Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Replacing relays with MOSFETs

Status
Not open for further replies.

Imagewerx

Member
Are there any disadvantages to replacing relays in a 12 volt automotive environment with MOSFETs?

I've already built a solid state indicator flasher using MOSFETs to drive high power Luxeon LEDs that has been in continuous use for a day on the bench flashing merrily away without any problems.

I'm now designing a BMW/Renault etc single button start/stop system that works with an aftermarket passive keyless entry system to totally eliminate the old fashioned ignition-key-in-the-lock system.Although easy to do with relays and a few flip flops it would be nice to get it working with MOSFETs instead.So what's the worst thing that could happen of I do this?

Chris.
 
If designed properly, a power MOSFET makes a great substitution for a mechanical relay. The design is a little more involved, however.
 
If designed properly, a power MOSFET makes a great substitution for a mechanical relay. The design is a little more involved, however.

Thanks,in what way does it need to be properly designed? Just with respect to over rating the current handling by a big margin or something more technical?

Chris.
 
A couple of things come to mind:

Voltage should be rated to accommodate load dump or the mos fets should be auto protected type.

Batteries do go dead. In some circumstances the fet may not be fully turned on and could be damaged. Under Voltage lock out can protect from this.
 
A couple of things come to mind:

Voltage should be rated to accommodate load dump or the mos fets should be auto protected type.

Batteries do go dead. In some circumstances the fet may not be fully turned on and could be damaged. Under Voltage lock out can protect from this.

Thanks,a few terms I'm not familiar with......

1.Load dump
2.Auto protected
3.Under voltage lockout

Chris.
 
Thanks,in what way does it need to be properly designed? Just with respect to over rating the current handling by a big margin or something more technical?

Chris.

The configuration of the MOSFET needs to be correct for power switching, properly driven and kept cool by heatsinking if necessary, protected from over-voltages by both the circuit and the automotive electrical system ( load dumps, etc ). Care has to be taken to not exceed any max operation specs, etc etc. If this all sounds new to you, then you either need to do alot more research or just use the relays, which should work fine anyway.

PS: There are some good threads on "load dump". I tried using the "Search" utility to find one for you, but there were so many hits, I gave up. ;(
 
Last edited:
It sounds new to me because I've not wanted to do this before,it's a case of wanting to rather than needing to.I am doing a lot more research by asking the question on here,this should get me the answers I need a lot more accurately than by Googling it.
So can anyone suggest some suitable MOSFETs please based on a nominal operating voltage of 14 volts and a maximum current of about 25 Amps?

Chris.
 
Is the switch to be high-side or low-side? If it's high-side you normally use P-MOSFETs and for low-side you normally use N-MOSFETs.
 
By "high side" you mean it switches the live supply to the load rather than ground?
Yes.

High-side switches the power and the load is grounded.

Low-side switches the ground and the load is connected to power.
 
Load dumping occurs when a large load is suddenly disconnected from the electrical system. The alternator, working hard to keep up with the load can not react instantly when the load is "dumped" and will put out a huge spike into the system. Sometimes as high as 60 volts.
Circuits should be designed to protect from reverse polarity too.
I believe there is an SAE guideline for automotive circuits.
Here is Hella's version of a S/S relay:

Amazon.com: Hella H41773001 Solid State Ceramic 32 Amp SPST Mini Relay: Automotive

Note that this a SPST relay. It might be a challenge to make a SPDT version.
 
search for intelligent mosfet, they have protections and made especially for automotive use
here are some but other brands may do the job also
International Rectifier - HEXFET® Power MOSFET Product Information

Thanks,read through the whole IR automotive MOSFETs section (I need to get out more!!!) and some interesting stuff to be found.I'm new to the HEXFET thing and only read up on them brieflly,but is it correct they're non-polarised (Neither P or N channel) so can be used high or low side? Also are they triggered when the input goes high or low?

Edit....Just read up some more on HEXFETs and can see there are N and P channel.

Load dumping occurs when a large load is suddenly disconnected from the electrical system. The alternator, working hard to keep up with the load can not react instantly when the load is "dumped" and will put out a huge spike into the system. Sometimes as high as 60 volts.
Circuits should be designed to protect from reverse polarity too.
I believe there is an SAE guideline for automotive circuits.
Here is Hella's version of a S/S relay:

Amazon.com: Hella H41773001 Solid State Ceramic 32 Amp SPST Mini Relay: Automotive

Note that this a SPST relay. It might be a challenge to make a SPDT version.
A nice neat 100% plug and play solution but if it looks like a relay I might just as well use an old fashioned one with several miles of copper wire wound round a bit of iron.One of the reasons for doing this is to make a nice low profile board that has to fit into an existing OEM housing so it all looks as original as possible.

ishh 30$! for the ssr relay
Agreed,a lot of money for something that I can buy that will do the exact same job for £1.

Chris.
 
Last edited:
Post #10 explained the difference between high side and low side switching. In post #11 YOU decided that you needed high side switching. Why don't you post your circuit so we can all figure out what you need together? Without it, we would only be guessing.
 
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.
 
Last edited:
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?
 
Status
Not open for further replies.

Latest threads

New Articles From Microcontroller Tips

Back
Top