Now, like any project with electricity, certain calculations are necessary. Although the equations might look a bit threatening, I can assure you that it's quite simple. Before starting your calculations, make sure you have your LEDs' detailed specifications from your supplier.

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.

[AC MAX] X 1.4 = A
A / [FV MAX] = [# LEDs]

[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.

125 X 1.4 = 175
175 / 3.8 = 46

110 X 1.4 = 154
154 / 46 = 3.3478
C is in range. (exhale)
LGProspects3 years ago
So I did the build I am in the US so based everything off here. 5MM leds, 3.0-3.2 FV. Did the math and got 55 LED's needed. During the check it came back as 2.8. So I figured it was UNDER voltage at 55 so went for it.

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?

Questions: in this series crt, 110-125Vdc/1.5A is initially applied to 1st Led (rated at 3.2-3.8Vdc/30mA, doesn't this cause damage to the Led? Thanks.
meissler6 years ago
Could you just use any 12V power chord (like a charger for a computer or something) and hook the +/- ends up to the right areas? Of course use resistors too. Would that work or no, not really?
chrwei6 years ago
where does the 1.4 value come from in AC MAX X 1.4 = A ?
wirecutter6 years ago
GOOD Ideas. But the thing about LED's is that as the current through the diodes heats them up the voltage across them drops, then the current rises, the diodes gets warmer, and warmer, the voltage across drops till PHUT! its called thermal runaway As an aside if you connect each LED to an identical LED but the other way round you dont need to have a bridge rectifier to give a dc supply just ac.
Hardwyre6 years ago
This is an excellent instructable; and thank you for answering my other question, just another one for ya. I noticed you made three sizes of lights; looking again at your calculations I THINK I'm understanding how you did it. For the small one you did one series of 46 LEDs, the mid was what, 2 series? And the large was 3? As long as you're using series of the number of LEDs you calculated, there shouldn't be much worry about blowing them, correct? I'm curious how the issue of maximum milliamps is handled.
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.
Al16 years ago
That's a good idea about using LEDs for the bridge rectifier. The only slight downside (I could see) is that those four LEDs would only illuminate for half of the AC cycle as compared to the main bank of LEDs so they may appear to flicker or appear dimmer.
charlieb0006 years ago
your rectifier can consist of leds too, in this case you have two arrays so you would need two led rectifiers