Using resistors will I be able to hook up more then 3 leds on a 12v adapter?
Yes. If you hook the LEDs up in parallel and use a resistor to limit current. You can use as many LEDs as your power supply can handle.
LEDs by nature will drop roughly 3.6 v across their leads. With a 12 v power supply that leaves 8.4 volts that you have to figure out what to do with. This is where the resistor comes in. By adding the resistor in series with the LED
, you can give the unwanted voltage a place to go and be used as heat. It also gives you a way to limit the amount of current the LED
gets passed through it. In this drawing you see a 500 ohm resistor in series with the LED
. Since we know the voltage across the resistor and the value of the resistor itself we can use ohm's law to calculate the current through it. 8.4v/500 ohm=.0168A (16.8mA) This is well below the max rating of the LED
of 30 mA.
In our circuit below you can see that the power supply is rated at 1000mA this means the we can use 1000mA/16.8mA=59 LEDs wired up the same way and the power supply will handle it.
And yes, the amount of current passing through the LED
is what controls it's brightness. You can even exceed the rated max if you like by using a smaller resistor, the device will just not last as long.
Keep in mind that these values only exist in the perfect world. Each resistor will not likely be 500 ohm and even the LEDs will not all drop 3.6v either. Some experimenting is always required.