I'm trying to design a protection circuit for my car. It's purpose will be to protect from shorts and surges from trailer wiring: basically running lights, brake lights and blinkers. I've come across several designs but am really insure as to what direction to go. Thank you in advance for the guidance.

 

Fuses protect from shorts. What surges do you expect from an incandescent trailer lighting circuit?

Ken

 

You cannot directly wire a trailer socket into most modern cars because:

1) Most new(ish) cars employ bulb failure warning, which precisely monitors the circuit loads and quite a few of them eg Mercedes, VW/Audi, Peugeot/Citroen, Ford employ bulb substitution in the event of a bulb failure.
Also some now use the same filaments for rear lights and brake lights using PWM to vary the bulbs intensity. eg 2005 --> VW/Audi

2) Some rear lamps now only employ LED lamps and the current drawn by the trailer lamps will damage or shut down the associated electronics.

There are commercially available trailer interface modules which are independently powered and effectively draw a minuscule current from the relevant circuit to switch on either a relay or Mosfet.

You can easily make something similar with suitable Mosfets , P channel are best as you will be switching on the + side. You will probably need some sort of bleed resistor to earth at the gates to prevent any possibility of any trailer circuit inadvertently latching on.
I will leave it to some of the more expert people on here to suggest suitable components & electronics circuits. You should be safe enough drawing up to 100mA from any of the car circuits.Check with your car manufacturer if you are in any doubt.

 

I believe this discussion is a no-no per forum rules.

 

Well sorry to say. But the site owners. Have decided not to allow any automotive topics on this site. So this thread will be closed. But you will be more than welcome to discuss any topic that allowed by the terms of service http://www.allaboutcircuits.com/l_tos.html
You may also take a look here
http://forum.allaboutcircuits.com/showthread.php?t=54400

 

To be fair to the original poster, he is seeking advice on "overload protection for a car with trailer" . "Automotive Topics" is quite different to "Automotive Modifications".
I am 100% in agreement with the banning of discussion of "automotive modifications" on this site.It is potentially dangerous and bad advice could lead to some very expensive "Cock-up", even death or injury.

However as electronics are so pervasive in modern vehicles I can see nothing wrong with a poster making enquiries or requesting information on "Automotive Topics". Surely this is a great way to help their understanding and broaden their knowledge.
It is not for me as a fairly recent member to suggest what can or cannot be discussed but I think we need clarity as to what is and isn't allowed.
I enjoy this site and would not like to stray outside the terms of service.

 

To be fair to the original poster, he is seeking advice on "overload protection for a car with trailer" . "Automotive Topics" is quite different to "Automotive Modifications".
I am 100% in agreement with the banning of discussion of "automotive modifications" on this site.It is potentially dangerous and bad advice could lead to some very expensive "Cock-up", even death or injury.

However as electronics are so pervasive in modern vehicles I can see nothing wrong with a poster making enquiries or requesting information on "Automotive Topics". Surely this is a great way to help their understanding and broaden their knowledge.
It is not for me as a fairly recent member to suggest what can or cannot be discussed but I think we need clarity as to what is and isn't allowed.
I enjoy this site and would not like to stray outside the terms of service.

I've seen the trend of posts which are closed and posts which are allowed to stay open so I know where "the line" is and this post is definately across it. Your post #3 was really good post; it told why it isn't a good idea, and left it at that. That is more closure than what is usually given to these type of threads and OP is lucky to have read your words. I think this thread has fallen through the cracks and would ordinarily have been shut down already. It will be soon (no, that doesn't mean I'm going to flag it).