Replacing relays with MOSFETs

[Imagewerx]

What will the consequences be if one or more of the mosfets connected to the 12 volt perm line short from drain to source?

Do you have the schematic for the flip flops?

You wouldn't be able to turn anything off,the starter won't spin unless you push the brake pedal down.But why would this happen randomly and be any more relevant because I'm using MOSFETs,the same could happen with sticking relay contacts surely? As long as I use the correct automotive MOSFETs surely there won't be any problems?

Flip flops are just standard 555 circuits,the top one with one button to set and another to reset and the bottom one has just one to set and reset.

Chris.

 

[Imagewerx]

Highest current draw is the ignition switched accessory circuit such as lights at 15 Amps,starter only draws 5 amps,the ignition run circuit 3 Amps and the stereo just 0.2 Amps.

Any further thoughts on what I'm trying to do?

Chris.

 

[Jaguarjoe]

Those IR "IPS" units should work well. Some 555 timing and logic circuits can be very tempermental. One lister on this forum made a 555 toggling some p-ch. mosfets to alternately select high or low beam headlights on his motor home. Without touching anything, lights would just switch at random. In the end, he replaced the 555 circuit with discrete components. Be sure the 555's have very stable clean power sources. They should have their own automotive rated regulator. National makes a nice one, but I don't recall the part number.

 

[kinarfi]

https://www.electro-tech-online.com/threads/flip-flop-lm555.112406/

I used and suggest the IRF4905 P FET, about $1.50, rated at 74 amps. I plan to replace all the relays in my off highway vehicle with these, maybe this winter, my plan is to make a positive bus out of aluminum and use it as a heat sink, the bus will be something like a 2" X 1/4" bar of the necessary length with each tab mounted, but insulated to the bar and the source lead connected to the bar or to a fuse block that is connected to bus.
Kinarfi
https://www.alliedelec.com/search/productdetail.aspx?SKU=2731094

 

[kinarfi]

Right,this is what I'm trying to do.There will be one button to push to turn the ignition on and crank the starter (you have to press the brake at the same time),then another press to turn the engine off.The accessory feed to the radio will stay until I open the drivers door when it will shut off automatically.

**broken link removed**

My aim is to replace the four top relays with MOSFETs,I don't know how much current each circuit draws YET but I will measure them all properly tomorrow.

Chris.

Most brake switches supply the brake lights with 12 v, does this have a multi poled switch? When is the 12 v from alarm hot and not hot? What turns it on? Any idea of what's inside the flip flop boxes.
Thanks,
Kinarfi

 

[kinarfi]

How's this look to you?
Kinarfi

 

[carmusic]

irf4905 does not have any protection (they may burn and starts fire if load become short-circuit) i would use as suggested before ips mosfet made for automotive with overload and overtemp protection

 

[Imagewerx]

Those IR "IPS" units should work well. Some 555 timing and logic circuits can be very tempermental. One lister on this forum made a 555 toggling some p-ch. mosfets to alternately select high or low beam headlights on his motor home. Without touching anything, lights would just switch at random. In the end, he replaced the 555 circuit with discrete components. Be sure the 555's have very stable clean power sources. They should have their own automotive rated regulator. National makes a nice one, but I don't recall the part number.

Thanks,already had some fun getting a few logic circuits to work that needed dedicated clean and tightly regulated power supplies.Been working all this year perfectly so hopefully nothing to worry about in that respect any more.

https://www.electro-tech-online.com/threads/flip-flop-lm555.112406/

I used and suggest the IRF4905 P FET, about $1.50, rated at 74 amps. I plan to replace all the relays in my off highway vehicle with these, maybe this winter, my plan is to make a positive bus out of aluminum and use it as a heat sink, the bus will be something like a 2" X 1/4" bar of the necessary length with each tab mounted, but insulated to the bar and the source lead connected to the bar or to a fuse block that is connected to bus.
Kinarfi
International Rectifier - IRF4905PBF - Discrete MOSFETs - Semiconductors - Allied Electronics

Oddly enough I'd already made the busbar for the relays that would work just as well for the MOSFETs.

Most brake switches supply the brake lights with 12 v, does this have a multi poled switch? When is the 12 v from alarm hot and not hot? What turns it on? Any idea of what's inside the flip flop boxes.
Thanks,
Kinarfi

This one does as well,that's why I'm using the relay to invert it to -ve switched.I already have a latching relay in the car that is switched on and off by the +ve central locking pulses from the alarm that will now also switch this lot on and off.The flip flops are nothing more than basic 555 circuits.

How's this look to you?
Kinarfi

Thanks,it looks like something I can work with.

irf4905 does not have any protection (they may burn and starts fire if load become short-circuit) i would use as suggested before ips mosfet made for automotive with overload and overtemp protection

I intend to fuse everything I'm doing so the chance of anything catastrophic like this will be impossible surely? But I am also looking at the IPS,something that will more likely be based on whether I have to buy 5000 of them or not.

Chris.

 

[Jaguarjoe]

I intend to fuse everything I'm doing so the chance of anything catastrophic like this will be impossible surely?

"A solid state device will always protect its fuse"

-Famous quote from 2 talented engineers at Bell Labs in 1947

Bullet proof voltage regulators and IPS type devices would not have been developed if their bare bones cousins were adequate.

 

[Imagewerx]

"A solid state device will always protect its fuse"

-Famous quote from 2 talented engineers at Bell Labs in 1947

Bullet proof voltage regulators and IPS type devices would not have been developed if their bare bones cousins were adequate.

Sorry but I don't understand this?

Chris.

 

[Jaguarjoe]

If you install a $0.50 fuse to protect a $100 part, the $100 part will blow to protect the fuse.

 

[Imagewerx]

If you install a $0.50 fuse to protect a $100 part, the $100 part will blow to protect the fuse.

How??????? If my load draws 15 Amps and my switching device is rated at 50 Amps and I fuse it for 20 Amps,how is this wrong?

Chris.

 

[BrownOut]

I think JJ is j-jabbing at you.

 

[carmusic]

fuse arent fast enough to protect semiconductor generally, they are there to protect wiring from long overcurrent, a 15 amp fuse can pass 100A for few milliseconds and that enough to kill mosfet and transistors

 

[BrownOut]

That's a great point, carmusic. Now I see what JJ means by a 50 cent fuse. For electronics, higher quality, fast acting fuses are required ( meaning: more expensive ) Look at datasheet for the IRFP460 power MOSFET as an example. The SOA (save operating area ) graphs shows that for this nominally 20A part, an over current of nearly 25A can be tolerated for 100ms, or nearly 40A for 10ms ( at around 10-11V ). To protect this part, the fuse must blow in less time at that current. There is no safe area for 100A. There must be sufficient resistance in the circuit ( wires, etc ) to prevent the current from reaching that level in the case of a shorted load. Heat sinking requirements must also get attention. For the TO-247 case, the junction to ambient thermo resistance it 30 degrees/Watt. Any more than a couple watts, and heat sink will be needed.

I'm only using this part as an illustrative example. I didn't bother to check the specs on whatever part we've been discussing.

 

[Imagewerx]

Ok thanks for all the advice guys and acting on what I've learnt on here I'll be using ALL relays instead of 555s and MOSFETs this time around.This is still something I'll look at doing in the near future once I've done some more research but as I'm starting to run out of time now,I'll stick with what I know about.

Chris.