LED bulbs for automobile brake & turnsignal lighting ? Resistors ?

[Riick]

I'm toying with changing rear light bulbs on my car from Incandescent to LEDs.
I know the turn signal mechanism relies on resistance to turn bulb on and off.

What people are selling to add resistance to the circuit are 25w and 50w
wirewound resistors in pretty aluminum casings, resistance is between 6 and 8 ohms.

I measured resistance of existing bulbs, which, *cold* measure around 2-3 ohms.
Have no idea what resistance might be once heated up in circuit, but willing to guess at 6-8 ohms
I'd think the draw of these small incandescent bulbs can't be anywhere near 25w, let alone 50w.
-then again I've been wrong before-
Are the guys selling this stuff ( at anywhere from $10 to $50+ per pair ! ) selling overbuilt expensive junk
to credulous know-nothings?, or would you really need ability to handle 50w draw? Maybe I'd better tap
into circuit and see what the draw is, or better, there *have* to be tables that show what a 921 or 7443 bulb draws.

Thanks in advance for your thoughts.

 

[(*steve*)]

Resistors are not too expensive.

1. Find out what the wattage of the old bulb was
2. Find out the wattage of the new LED bulb
3. Subtract 2 from 1 (this gives you the maximum amount of power you need to waste to keep the flasher happy.
4. The resistance you require is 144 divided by this power. (because P = V^2/R)
   
5. Go find a pair of resistors of approximately this resistance and at least double the power calculated in (3) above.

Get all the figures, do the calculations, let us check them, then tell us where you are and we may be able to recommend a supplier.

$10 per pair would be reasonable, $50 isn't.

Let's assume you're replacing a 15W bulb with a 3W LED.

1. old = 15
2. new = 3
3. 15 = 3 = 12W to dissipate
4. 144/12 = 12
   
5. Need to find two 12 ohm 24W resistors (wattage can be greater, and resistance needs to be within about 20%)

So if I was doing this, I would get two of these. They are the correct resistance. Whilst I could get 25W for a few cents less, going for a higher power means they'll run cooler. The supplier might be very wrong for you, and the value is probably also wrong, but it gives you an idea that $10 per pair is probably not too bad -- assuming they're the correct resistance and power).

When you come to connecting them, you'll need to connect them to some metal, and also to insulate the leads. I recommend you solder the wires to the resistor and then put some heatshrink tubing over the connections.