Here, in part 3, we combine what we learned before to design a LED light that operated directly off AC mains.
Warning: AC mains is hundreds of volts, and is potentially lethal. Please take all necessary precautions before you start working with it!
Step 1: The no-transformer transformer.
Vac / 3.3
to give us the number of LEDs we need to be able to properly handle the power without additional resistors and other parts.
What if we bypass the transformer completely and consider AC mains? In some ways it is simpler - the voltage from transformers could vary greatly with the load we put on it, whereas AC mains are much more stable.
If we use the 110v standard of the US, we first calculate the peak voltage, 1.4 * 110 = 156 and we can find the number of LEDs it can support:
156 / 3.3 = 47 LEDs
So, does that mean that if we put 47 LEDs in series, we can run the whole string directly off a 110v AC socket?
The answer is Yes! As long as we maintain the voltage across each LED at 3.5v or less, it will operate within its limits.
But then, let's not forget that for each positive cycle, there is a negative cycle! That means we need a mirror circuit like in (1).
Wow, that's an awful lot of bulbs!
However, if we add a blocking diode like in circuit (2), then we can safely operate our circuit. The 1N4003 is capable of handling 200 volts so is fine for US power.
For EU countries, the magic number is 103 LEDs (double if you want to use both cycles) and the diode for ckt (2) should be a 1N4004 or better.
Step 2: Pushing the envelope
With a simple part called a Bridge Rectifier this can happen. This device is actually 4 diodes connected in a criss-cross way to make both cycles go in the same direction. Electronic fans will know this as part of the 'Full-wave rectification' circuit (as opposed to Half-wave).
With this addition, our LEDs will be turning on twice as often and we WILL get twice as much light from them.
Step 3: Build time!
You will need:
Lots of white LEDs - naturally! And TEST them all!
AC line cord
1N4003 diode or 200volt bridge rectifier
The first picture is what my circuit looks like when finished. Quick eyes will note that there are only 42 LEDs on board. Because of the need to accomodate the bridge on the board, and because of the relatively stable nature of our mains, we can run our lights a tad over 20mA.
The Bridge has 4 leads: 2 marked (~), a (+) positive and a (-) negative. The (~) ones go to AC Mains.
Start by connecting the Bridge (+) to the longer (+) lead of the first LED, then take the short lead to the long lead of the next LED. Do 1 row, double and triple check before soldering! Work your way down, ALWAYS connecting shorter to longer.
I have additional pictures below showing the various stages of completion. Print them out to help you do the wiring.
Step 4: Behold!
And there is light!
Because of the hazardous nature of the components when plugged in, I covered the circuit board with a triple layer of parchment paper, which has a good dielectric value, and can withstand over 400F of heat.
Then I mounted the board on the lid of a take out container, using a foam spacer from a DVD spindle, with a cutout for the power cord.
The light output is equivalent to a 40-watt frosted bulb, but the container is barely warm.
Remember: Always unplug the circuit before you touch any exposed parts.
Also, the LEDs will be running close to their rated current, which could mean temperatures as high as 85C on their surfaces.
Step 5: Variations
You can combine circuits (2) and (3) to give our light a Hi/Lo switch. In Hi, the switch shorts the diode so that it operates in Full-wave mode as in (3). Opening the switch only allows current to flow half the time, just like (2).
Ozzies and Brits: You too can use the 42/47 LED circuits - just combine the US version (.4uF and 1K-ohm) circuit presented in part 2 and you too can make a AC-mains light with just 42 LEDs! Or check out the calculations in the following step.
Oh yeah, our 'big' light is super thrifty - running off 110-volt mains, it barely consumes 3-watts.
Find out about more ways to light your house with LEDs off A/C mains here!
Step 6: Crunching Numbers
To operate white LEDs (nominal voltage 3.3v) safely off AC Mains without using any regulation (other than the diode bridge), the magic number is: Vac * 1.4 / 3.3. Which is the minimum number of LEDs in series that will run off AC without exceeding its 'comfortable' operating range. The choice of LEDs can be 20mA or higher - AS LONG AS they are all the same type and attached in series.
If you are using the full number of LEDs calculated above, that is all you need, but for arrangements using fewer LEDs (but no fewer than 30), we need to add the voltage dropping RC combination. R is always a 1K, 1Watt resistor, while the value of C is calculate as:
Vpk= Vac * 1.4
Vdd= N * 3.3, where N is the number of white LEDs we wish to use in series.
Iled = 0.02, the current we want for our LEDs.
C = 1 / (2 * pi * f * (Vpk-Vdd) / Iled), where f is the mains frequency, but you can simplify it to: (58 / (Vpk-Vdd)) in micro-farads (uF), and should range between .1 and .5 uF. Make sure it is a non-polar capacitor.
IMPORTANT: Parts must be rated for at least Vpk, and enough current to handle Iled.