MOSFET/transistor vs magnetic relay

Thread Starter

JonnyS

Joined Dec 28, 2015
27
hi all and happy holidays. New to the forum and was wondering, when it comes to a pulsed circuit such as a low amp 12v indicators in a car why they wouldn't use a transistor to "flash" the indicators rather than a relay that you can hear clicking on and off? If I was building a similar circuit that requires a pulse with no different power requirements I would normally opt for a cheaper transistor. Perhaps they do now and I just drive an old jeep :)
 

Thread Starter

JonnyS

Joined Dec 28, 2015
27
Modern cars use mosfets for flashing and interal door lights fading off, the clicking sound is made electronicly.
I have been using mosfets longer than my jeep has been around and just always presumed that it was designed to wear out faster.
 

Picbuster

Joined Dec 2, 2013
982
hi all and happy holidays. New to the forum and was wondering, when it comes to a pulsed circuit such as a low amp 12v indicators in a car why they wouldn't use a transistor to "flash" the indicators rather than a relay that you can hear clicking on and off? If I was building a similar circuit that requires a pulse with no different power requirements I would normally opt for a cheaper transistor. Perhaps they do now and I just drive an old jeep :)
A (mos)fet's internal resistance when in conducting phase is milli ohms a xtor in ohms.
power in a device = I^2 x resistance and will produce heat.
That's why.
 

Thread Starter

JonnyS

Joined Dec 28, 2015
27
The small amount of heat can't be dissipated by a heatsink like in an amplifier? It is in an engine bay so you wouldn't think heat would be as bad as arcing from contact bounce of a relay. You would think a solid state solution would be safer and last longer.
 

Thread Starter

JonnyS

Joined Dec 28, 2015
27
And like dodgydave said, some modern cars don't use relays and have an electronically generated click that comes from a speaker as a reminder that they are on. I think bmw might have been the first model I saw in the 90's like that.
 

Picbuster

Joined Dec 2, 2013
982
The small amount of heat can't be dissipated by a heatsink like in an amplifier? It is in an engine bay so you wouldn't think heat would be as bad as arcing from contact bounce of a relay. You would think a solid state solution would be safer and last longer.
Why swing energy, converted, to heat away? please calculate the number of estimated blinks per year with 30 x per minute on for 1 sec.
convert this to energy 1.5Amps per blink @ 13.4V ( alternator voltage).
10 Amp (mos)fet cost the industry @ 50000 5 cent approx. solid state relays multiple.
 

dl324

Joined Mar 30, 2015
8,950
when it comes to a pulsed circuit such as a low amp 12v indicators in a car why they wouldn't use a transistor to "flash" the indicators rather than a relay that you can hear clicking on and off? If I was building a similar circuit that requires a pulse with no different power requirements I would normally opt for a cheaper transistor. Perhaps they do now and I just drive an old jeep
My newest vehicle is 12 years old and it still uses a bunch of relays. I think it comes down to human inertia. The auto industry has used relays for many decades, so the designers are comfortable with the parts. That and the fact that they have a propensity for designing in as many points of failure as they can to provide a steady stream of revenue for their dealership service and parts departments.
 

benta

Joined Dec 7, 2015
101
My newest vehicle is 12 years old and it still uses a bunch of relays. I think it comes down to human inertia. The auto industry has used relays for many decades, so the designers are comfortable with the parts. That and the fact that they have a propensity for designing in as many points of failure as they can to provide a steady stream of revenue for their dealership service and parts departments.
I think you see this from the wrong angle. As a semiconductor specialist, I've worked with automotive companies for many years, and the engineers are in no way averse to using new technology, and the slur about "built in failures" I can not recognize.

However, car manufacturers have many factors that need to be taken into account:
:- can it be repaired by our workshops?
- do the mechanics understand the technology?
- is it more or less reliable?
- what does it cost?
- do we need to build up new spare-parts warehouses in addition to the existing ones?
- will the authorities understand this technology?
- will the authorities approve this technology?
- ... the list goes on and on, and it's mainly practical and safety requirements that dominate.

Relays are well known by authorities and workshops, and are also a no-brainer for home mechanics. And they work really well. I drive an 18-year old car that has never had electrical problems.
You can weld everything together in a car (like an unrepairable iPhone), but I prefer bolts and fasteners and the option of doing my own work on the car.
Finally, you will not find a smart MOSFET switch or whatever that is more tolerant of abuse (eg, lightning strike, short-circuits, jump-starting) than an old-fashioned relay.

Cheers,

Benta.
 

