Using AC instead of DC to power LED's

NWCCTV

So I was just watching a show on Science Channel that demonstrated how strands of LED Christmas lights are made. First, for some reason they only used resistors on 1/3 of the 60 LED's which are bright white. No explanation as to why only 1/3. Next, they did not show anything about what is used to convert the LED's to be able to use AC current instead of DC. So, there in lies my question. If I want to make a strand of LED's what would I need to use in order to run them on AC voltage?

Phil Pilgrim (PhiPi)

Be sure to wire them back-to-back in pairs or in series strings: i.e. anode to cathode, cathode to anode. Otherwise, even with resistors, the reverse voltage on the opposite half-cycle will be way beyond spec for the LEDs.

-Phil

NWCCTV

But is there anything special I need to power with 110V AC as opposed to DC?

xanadu

You could check out a basic tutorial on bridge rectifiers, that will answer a ton of questions. The rest would be series and parallel circuits. You can use an online LED calculator for the resistors.

Or you can Google image search "led string schematic" and let the sparks fly

prof_braino

One experiment I did used pairs of LEDs anode to cathode and cathode to anode in a big ol' string. I put enough pairs in series until the oltage drop for all the pairs was greater than 110. I think I put one extra pair. I also put one current limiting resistor at the end.

It was very bright initially. I ocasionally plug it in for reference, and check the current. It drraws very little current. It seems to have gotten dimmer (after a couple years?). I never leave it on too long as I'm afraid of 110 volts.

I think I should have had a resistor and capacitor circuit, but at the time I figured I was better off not messing with mains circuits, so I never figured out the proper capacitor values, to resist the temptatio to keep messing with it.

Ron Czapala

Here is a transformerless circuit to power a single LED using a full-wave bridge and a non-polarized 275 VAC .22uf capacitor

prof_braino

Ron Czapala wrote: »

Here is a transformerless circuit to power a single LED using a full-wave bridge and a non-polarized 275 VAC .22uf capacitor

That is similar to what I looked at previously. How would we go about adding more LED's , get flashes from both the up cycle and down cycle, and eliminate the non-LED diode in the rectifier circuit?

I looked at this:

http://www.electricaltechnology.org/2013/03/230-v-ac-main-operated-led-powerful.html

but I don't have the backgound to mod the circuit for more LED and or 110 volts. I just added more LED's until the voltage drop was about 110 volts, and onitted the capacitor. Which is probably why it getting dim quickly.

Ron Czapala

prof_braino wrote: »

but I don't have the background to mod the circuit for more LED and or 110 volts.

I'm in the same boat. A friend of mine is an electrical engineer and explained how this circuit works (several years ago) but I forget the details.

I do remember it is based on capacitor reactance -

http://www.electronics-tutorials.ws/filter/filter_1.html

http://electricianeducation.com/theory/capacitive_reactance.htm

whicker

To be correct, you can use a full wave current regulator that is specific to powering LED's off of 120V (one example has line, neutral, and current output). Requires all the LED's to be in series and all of their forward voltage drops added up to be somewhat less than 120V. Because of the low current (21 mA) it doesn't get that hot from the power it has to dissipate.

Not that the chinese manufacturers would use it. They just nominally add up all of the voltage drops and resistors in series for a little over 120V and call it good. Double the string length for 240V. added bonus don't worry about lopsided power factor with all of the LED's facing one way, but still throw "energy efficient" and "lasts 10000 hours" on the box and change the company name every 2 years.

LoopyByteloose

whicker wrote: »

.... and change the company name every 2 years.

LOL, but likely to be very true. Sometimes it pays very well to put a lot of distance between yourself and your customers.

On the other hand, there is a lot that can be learned about electronic basics by building a good string of 120VAC Leds. So please focus on the good solution and try to come up with an answer that we can all learn from.

A full wave 120VAC current regulator? Does such a thing really exist?

kwinn

The circuits in post 6 and 7 both depend on the capacitive reactance to limit the current. The resistor is not really required. If you use a single series string of leds you will need a bridge rectifier to prevent damage to the leds from high reverse voltages. If you use two series strings in parallel as per the link in post 7 a bridge rectifier is not needed. One string is always forward biased so the voltage on the other string is never high enough to be a problem. Only problem with two strings is they alternate being on so under some conditions there is a noticeable flickering.

I researched using series leds powered from 120VAC to build a traffic light/color organ for my kids years ago. Pretty sure I still have all the notes, calculations, and schematics somewhere. I can dig them out and post them if anyone is interested.

bsnut

That would helpful if you four them I would like to see them and everyone else may want to see too.