Simple LED wiring help

[guitarguy89]

Alright so,
I've done a few small wiring projects before but I can't really figure out what's going on.
I just bought my girlfriends son one of those 12V ride on cars for kids. It works great and he loves it. But I got the wise idea to add lights to it. So I drilled out the backs of the headlights and side mirrors, as well as the brake light in the spoiler. Altogether I'm running 6 5mm red led's and 2 jumbo white ones. (I believe they are 10mm) so when I hook up the power to the 12v battery, nothing.
Do I just need more power?
I have all of the positives wired together and all the negatives wired together

[pebe]

Have you got a specification for the LEDs? Voltage? Current?

[JMACgyver]

You wrote:
Altogether I'm running 6 5mm red led's and 2 jumbo white ones. (I believe they are 10mm) so when I hook up the power to the 12v battery, nothing.
Do I just need more power?
I have all of the positives wired together and all the negatives wired together.

Greetings,

My guess is you need *less* power. Assuming these are normal high-brightness LEDs all wired in parallel, they can take a higher then normal current, but voltage needed should max out at 5V.
In my experience, a normal Power Wheels battery supplies 12V at 4 to 6 amps. I would expect them to burn out VERY fast. I would visit some sites containing LED calculators to figure out how to wire them to take 12V and what size current limiting resistor you will need.

--Electro--
aka The Other David

[hevans1944]

I have all of the positives wired together and all the negatives wired together

That is the problem.

LEDs are not interchangeable with incandescent light bulbs (which you CAN wire in parallel if all bulbs are rated at the same voltage). LEDs need a current-limiting resistor wired in series with the LED. Each LED needs this resistor. You cannot take the paralleled LEDs, as you have currently wired them, and use just one current-limiting resistor. As a rough rule-of-thumb each LED will drop about 2 volts when it is conducting. If the battery provides 12 volts, you need to drop 12 - 2 = 10 volts across the resistor to limit the current supplied to the LED.

What value resistor to use? That depends on the LED current as well as the actual LED voltage drop, which you should measure. See http://dangerousprototypes.com/docs/Basic_Light_Emitting_Diode_guidearticle. A typical current might be 50 mA (0.050 A). So to drop 10 volts at 50 mA, you need a resistor with a value of E/I = 10/0.05 =200 ohms. This resistor will dissipate a power of E x I =10 x 0.05 = .5 watts. So pick a 1 watt (or larger) resistor.

The two white-light LEDs probably draw more current than the red LEDs, so their series resistor will be smaller in value. You can get in the right ball park by using a handful of resistors and a multimeter to measure the current in each LED when it is powered by the battery. Radio Shack sells an assortment of 1/2 watt resistors that you can start out with. If you know the maximum current rating of each LED, no calculations are necessary. Just start with a large value resistor, say 1000 ohms, and work your way down in value while observing on the multimeter the current through the LED. Note that the multimeter is connected in series with the LED and resistor when measuring current. If using an analog multimeter, pay attention to polarity. When a selected resistor results in a current near the rated current for that LED, that is the resistor value to use.

You do need to pay attention to how much power is being dissipated in the resistor, so as not to burn it up. Do the tests quickly until you find the right value resistor, then measure the voltage drop across the resistor and multiply by the current to determine the power dissipated in the resistor. Pick a resistor of the next higher power rating.

If there is no fancy switching involved, i.e., all LEDs light at the same time, you can wire the four red LEDs in series with just one current limiting resistor. Pay attention to LED polarity: anode of one to the cathode of the next. Similarly, you can wire the two white "headlight" LEDs in series with just one current limiting resistor. This series-parallel combination of LEDs can then be connected to your battery terminals.

Again, the simple way to determine the resistor value is to just measure the LED current with an inexpensive multimeter. Start with a higher value resistor and work your way down until the desired LED current is reached.