Resistance requirements for LED switch position indicator lights

Tom MohrOGR Forum Member DIGITAL SUBSCRIBER

Let’s say I want to use a 9V / 5A DC power supply to operate tortoise switch machines, and also to power some bicolor LEDs to indicate the switch position(s). According to the LED specs, each LED pulls 20mA of current. The bicolor LEDs use 2-2.2V (for the red light) and 3-3.2V (for the green light). Can I use inline resistors (of the proper values) to lower the source voltage (9V) for the two different lights? This is an Ohm’s Law exercise, right (V = I x R) ?

Does this set up of the problem to get the correct resistance for each color seem logical?
Using the midrange value of the voltage for the red light:

2.1V = 0.02A x (?)Ω. Then 2.1V / 0.02A = (?) Ω, and (?) = 105 Ω

Likewise, for the green light:

3.1V = 0.02A x (?)Ω; 3.1V / 0.02A = (?)Ω, and (?) = 155 Ω

Does this seem to be right? (I haven’t used higher maths for awhile…) 😏

3/27/255:42 PM

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Mellow Hudson MikeOGR Forum Member DIGITAL SUBSCRIBER

Yes it is Ohm's Law. But not your way.

R = V / I, where V is the difference between 9v and the voltage of the LED, and I is the current draw of the LED.

Try this:

A.) Red

Step 1.) 9 - 2.1 = 6.9

Step 2.) 6.9/0.020 = 345 ohms

B.) Green

Step 1.) 9 - 3.1 = 5.9

Step 2.) 5.9/0.020 = 295 ohms

When you buy your resistors pick the closest standard resistor value for each, i.e. either 330 or 390 for 345 above, and either 270 or 300 for 295 above.

Mike

3/27/256:16 PM