LED circuit design?

Question

I am designing a science project. I want to connect a red LED, a yellow LED, and a blue LED to a DC voltage source I want to demonstrate the effect on the lights when the voltage is reduced. Please explain the math to calculate the ohms and which wiring method is the simplest to use that will not adversely effect the life of the LED.

Answer 1

Assuming: Voltage drop across one LED is 1.2 volts, and the MAX current for the LED is 15mA. Also assume your max applied voltage is 15 volts.

Put 3 LEDs in series, and use a resistor to limit the current to 15mA max.

15Volts - (3*1.2) / 15mA = Resistance

Resistance = 760 ohms

If your applied voltage changes, adjust the formula accordingly.

Answer 2

Based on what you are describing here, I suggest that you connect all your LEDs in parallel to a signal source for your purposes. This is due to that each color of LED has a different Forward Voltage (Vf) with it will light up.

Red & Yellow LEDs have a typical Vf of 1.8 to 2.5 Volts, 2.2 Volts typically.

Blue & White LEDs have a typical Vf of 3.0 to 3.5 Volts, 3.2 Volts typically.

For most applications a LED should have a Forward Current (If) of .02 to .03 Amperes.

Here is a formula to help you calculate your resistor values for any given LED color.

Vr (ohms) = (Vs – Vf) / If

Here’s an example for you:

Vs = 12 Volts
Vf = 2.2 Volts
If = .02 Amps

(12 – 2.2) / .02 = 490 ohms

The value that you obtained from the above formula is to be used as a guide for selecting your resistor value. Resistors are commonly available to 2 tolerances, 1% & 5%. Select the closest resistor value in that is available in the desired tolerance.

Also be certain to select the correct wattage rating for application.

To find the circuits power dissipation use the following formula:

Watts = I(2) X R or V X R

Select the resistor’s wattage based your answer from the above formula. Resistors also come in standard typical wattages, these are 1/8, 1/4, 1/2, 1, 2, etc… Ways select the larger standard wattage value rather than the preceding value.

Answer 3

I would wire each LED in series with a resitor and apply the DC source across the two. That way, you'd have some current limting and you could tell what the LED current was by measuring the voltage across the resistor.

Answer 4

Assumptions: a million) Vce is negligible with the BJT on 2) beta for the BJT is extreme adequate, which it in all probability is 3) the dynamic resistance of the LED is negligible (or the 2V given is for the proper cutting-edge you're finding for) Then that's an basic cutting-edge calculation: I = V / R, and V = 13V since the different 2V drop around the LED. in case you decide directly to incorporate Vce, basically subtract that much extra out of your voltage.