headlamp flashing circuit, modern cars, schottky?

[timthumb]

hi all

i am thinking about making a device which fitted to any modern car will illuminate the headlights for a delay after the ignition is switched off, but i am concerned about modern cars no longer being wired in parallel (possibly things like the CANBUS systems etc).

I am guessing that it's not really practical to connect the +ve terminals of the left headlight with the right headlight, in case it flags up some error on the dashboard

my solution would be a high current diode on each light

My Relay ----->|---- (+ headlight 1) -----Gnd
/
Car's circuit --->|---


My Relay ----->|---- (+ headlight 2) -----Gnd
/
Car's circuit --->|---


which would effectively isolate the lights from each other. However this presents another problem in that a standard 55 watt 12 volt car halogen headlamp is (obviously) nearly 5 amps, and I can't find a diode suitable, that isn't a schottky one

so the question is - does anyone know of a suitable diode?

i assume schottkys aren't applicable because of their low forward voltage, and low reverse current handling (they will need to handle considerable reverse current i would have thought).

i guess the specs would need to be (but i could be wrong, im very much a novice):

-Vfwd = 12+vdc
-Vrev = 12+vdc
-Ifwd = >5A
-Irev = >5A



thanks!!

 

[timthumb]

If it is of use, this is the circuit I am planning to build:

P+Hc and P-Hc are high current permanent supplies of 12VDC
C+ is the positive feed from the courtesy (interior ceiling) light
H1 and H2 are the two headlight bulbs, each at 55W

The relay is a standard 12VDC SPDT with 10A switch current

**broken link removed**

Any critique welcomed!!

 

[picbits]

Firstly the inrush current of a 55w halogen bulb is a lot greater than 5 amps ......

Personally assuming one side of the headlight is grounded, I'd use an SPDT relay with the common connected to the headlight,the normally open side of the switched terminals connected to +12v (through a fuse of course) and normally closed side connected to the existing feed to the headlight.

When the relay is energised, it will break the existing wiring and connect the headlight directly to +12v. When the relay is off the system is wired in the same way as it should be. You can do this with both sides.

Failure modes are fine - if the relay fails to actuate, your headlights still work as normal. If it sticks in the "on" position, your headlights stay on and you get a flat battery but there is no failure where you will lose your headlights while you're driving.

 

[picbits]

If fact, with brain now into gear you could do it with a DPDT relay - same method as above but use the second set of terminals to drive the second headlamp.

I'd go for a relay with at least 15-20 amps of switching current.

 

[timthumb]

Sweet idea, DPDT relay to do it. One other Q tho, if modern cars have a "light bulb blown" circuit by measuring the resistance across the bulb constantly (even when "off") how would i incorporate that into the above circuit, assuming the ground remains grounded? i guess one solution would be to have a resistor, say about 300R, across the relay from the permanent + to the headlight feed, but then that might basically prevent the whole circuit from isolating the car's circuitry from this?

 

[picbits]

You'll probably find the car doesn't speak much to the headlights with the ignition off.

When the relay is in its off position the blown lamp sensing circuit will see it as a valid bulb anyway - try it before you try and make life harder for yourself

 

[timthumb]

indeed! just need to get myself a high current DPDT relay now... ebay first, then maplin (the usual order of things)...

thanks picbits for the help with this!