Question about headlights relay in car.

[moses123]

Hello,

I was wondering if I can substitute the relay in the car head lighting system with some other device like mosfets or triacs. If it is possible what is the best way to use it?

Thanks!

 

[MikeMl]

Unless you are planing to flash the lights, why bother?

 

[debe]

If it was cheaper than Relays to switch large currents, then manufacturers would be doing it. Relays seem to be pretty reliable.

 

[ClydeCrashKop]

N channel MOSFETs ground a circuit. If the relay grounds the lights to operate, go for it. Just find one that will accept battery voltage as input. Use a pull down resistor on the input to keep it turned off when it should be off.

 

[kubeek]

Almost all cars have the chassis grounded, so the realy switches the +12V, so you would need a P channel mosfet.

 

[crutschow]

In general, the best way would be to use a solid-state DC (not AC) type relay since they will switch any type of load. But they tend to be expensive.

 

[picbits]

ST do high side MOSFETs suitable for use in cars. They even have feedback controls to show any issues such as shorts or overheats and can be driven by a logic signal. They aren't cheap but are pretty well protected against most nasties in the automotive environment.

Have a look here - some interesting high side switches

https://www.st.com/internet/automotive/subclass/1037.jsp

 

[4pyros]

ST do high side MOSFETs suitable for use in cars. They even have feedback controls to show any issues such as shorts or overheats and can be driven by a logic signal. They aren't cheap but are pretty well protected against most nasties in the automotive environment.

Have a look here - some interesting high side switches

https://www.st.com/internet/automotive/subclass/1037.jsp

Nice but most of these are low current devices that can not drive a headlight.
I did not look at all the data sheets but thay seem more geared towards powering the engine control systems.

 

[picbits]

The VN16 will happily switch 20A at 12v - more than enough for a headlamp.


They have 63 parts listed on that page that handle 20 amps or more (up to 100 amps) - that is almost half of the parts listed on that page.

How much more current do you need ?

 

[4pyros]

The VN16 will happily switch 20A at 12v - more than enough for a headlamp.


They have 63 parts listed on that page that handle 20 amps or more (up to 100 amps) - that is almost half of the parts listed on that page.

How much more current do you need ?

Sorry they should put max current on the 1st page. As I said I did not look at all the data sheets.
20 amps should do.

 

[picbits]

I must say it took me a few minutes to find the current capacities on that page but they are well worth looking at if you need an automotive high side driver. For a low side switch, a decent low rds on will do fine

 

[kinarfi]

Hello,

I was wondering if I can substitute the relay in the car head lighting system with some other device like mosfets or triacs. If it is possible what is the best way to use it?

Thanks!

YES, I have the head lights in my camper running off IRF4905 55V-74A, you apply 12 v to the gate via a resistor and the FET conducts.

 

[ClydeCrashKop]

Thanks a lot Picbits. I have been waiting years for a real high side driver. I think $3.54 for 85 amps is a great price.
https://www.digikey.com/product-detail/en/VN5E010MHTR-E/497-11471-1-ND/2713728

 

[ClydeCrashKop]

If the IRF4905 is a real, high power P channel FET, why don't they say so?
I'm not an engineer so when I see Vdss = -55V and ID= -74A, I go away disappointed.

 

[crutschow]

If the IRF4905 is a real, high power P channel FET, why don't they say so?
I'm not an engineer so when I see Vdss = -55V and ID= -74A, I go away disappointed.

Why are those values disappointing?

 

[picbits]

Thanks a lot Picbits. I have been waiting years for a real high side driver. I think $3.54 for 85 amps is a great price.
https://www.digikey.com/product-detail/en/VN5E010MHTR-E/497-11471-1-ND/2713728

Do yourself a favour though and have some way of bypassing it if it does fail - maybe a couple of terminals on the board you can jumper over with crocodile clips or an emergency switch arrangement.

Losing your headlights in the middle of nowhere may be unlikely but it's good to have a "plan b"

 

[kubeek]

Why are those values disappointing?

Do you really believe that the TO-220 package can withstand 75A for longer than a second?

 

[4pyros]

If the IRF4905 is a real, high power P channel FET, why don't they say so?
I'm not an engineer so when I see Vdss = -55V and ID= -74A, I go away disappointed.

Whats wrong with that?

 

[ClydeCrashKop]

Hi Crutschow
I never work with negative voltages and don't intend to for any of my projects. So when I see negative voltages on the data sheet, I assume that won't work for me.
Yes, I know about the word ass u me. My brain does well with logic but it doesn't go negative.

Why aren't we using that IRF4905 instead of bootstrap circuits for driving the high side on stepper motors?

 

[()blivion]

@ClydeCrashKop

The negative sign is just a notation to illustrate that those parts are the inverse of N-channel/NPN transistors. It has nothing to do with "negative voltages" as you put it. It's more about current flow perspective and proper engineering etiquette.