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How to work out what resistor to use

As I’m new to the world of LEDs, every time I make a circuit, I have to figure out all over again what resistor I need.  It generally takes a while, so, for my benefit as much as anyone else’s, I give you: The Process.

1. What colour LED are you using?

If it’s red, orange, or yellow, it’ll need 2.2 volts (v).

If it’s any other colour (including colour change LEDs and any white LED), it’ll need 3.4v. Both of these voltages are per LED.

2. Then work out how much voltage all your LEDs will need. (For example, for a circuit with 3 white LEDs: 3 x 3.4v = 10.2v)

3. The next step is to work out the residual voltage. (This means the voltage leftover from your power supply that your LEDs won’t need.) As we’re working with LEDs, the power supply will usually be supplying 12v. So the equation is: power supply voltage (12v) – voltage used by LEDs  (10.2v) = 1.8v residual voltage.

4. Divide this number by current. (This example is using 5mm LEDs, which always use 0.025 amps of current.) 1.8 ÷ 0.025 = 72. This means you need a 72 ohm resistor.

If you don’t have the exact resistor you need, then it’s always better to have a resistor that’s a higher value than you need (for example, a 85 ohm resistor instead of a 72 ohm one), rather than a lower one. A lower one won’t provide enough resistance, and reduce the  lifespan of your LEDs. You can place two resistors in a line (“in series”) to add the values up (for example, two 40 ohm resistors in a line would suffice in this example). You can also place two resistors in a stack (“in parallel”) (to do this, lie one above the other, and twist the legs together), which makes the values divide. (In this example, you could do this with two 160 ohm resistors; 160÷2=80.)

For more information about this kind of thing, have a look at the knowledge base on our website. Dave’s written a bunch of really useful articles on many (LED-related) topics; it’s a great learning resource.

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This entry was posted on August 16, 2012 by in LEDs and tagged , , .

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