high level brake light switchable to rain light (racing)

[mcerbm]

I have a LED rain light which is required for the time attack style championship I am doing at knockhill this year. Its mounted on the roll bar where a high level brake light would be placed.

The rain light is planned to be soldered into the fog light wiring at the back of the car. It is the MSA / FIA approved type and I was wondering if it required a resistor and if so what size? also if I wanted to use it for a high level brake light also how could it be wired up so the brake light wires would supply power if I'm on the roads, but I wouldnt want to turn the fog light on and have this inadvertantly turning the brake lights on as well since its a common circuit. would that be a diode I would need? excuse my lack of electrical knowledge!

 

[Jonathan Kay]

Do you want to actively switch it between road and track modes, or have it so that it always works whichever circuit is activated?

Jonathan

[mcerbm]

ideally both active, but if its not easily done I would accept a switch method would be a lot easier to implement.

[elie boone]

i am no elec. expert but a diode in both power feeds to the light would work i think.

[Jonathan Kay]

Yes... hoping that an expert will be along in a minute with the specs, and advice on whether a fuse or relay should be used, added or moved.

If it's permanently wired both ways is there any problem with road legality and the MoT? My first thought is that it's OK but it's worth a moment's thought.

Jonathan

[Roger Ford]

I have mine switchable between high level brake light and rain light.

Yes, it does need diodes in the feeds. Otherwise the rain light will light up the brakes, and vice versa.

The rain light will already have the necessary resistors in circuit. The diodes will drop the supply voltage a bit, typically half a volt, so if you were doing it from scratch (I did) it would be preferable to use a slightly lower resistance. But it probably won't matter - it will be slightly dimmer, but not much.

Get schottky diodes as they have a lower voltage drop than other types. 20V ones will be sufficient, then you need to figure out what amp rating you need based on your rain light.  I don't recall how many amps mine uses, but I suspect 1A is probably sufficient for most.

Edit: Having actually read the requirements, I see that you'd like it working both ways.  That actually simplifies things because you don't need an extra feed from the switch in the dash.  All you need to do is take the brake light feed and the rain light feed, connect a diode to each and join them together after the diodes. Then where they are joined becomes the feed into the dual-purpose light.

Checked the specs of my rain light (the simple Maplin one, widely sold by Motorsports suppliers for two or three times the price with a resistor in circuit) and it takes a maximum current of 0.25A.  So it's unlikely other rain lights will use more than 1A. Therefore these diodes should be fine. The striped end is the positive, so should go at the light end of the circuit. Note the "forward voltage" of 0.45V - that's what you'll be losing from the input voltage.

[mcerbm]

the maplin rain light you linked to is the same one I have but I cant remember where I got it from (had it for over a year) and therefore if it has a built in resistor. I'm guessing not as I bought a 22ohm 3W resistor at the same time. If I have a dual function light should I lower the resistor is this the figure you had in mind?

[Roger Ford]

You can get away with an 18 ohm resistor at full voltage without exceeding the max current, so will definitely be fine with the diode. Would need to figure out the calculation, but I'm pretty sure a 15 ohm (15R) would be OK as well, and give you better brightness.