this is how would you design this circuit to safely and reliably switch the high current (40A for starter and 85A for ignition)
So you'll be doing this different than the way they do in cars today? My 2021 Buick remote start only energizes the engine, and headlights, most everything else is not energized when remote is usded. Before you can actually drive the motor must be restarted to have the normal systems working.
 

Thread Starter

DBTech7

Joined Apr 21, 2022
30
So you'll be doing this different than the way they do in cars today? My 2021 Buick remote start only energizes the engine, and headlights, most everything else is not energized when remote is usded. Before you can actually drive the motor must be restarted to have the normal systems working.
Yes its due to the fact that its for older diesels that don't operate like modern counterparts or gasoline engines. Its simply a matter of powering the glow plugs, lift pump, injection pump, and starter. The glow plugs and starter are temporary but the other 2 remain powered as long as the vehicle is on. My system will require the user to put the key in and turn the ignition to the run position which will disengage the remote start circuit and it will shutdown the vehicle if the brake pedal is depressed before turning the ignition on to prevent theft.

My reason in accounting for the higher amperage on the Pink wire is that the ECM may not like it if the transmission, lights, or even the gauges aren't powered when the vehicle is running. GM Engineers were on something good back in the 90s and made some questionable design choices in their electronics so I wouldn't put it past them.
 

LowQCab

Joined Nov 6, 2012
2,054
Completely rebuilding the Electrical-System is no trivial task,
also, it seems that You want to build and sell a customized system designed around
one brand and model of a 30-year-old Diesel-Truck that wasn't all that popular when they were new.
I don't see how you're going to stay in business doing that.

The way GM did it back in the day, would last ~10 to ~15-years, and then fail.
This is a ~400.oo repair because of the labor costs, but the parts are cheap and still available.

The best way to approach this is by using any one of the great systems that are available for
Race-Cars and Hot-Rods, they are generically called PDU's or "Power-Distribution-Units".
They are modular, and can be adapted to any situation, and most use CAN-Networking to
minimize the Wiring to the customizable, back-lit, Push-Button Control-Panel(s).
Some of them also accept existing external Switch Inputs.
Most have built-in Electronic-Current-Limiting / Circuit-Breakers,
and use MOSFETs for Switching, so there's no mechanical-Contacts to eventually fail.

It may take You a while to wrap your head around the unbelievable variety that's available ...........
Here's a place to start looking ......
https://duckduckgo.com/?q="Race+Car"+Power+Distribution+System&t=brave&iax=images&ia=images
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keypad_500.jpg
 

Thread Starter

DBTech7

Joined Apr 21, 2022
30
Completely rebuilding the Electrical-System is no trivial task,
also, it seems that You want to build and sell a customized system designed around
one brand and model of a 30-year-old Diesel-Truck that wasn't all that popular when they were new.
I don't see how you're going to stay in business doing that.

The way GM did it back in the day, would last ~10 to ~15-years, and then fail.
This is a ~400.oo repair because of the labor costs, but the parts are cheap and still available.

The best way to approach this is by using any one of the great systems that are available for
Race-Cars and Hot-Rods, they are generically called PDU's or "Power-Distribution-Units".
They are modular, and can be adapted to any situation, and most use CAN-Networking to
minimize the Wiring to the customizable, back-lit, Push-Button Control-Panel(s).
Some of them also accept existing external Switch Inputs.
Most have built-in Electronic-Current-Limiting / Circuit-Breakers,
and use MOSFETs for Switching, so there's no mechanical-Contacts to eventually fail.

It may take You a while to wrap your head around the unbelievable variety that's available ...........
Here's a place to start looking ......
https://duckduckgo.com/?q="Race+Car"+Power+Distribution+System&t=brave&iax=images&ia=images
.
.
.
View attachment 265581
This was mainly a project to practice circuit design, pcb design and object oriented programming, it started out as a one time thing but after gauging interest in online communities for my truck I decided to make it available to purchase to get a little extra money. I have lots of free time on my hands because of my job and my lack of friends lol so the business aspect doesnt worry me too much. They will be made to order so I wont be stocking boards Im never gonna sell, and I want the per unit cost to be as low as possible to make it profitable and so I can pass some of those savings onto the customers

I want that button panel in my truck I would love to have a "drink" button XD
 

shortbus

Joined Sep 30, 2009
9,372
I want that button panel in my truck I would love to have a "drink" button XD
I don't think those panels work like he thinks they do. They are still going to take so rewiring of the truck, they aren't plug and play at all.

What are you going to use as the remote transponder or key fob?
 

Thread Starter

DBTech7

Joined Apr 21, 2022
30
I don't think those panels work like he thinks they do. They are still going to take so rewiring of the truck, they aren't plug and play at all.

What are you going to use as the remote transponder or key fob?
Yeah its a good solution for a custom setup maybe but not quite what I need to make happen.

I plan on using a smartphone as the remote with an esp32 because this will not only be a remote start but also provide real time vehicle data to the user and If I feel like going the extra mile I might add a diagnostics program as well for simple error code identification, no more paperclips and watching the lights blink haha
 

Thread Starter

DBTech7

Joined Apr 21, 2022
30
Well Wendy locked my other thread which was more relevant than this one so thanks for the help everyone else, I'll figure it out
 

shortbus

Joined Sep 30, 2009
9,372
Well Wendy locked my other thread which was more relevant than this one so thanks for the help everyone else, I'll figure it out
Your other one should have been transferred to this one. They are about doing the same thing and the forum only allows one thread on a subject per member.
 
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