Thread Starter

JonnyS

Joined Dec 28, 2015
27
My newest vehicle is 12 years old and it still uses a bunch of relays. I think it comes down to human inertia. The auto industry has used relays for many decades, so the designers are comfortable with the parts. That and the fact that they have a propensity for designing in as many points of failure as they can to provide a steady stream of revenue for their dealership service and parts departments.
That's what I was thinking.
 

#12

Joined Nov 30, 2010
18,076
in a car why they wouldn't use a transistor to "flash" the indicators rather than a relay
The basis of this question is partially mind reading...which most men are of dreadful quality. When The Dodger said they use transistors and add a speaker to make a clicking sound, I thought he was joking.:D I disagree with half of what benta said, but one part comes through loud and clear:

car manufacturers have many factors that need to be taken into account
This only makes the mind reading factor worse.:(

One automotive engineer said, "It's easy to design a truck that will last for a million miles. Designing them to wear out in 50,000 miles is a lot more difficult." Why would he even be thinking in those terms if he wasn't faced with making sure his designs wouldn't go for a million miles?
 
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Thread Starter

JonnyS

Joined Dec 28, 2015
27
I think you see this from the wrong angle. As a semiconductor specialist, I've worked with automotive companies for many years, and the engineers are in no way averse to using new technology, and the slur about "built in failures" I can not recognize.

However, car manufacturers have many factors that need to be taken into account:
:- can it be repaired by our workshops?
- do the mechanics understand the technology?
- is it more or less reliable?
- what does it cost?
- do we need to build up new spare-parts warehouses in addition to the existing ones?
- will the authorities understand this technology?
- will the authorities approve this technology?
- ... the list goes on and on, and it's mainly practical and safety requirements that dominate.

Relays are well known by authorities and workshops, and are also a no-brainer for home mechanics. And they work really well. I drive an 18-year old car that has never had electrical problems.
You can weld everything together in a car (like an unrepairable iPhone), but I prefer bolts and fasteners and the option of doing my own work on the car.
Finally, you will not find a smart MOSFET switch or whatever that is more tolerant of abuse (eg, lightning strike, short-circuits, jump-starting) than an old-fashioned relay.

Cheers,

Benta.
That makes more sense. Cheers
 

Thread Starter

JonnyS

Joined Dec 28, 2015
27
One automotive engineer said, "It's easy to design a truck that will last for a million miles. Designing them to wear out in 50,000 miles is a lot more difficult." Why would he even be thinking in those terms if he wasn't faced with making sure his designs wouldn't go for a million miles?
I think they learnt the importance of selling replacement parts after the collapse of The Delorean company.
 

#12

Joined Nov 30, 2010
18,076
I think they learned the importance of selling replacement parts after the collapse of The Delorean company.
I don't think so. The automotive manufacturers knew that service is the repeat business cash cow since before DeLorean was born. The only lesson from the DeLorean failure was, "Don't get suckered into a cocaine deal when you know your competitors are terrified of your product, and will do anything to destroy your company."

I love the idea of a stainless steel body. If I could get one of those, I'd never sell a car for scrap metal prices, but I would be repairing it every year, regardless of the fact that it wouldn't be body work I was doing.
 

Thread Starter

JonnyS

Joined Dec 28, 2015
27
I love the idea of a stainless steel body. If I could get one of those, I'd never sell a car for scrap metal prices, but I would be repairing it every year, regardless of the fact that it wouldn't be body work I was doing.
Me too, should last forever and I hate rust. Just make me a 4x4 version ;)

It does seem however that anything before 1980 had a lot less components to replace, all I ever remember replacing in my 1977 LX Holden torana (electrically) was the coil and a couple of fuses. Then as soon as we got into the 80's they used the electronically controlled fuel injection system as an excuse to make everything electrical including the windows all with relays.
 

hp1729

Joined Nov 23, 2015
2,304
hi all and happy holidays. New to the forum and was wondering, when it comes to a pulsed circuit such as a low amp 12v indicators in a car why they wouldn't use a transistor to "flash" the indicators rather than a relay that you can hear clicking on and off? If I was building a similar circuit that requires a pulse with no different power requirements I would normally opt for a cheaper transistor. Perhaps they do now and I just drive an old jeep :)
Re: flashing LEDs
You can buy LEDs that flash their own.
 

Thread Starter

JonnyS

Joined Dec 28, 2015
27
Re: flashing LEDs
You can buy LEDs that flash their own.
That would look interesting when you put your hazard/warning lights on :) it would look like party lights with each indicator slightly out of time with the others. I don't think that would be legal.
 
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