What do I need to know? Such as, how would these be best wired up? And what resistance would I need?
It's for a computer case modification I'm planning on doing in the near future, Will be powered from a single 12v source (Computer Powersupply).
Electronic – 20 3.2v – 3.8v LEDs in a line on a 12v circuit
12vledresistors
Related Solutions
By decoding your question, this is what I arrive at:
You are using the two resistors as a voltage divider, with the LEDs wired in parallel connected to the junction of these two resistors.
simulate this circuit – Schematic created using CircuitLab
This is not a valid approach, the voltage divider will not provide the 1:4 (approximately) voltage division your calculations indicate.
Firstly, either apply the three LEDs in series, with one resistor in series, or if you must have them in parallel, each will need a series resistor.
The above indicative values of resistors are assuming 20 mA per LED as optimal. Do your own calculations for your specific LEDs.
For a ready to use online calculator, use an "LED resistance calculator" instead of a voltage divider one. This one even provides the correct schematic or wiring diagram as you prefer, to wire the LEDs up.
If you have no data sheet information about the expected current of the LED (because you extracted it from an existing appliance, or something like that), then you should measure the voltage and the current while still in its original position and use those values for further calculations.
I do not recommend that you base your calculations on the nominal voltage and nominal wattage figures found in non-technical documents.
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Best Answer
To avoid having to burn too much power in a load resistor, you should put some of the LEDs in series. For example:
Assume each LED drops 3.5V. Then, the resistors will have (12V - (3.5V * 3)) = 1.5V across them. If you want the current through the LEDs to be 10mA, then the resistor value should be (1.5V / 10mA) = 150 Ohms.
Each resistor in this case would be dissipating (i^2 * R) = (0.01 mA) * (150 Ohms) = 0.015 W, so any size resistor (1/8 W, 1/4 W, etc) would have no problem handling the power.
Note that this diagram has 21 LEDs, while you asked for 20. To keep things simple and symmetric, it would be easiest to just install a 21st LED in the circuit and hide it, if you really only want 20 showing.