***** LED's IN SERIES *****

So.. To Calculate Resistor Value for LED's in Series..

LED Data Sheet : 2V Vf, 0.02A If, Two LED's of the same kind, with a supply voltage of 5vdc.

With LED's in series, you add the forward voltage of all the LED's in series, and the LED's forward current stays the same.

**Step 1.**Add LED's total Forward Voltage (VF). 2 x 2 = 4vdc.

**Step 2.**Subtract LED's Forward Voltage from Supply voltage to find the amount of voltage to drop across the resistor.

4Vdc - 5Vdc = 1Vdc.

**Step 3.**To find the resistor value for the LED's in series, divide the Voltage to be dropped across the resistor by the forward current of the LED's.

1Vdc / 0.02A = 50Ω (I will need a 50Ω resistor)

***** LED's IN PARALLEL *****

So.. To Calculate Resistor Value for LED's in Parallel..

LED Data Sheet : 3.5Vdc Vf, 0.035A If, Twenty LED's of the same kind, with a supply voltage of 5Vdc.

With LED's in Parallel, you add the forward current, and the LED's Forward Voltage Stays the same.

**Step 1**. Add LED's Forward Current ( IF). 0.035A x 20 = 0.7A

**Step 2.**Subtract LED's Forward Voltage from Supply Voltage, to find how much voltage is to be dropped across the resistor.

3.5Vdc (VF) - 5Vdc = 1.5Vdc

**Step 3.**To find the resistor value for the LED's in parallel, divide the voltage to be dropped across the resistor by the forward current of the LED's.

1.5Vdc / 0.7A = 2.142Ω (I will need a 2.15Ω resistor)

Doesn't exist..? Sure it does.. Mouser Electronics Part # 660-MF1/4DCT52R2R15F (2.15Ω, 250mW, 1% Tolerance, KOA Speer, Metal Film Resistor, $00.10 ea, & 1,754 Can Ship Immediately)

If that's all correct.. then how do you calculate the Power dissipation of the LED's each, LED Total, and the Resistor Power Dissipation in Watts?

Knowing that, you will be able to reinsure that 1. The Resistor you picked out 1/4 Watt, is good enough for your simple circuit, and 2. It also shows that you have a good understanding of what's going on with your LED and Resistor. Not know it, will cost you more money.

**1.**You will not only be charged $00.10 per resistor, but also S&H charges again.

**2.**You might also have to buy more LED's because you didn't know what you were doing in the first place.

**3.**You will have to wait again.. For your electrical components to come in the mail.. So.. If you were working on this for someone, you've not only wasted your time and money, but you've wasted the clients time, and you spent more than you had to on the project.

**NOTE :**Since we choose a higher resistor value, the current for the LED has went down.

***** LED's IN SERIES, POWER DISSIPATION *****

So.. Let's calculate LED and Resistor Power Dissipation, shall we.

To Calculate Power Dissipation of each LED using the Data we already calculated in the above text.

**LED's In Series. SIngle LED Power Dissipation.**

Step 1.Using Ohm's Law, find the Power Dissipated in Watts. (LED Forward Voltage x LED Forward Current = Watts)

Step 1.

2Vdc x 0.02A = 0.04 Watts, or.. Since there's 1000 Milliwatts in 1 Watt, it's a power dissipation of 40mW for a single LED.

**LED's in Series. Total LED Power Dissipation.**

Step 1.Using Ohm's Law, find the Power Dissipated in Watts. (LED Forward Voltage x Number of LED's in series) x LED Forward Current = Watts

Step 1.

(2Vdc x 2ct.) = 4Vdc x 0.02A = 0.08 Watts. or.. Since there's 1000 Milliwatts in 1 Watt, it's a power dissipation of 80mW for all the LED's total.

**Resistor for LED's in Series Power Dissipation.**

Step 1.Using Ohm's Law, find the Power Dissipated in Watts for the Resistor. (LED Total Forward Current Squared divided by the Resistor Value)

Step 1.

(0.02A x 0.02A) = 0.0004A * 50Ω = 0.02 Watts. or.. Since there's 1000 Milliwatts in 1 Watt, it's a power dissipation of 20mW for the Resistor.

**Can someone please verify that the above calculations are correct ?**

***** LED's IN PARALLEL, POWER DISSIPATION *****

LED's in Parallel.. Single LED Power Dissipation.

**Step 1.**To find the Power dissipated in watts for a single LED in Parallel, you multiple the LED Forward Voltage by the LED Forward Current.

3.5vdc x 0.035A = 0.1225 Watts.

LED's in Parallel.. Total LED Power Dissipation.

**Step 1.**To find the total power dissipated in watts for ALL THE LED's in Parallel, you multiple the Total Forward Current of all the LED's in parallel by the LED's Forward Voltage.

(0.035A x 20ct.) x 3.5Vdc = 2.45 Watts

LED's in Parallel.. Total Resistor Power Dissipation.

Step 1. To find the total power dissipated in watts for the Resistor. It's LED's Total Forward Current Squared multiplied by the Resistor Value used.

In this case, we went with a 1.5Ω Resistor.

In this case, we went with a 1.5Ω Resistor.

So it's LED's Total Forward Current Squared x Resistor Value used.

(0.7A x 0.7A) x 2.15Ω = 1.0535 Watts

*So it looks to me that the part number that I picked out for my project just isn't going to cut it..*

That's ok.. I'll just use a 2.2Ω, 2 Watt Resistor from Mouser Electronics. Mouser Part # : 594-5083NW2R200JA100

Or.. I could also use.. Mouser Part # : 594-5093NW2R200J , which is also a 2.2Ω Resistor, but with a power rating of 3 Watts.

So.. the calculations for the Resistors Power dissipation would be something like this..

(0.7A x 0.7A) x 2.20Ω = 1.078 Watts. PERFECT.. Right..?

Or.. Using Ohm's Law yet again.. Instead of using the current.. I'll use the voltage to be dropped across the resistor and the resistors value, to find out exactly how much power is to be dissipated on the resistor.

So.. according to Ohm's Law.. There's going to be 1.5Vdc dropped across the resistor. Using a 2.2Ω resistor the formula is as follows.

Power = Voltage Squared divided by the resistance of the resistor.

(1.5Vdc x 1.5Vdc) / 2.2Ω = 1.0227 Watts. PERFECT

Using Ohms Law once again, using the same information.. 1.5Vdc & 2.2Ω, let's find out how much Current there will be.

Ohm's law says that Current = Voltage divided by Resistance.

1.5Vdc dropped across the resistor x 2.20Ω = 0.68 Amps.

THAT'S AWESOME !!! Our LED will be at..

0.68A / 0.7A = 0.97402 x 100 = 97.402% Brightness !!!

Awesome right..?

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