Large Array LEDs on 120VAC

[Cliff Rohrabacher]

A remarkably interesting post I d found here asks:
"How many engineers does it take to light a LED? After all these posts, the OP ended up picking components from different posts."

And the guy asking the question still got it wrong. and even after no one it seems could agree on anything. I read the whole thread and my head never stopped spinning.

And Yet I have pretty much the same question.
I'd buy a kit if I knew of one.
Which brings the title of this post to bear:

I want a few panels of LEDs with a 500 or so LEDs of different colors on each panel. (Yah I know the different colors have different power draw) And to drive them with 120VAC which is smarter:
1.) Getting a cheap transformer that'll run off 120 VAC and running the LED arrays on DC
- - or - -
2.) Trying to figure out how to build a circuit for each panel to run off of 120VAC


I understand what a resister is, I know that a good design should not try to run more than a few LEDs from one resister because of voltage drop, and the best design will have one resister for each LED.


It's the power supply that perplexes me.

Any electronics Wizards care to enlighten a novice?

[Jason Beam]

I don't think you can take AC to DC with just a transformer - you can step the voltage up or down, but it'll still be AC. I'm not the least bit of an expert but I think you need a couple diodes at a minimum or a rectifier to make it DC voltage.

THat's about all i (think i) know about it - I'm looking forward to seeing the experts chime in :)

[Greg Peterson]

You need to do some rectifying. Bridge rectifier. Four diodes and a hand full of capacitors. The trick is to calculate how much voltage you need.

[Mike Henderson]

I guess the way I'd approach it (but note that I'm not a power supply designer) is to get the operating voltage of the LEDs I'd want to use. You can use a series resistor to drop the voltage to what you need (knowing the current draw you can calculate that) but the more voltage you drop across the resistors, the more waste heat. You want most of the electrical energy going into the lights and not into the resistors so you want your supply outputs to be as close to the LED voltages as possible.

So if you have some LEDs that operate at 1.8V, plan to operate them from the 1.8V output of the power supply (but most engineers recommend a series resistor, rather than directly connected like that, to limit runaway current). If you have some LEDs that operate at 2.5V, plan to operate them from the 3.3V output of your power supply with an appropriate series resistor. There are certain voltages that are "standard" in electronics and you'll find power supplies with those output voltages. Back when I was doing electronics, 1.8V and 3.3V were pretty standard. Check to see what's available now in power supplies.

Once you know the voltages you need, and how many lights you plan to use of each draw, I'd then go hunting for a multi-voltage power supply that will meet your needs.

So if you need 5 amps of 1.8 volts and 5 amps of 3.3 volts (for example, your needs will likely be larger), I'd find a power supply with 120 volts input and two DC outputs, one of 1.8 volts and the other of 3.3 volts, each with more than 5 amps.

Then start wiring up.

Note on power supplies. The life of a power supply is directly related to how close you run it to rated currents. So if you get a power supply rated at 5 amps output each at 1.8V and 3.3V, you'll get the rated lifetime. If you buy a power supply rated at 10 amps output each at 1.8V and 3.3V, the power supply will generally last a lot longer than the rated life (assuming you only take 5 amps each).

Mike

[P.S. Don't try to design your own power supply. It's a very specialized field and difficult to do when you have to meet a bunch or requirements. For example, the output needs to be regulated to keep the voltage within tight tolerances, but there are a bunch of other things you also need to consider.]
[Also, each LED needs to have it's own resistor (if one is needed). If you try to put several LEDs on one resistor, over time the current draw will likely change, or one or more LEDs will fail. If the current draw decreases, the voltage to the LEDs will increase. If it's sufficient, it will destroy all the LEDs on that circuit.]
[One more thing, remember that LEDs have polarity. Don't wire them backwards.]

[Dan Friedrichs]

FWIW, I'm actually a power supply design engineer, and the lab I work for does extensive work on LED lighting and power supplies for LED lighting (although that is not something I personally work on). I'd recommend just finding something you can buy, rather than trying to build your own. It's just too much trouble, and you'll end up spending more money. Even with the knowledge I have in this field, I wouldn't try doing it myself.

A few notes:
-You'd need to decide if you want to run the LEDs in series or parallel. Series results in the whole string failing if one LED fails open, though. But it also lets you add the voltage drops up, so your transformer ratio doesn't need to be so large.

-The LED is, by definition, a diode, so you don't necessarily need a rectifier after the transformer, but it's a good idea. Not using a separate rectifier will result in you half-wave rectifying the AC line, though, and the LEDs will have noticable flicker.

-With as many LEDs as you are suggesting, the peak currents could be high, which can stress components more than you'd expect.

