Little Xmas Project: Wiring LED Lights to a Vehicle Headlight

[mackconsult]

I have a little project that I am working on. Would like to put some LED lights on a wreath on the front of my vehicle. Planning on wiring it into the head lights so when the 12 volt head lights are turned on the wreath will light up on the front of the vehicle.

I bought a cheap string of 20 white led lights that run normally on 3 AA batteries. Opened up the battery case and noticed a resistor inside the case, but did not take note of it.

Figured I could wire up some resistors in series to drop the voltage down from 12 volts to 4 volts and get away with it. I had a power supply hooked up to this circuit and am able to slowly raise the voltage from 0 to 4 to 12 volts.

On my first try I used three 25 ohm resistors wired in parallel, which are then in series with the connections in the battery box going to the LED light string.

On my second try I used five 50 ohm resistors wired in parallel, which are then in series with the connections in the battery box going to the LED light string.

On my third try I used five 75 ohm resistors wired in parallel, which are then in series with the connections in the battery box going to the LED light string.

In each of the mentioned cases above smoke is coming from the resistor in the battery box. In the 75 ohm test above I was able to keep my finger on the 75 ohm resistors, so I think I am going with those for drop down. Do I need to put another resistor in parallel with the resistor that is in the battery box to distribute current across the resistors? Do they need to be of equivalent resistance?

More information: When I measured the resistance at the battery box connections it came out to be 14 ohms. Not sure where to go from here to make this work, any help appreciated ...

 

[turbo]

Go to Radio Shack and get a step-down voltage adaptor that plugs into the cigarette lighter and outputs 4-1/2V. Just shove it into the lighter-jack when you want the lights on.

 

[dlgoff]

When you parallel two 50 Ω resistors, the equivalent resistance will be 25 Ω. When you series two 50 Ω resistors, the equivalent resistance will be 100 Ω. So the more resistors you parallel, the more current will flow; hence why the smoke.

See the difference in the sourced current in these two examples:

 

[vk6kro]

You need to measure the current this device takes at 4.5 volts. Set your power supply to 4.5 volts.

Then put a current meter (ammeter) in series with the supply.

Once you know this, you can calculate the size of resistor you should place in series with the lights to run them off 12 volts. Actually, assume the car battery voltage is 14 volts.

The resistor will have (14 - 4.5) volts across it (or 9.5) and (the current you measured), passing through it.

So, suppose you measured 400 mA current. This is 0.4 amps.
The resistor size (using Ohm's Law) would be R = (9.5 volts divided by 0.4 amps), or 23.75 ohms.

Then you need to work out how much power it is going to dissipate.
Power = Voltage times current
Power = 9.5 volts times 0.4 amps, or 3.8 watts. So, you could buy an 27 ohm, 5 watt resistor.

Notice that this depends on your current measurement, so you need to substitute your figure for the 0.4 amp figure I guessed above.