## Step 2: [CALCULATIONS]

**FV**(Forward Voltage) - This is the voltage used by each separate LED. It is expressed as a range, so a minimum and maximum value is present.

**AC MAX/MIN**- AC Mains are not always at a constant voltage and are not always the same across a whole house. There is actually a range present. In the US, the range is 110-125VAC. In other nations, the range is 220-250VAC.

**EQUATIONS**

[AC MAX] X 1.4 = A

A / [FV MAX] = [# LEDs]

**CHECK**

[AC MIN] X 1.4 = B

B / [# LEDs] = C

C represents the forward voltage and must be within the range.

Your final result represents the number of LEDs you can put in each series. Think of this as a basic unit. The total amount of LEDs on the light bulb must be a multiple of this number. In each "unit," the LEDs connect positive to negative in order to distribute the voltage. All the series may then be connected together, positive to positive and negative to negative. Below is a sample of my calculations.

**EQUATIONS**

125 X 1.4 = 175

175 / 3.8 =

**46**

**CHECK**

110 X 1.4 = 154

154 / 46 = 3.3478

C is in range. (exhale)

**Signing Up**

I did everything following your diagram and it was a nice glitter show for a microsecond. It appears about 10 LED's are dead.

What did I do wrong?

Lance

I was trying to use the LED calculator at http://led.linear1.org/led.wiz to give me an idea, using 120volts as the input voltage, and trying different configurations, and it keeps telling me I need giant resistors at multiple wattages.

I'm just a little wary of spending lots of time to make a light and then have it over-driven and die after a handful of hours. I greatly appreciate the answers though.

I've taken a look at the Linear calculator and it seems to work okay (except that won't calculate for UK mains voltages - they're too high apparantly and so I'll just scale the figures anyway). Try:

Source voltage 150

diode forward voltage 2

diode forward current (mA) 30

number of LEDs in your array 72

You should get:

- 220 ohm resistor dissipates 198 mW
- the wizard thinks 1/2W resistors are needed for your application
- together, all resistors dissipate 198 mW
- together, the diodes dissipate 4320 mW
- total power dissipated by the array is 4518 mW
- the array draws current of 30 mA from the source.

Is this similar to your calculations?The trick is to build a LED array that has a comparable forward voltage to the supply so the voltage differential is small so only small resistor is needed.

I hope this helps.

Matt