-Using series resistors works fine for a few LEDs, but for this many, you could end up burning a LOT of power as heat in the resistors. What we usually do is design a specific LED power supply that regulates itself by limiting the current it puts out - this eliminates the need for a resistor entirely.

If you're set on the idea of doing it yourself, you should look for a power supply specifically designed to drive LEDs that using current-mode regulation. Again, though, I wouldn't not attempt this myself...

[Dan Hintz]

My main business is LED-based lighting systems, so I can help, but I need a better sense of what you're trying to accomplish.

1) Do you need the various colors tightly mixed, or can you get away with a strip of one color next to a strip of another color?

If it's the latter, there are plenty of pre-made LED strips that you can use, designed for 12V, 24, and 48V "low-voltage" lighting systems. These will (usually) have the proper current-limiting hardware in each strip.

If it's the former, you're (most likely) looking at a custom design.

2) Are the panels widely separated or close together?

If the latter, you may be better off with one power supply split to multiple panels. If it's the former, you're probably better off with multiple power supplies, each running off of 120V.

3) Not a question, a statement... as previously mentioned, do not try to create your own power supply, particularly if it connects to the mains. LEDs are current-controlled devices, and as such it is not a wise design to run a large number of them off of just a current-limiting resistor. As the supply voltage decreases and the LED forward voltage variability increases, you run into issues with reduced life in some branches and reduced brightness in others.




If you choose to go with a custom solution (and it sounds like you have an interest in building your own), I suggest choosing a UL-listed power supply of 24V... these are very common for under-cabinet lighting systems and should be relatively inexpensive (compared to a roll-your-own power supply that will be dangerous). This will ensure you have a fairly regulated/stable output voltage, soething you'll need if you use resistors to set the current levels.

Determine what current you want to run the LEDs at, find their nominal forward voltage at that current (datasheet), and size a resistor accordingly for each leg. Your mention of 500 or so LEDs tells me it's most likely the standard T1-3/4 through-hole type (and not the 1W+ types), so I imagine 20-30mA per leg will be about right.

Use up about 20-21V of the 24V supply with LEDs, and use the remaining 3-4V to calculate your resistor. With a mixture of RGB LEDs, you should be able to get around 9 LEDs per 20-21V leg. At 50 or so legs, you're only looking at a supply that can handle 1-1.5A... relatively small.

Personally, I would design the panels from scratch, but I can do that because I have the equipment and knowledge. YMMV...

BTW, I'd be interested in reading the thread you mentioned previously ...



P.S. This is the first time I have ever visited this particular forum, so don't expect fast replies from me to this thread.

[Rob Young]

Dan's on the money, LEDs are current mode devices and it is easier to design with them if you stop thinking of them as requiring a particular voltage except that the voltage potential needs to be greater then their forward drop (1V to 3V depending on LED type, phase of moon, direction of black cat hair, i.e. it isn't a precision specification).

In the long run, the more efficient designs are based on controlling the current supplied to the LEDs.

And in a on-off project, it might be more efficient, time-wise, to buy an appropriate power supply once you have your real requirements figured out.

Not a lot of help from my post but I seem to remember the guys that run www.sparkfun.com (http://www.sparkfun.com) did some large panel LED stuff. I think there were some LED array projects as well as a project that made a giant (many feet tall) 7-seg LED clock. It might be worth a few emails to them.

[Cliff Rohrabacher]

This is not going to work.
The more I research it the more involved it becomes and if it wasn't worth the money to just go buy the things then it's not going to be worth the effort and money to try building them. Plus I'll have got nowhere with respect to all the other elements of the project.

I was hoping for tomatoes, hot peppers, and herbs year round but space. cost , effort, yadda yadda yadda ~ ~ ~ it's just not going to be worth it.
I'm better off going to Wegmans for high end stuff than I am trying to grow it out of season.

[Dan Hintz]

This is not going to work.
The more I research it the more involved it becomes and if it wasn't worth the money to just go buy the things then it's not going to be worth the effort and money to try building them. Plus I'll have got nowhere with respect to all the other elements of the project.

I was hoping for tomatoes, hot peppers, and herbs year round but space. cost , effort, yadda yadda yadda ~ ~ ~ it's just not going to be worth it.
I'm better off going to Wegmans for high end stuff than I am trying to grow it out of season.
I brought this up again because I wanted to know what was happening with it (I rarely come to Off Topic). I didn't realize this was an attempt to create a grow light... that's a whole 'nother ball o' wax as you can't just use a white LED and expect plants to grow (they need a certain spectra to convert minerals, water, and CO2 into energy). We could have gone through a lot of design work, regardless of price, only to find your intended use wouldn't work.

It pays to spell out your intentions from the beginning ;)