Wall transformer for LED lighting?

[OdinsDream]

I'm going to wire up a nice lamp for valentines day using some shiny objects and white LED's courtesy of allelectronics. I was hoping to make a fairly easy job of the power supply by also getting it from allelectronics.

What type would best suit this application? I'm hoping to wire up 12 to 24 LED's and have the lamp plug into a standard 120V outlet.

What voltage, amperage should I get? Also, would it be possible to wire in a simple dimmer? Potentometer for this?

 

[mattg2k4]

12v 150ma should be plently. If you've got a spare ATX that will work in a pinch. You could easily use a lower or higher voltage and adjust how the leds are wired. If the specs on your white leds are like mine (~3v drop) you can safely run 4 or 5 in series on 12v wall transformer with no resistor, but YMMV depending on your leds. You could also set up a transistor with a potentiometer driving the base, but that's probably more complicated than you want to get.

A potentiometer could easily be used to control brightness put it in series with your 4 or 5 leds, but check the wattage since you're using so many leds. Off the top of my head, something around 100 ohms would probably work well.

 

[OdinsDream]

Thanks for the tips. I'm looking at these:

http://www.allelectronics.com/cgi-bin/category.cgi?category=340500&item=LED-75&type=store

The spec sheet notes the following characteristics:

Reverse Voltage (<50uA) = 5.0V
D.C. Forward Current = 20mA
Pulse Current(1/10 Duty Cycle, 0.1ms Pulse Width) = 100mA

Is the Reverse Voltage what you mean when you talked of Voltage Drop? Am I right to just add up these amounts to get the total power supply voltage needed? Serial wiring, yes?

If I were to have eight of these, that's 40 volts, and 80mA, if I were to pick a 12V power supply, would I just pick a higher mA rating to offset this?

 

[xonik]

Nope, what you want is the forward voltage drop. The reverse breakdown voltage is completely different and yields undesirable results. If you read further in the specification PDF, it says that the forward voltage drop should be between 3.6 and 4.0 volts. Using a 12 V supply you could have three LEDs in series, with additional strings in parallel configurations as needed. Just watch that current draw, especially on a wall-wart or the like; each LED draws approximately 20 mA.

 

[mattg2k4]

I think a link which explains the difference between serial and parallel and how it affects voltage and current is in order: http://metku.net/index.html?sect=view&n=1&path=mods/sahkooppi/index_eng

In short, in series you add the voltage, current is the same. In parallel it's the opposite.

And the reverse voltage is how much voltage it can stand in the wrong direction, the forward voltage drop (Vf) is what you need to know. And 8*20ma = 160ma, not 80. You didn't mean 800ma did you? That is the surge rating, you will kill it running at this continuously.

 

[e_lectro]

if you are looking to buy a quanity of leds, check out ebay. I have been very happy ordering my leds there, and they are dirt cheap. I'll happily pay $15 for 100 white LEDs vs $200 from somewhere else.

A wall transformer will work great for someting like this, I'm using several for LED lighting around the house.

 

[racercarl]

Wall transformers are easily found at thrift stores, they usually havea bin full of used transformers from different devices, and you can pick and choose.

I picked one up nd hooked it to the LED's on my paper shreader. ( need to take pics and show everyone ) also picked up one of those cheap plug in timers so it comes on and goes off by itself I spent more on the LED's than the transformer and timer and still had it repainted, lit, and timed for under ten bucks

 

[junkml]

this is an interesting idea. Does anyone have any idea what the electrical use would be on an led lamp such as this compared with fluorescent or incandescent bulbs?

 

[nanaki]

Originally posted by junkml
this is an interesting idea. Does anyone have any idea what the electrical use would be on an led lamp such as this compared with fluorescent or incandescent bulbs?

Well it takes 1000 ma for 1 amp so at 20 ma per led it takes 50 such leds to draw 1 amp of current. so
Volts (multiply by) Milliamps (divide by) 1000 = Watts
5 x 20 = 100/1000 =.1 watt so for you to get the same useage as a 100 watt light bulb would meen you would need 1000 of thoose leds.

All i can say is damn that would be one very very bright light

 

[OdinsDream]

So if I wire these in series...

to determine how many lights I can put on the power supply, I should do:

Number of Lights = Power Supply Voltage / Vf per Light

Series would mean that the current (mA) would just need to be equal to the mA rating of any one of the bulbs?



Parallel, I would need Power Supply Voltage = Vf of a single light...but I would need Power Supply mA = Sum of mA of All Lights ?


Please, let me know if I got this wrong, I wouldn't want to blow up my girlfriend's gift ;-)

 

[racercarl]

Go to Lsdiodes.com, they have good prices and a " How to hook up LED's" article that will be very helpful

 

[T-Bird 151]

Thanks for the lsdiodes link. I just placed an order for 12 white. To e_lectro, thanks for mentioning ebay, but at the moment, I only need a small quantity of solid white. Likely I'd want to buy cheaper greens in bulk, since, I happen to like the green glow ;-)

But, if anyone would mind looking at the previous post I made, about the formulae for series and parallel wiring, I'd really appreciate hearing whether that's correct or not.


Would this be an appropriate PS:

http://www.allelectronics.com/cgi-bin/category.cgi?category=480&item=PS-578&type=store


...for LED's with the following characteristics:

20 - 30 mA
3.3 - 4.1 Vf

...I would guess that I'd need to throw in a few resistors.. just not sure how many?

...from this page: http://metku.net/index.html?sect=view&n=1&path=mods/ledcalc/index_eng

I guess I'd need an 8.75OHMS ?

 

[OdinsDream]

eh, that last post was me... at a friend's computer by accident.

 

[linear]

Originally posted by T-Bird 151
Would this be an appropriate PS:

http://www.allelectronics.com/cgi-bin/category.cgi?category=480&item=PS-578&type=store


...for LED's with the following characteristics:

20 - 30 mA
3.3 - 4.1 Vf

...I would guess that I'd need to throw in a few resistors.. just not sure how many?

That's a fine supply. It's regulated, whiich not all wall warts are. That means it should change its output voltage only slightly under load.

Definitely drive your LEDs at 20 mA or below. Using the specs you provide:
R = (5.7 - 3.3) / 0.02 = 120 ohms, which happens to be a standard value.
P = (5.7 - 3.3) * 0.02 = 48 mW, so any resistor will be fine (1/4W are common and cheap)

That's the calculation for a single LED, or if you use one resistor per parallel LED, which is how I'd recommend you do it.

Now, your series and parallel assumptions are reversed. For the series combination, the forward voltages combined can't exceed the supply voltage. So 3.3 + 3.3 > 5.7, so you won't get away with series using that supply (Actually, you may, but I'd be guessing about how bright the LEDs would light, and I'd rather steer you toward a definite solution).

For the parallel case, you add the currents. So for example, doing all 12 in parallel with a single resistor would look like
R = (5.7 - 3.3) / 0.24 = 10 ohms, but,
P = (5.7 - 3.3) * 0.24 = 576 mW so you'd need a 1W resistor. Thta's not worth the hassle, and if there are slight differences in your LEDs, they may not all light evenly.

So, I recommend you use 12 120 ohm, 1/4W resistors, one in series with each LED, and wire them all in parallel across your power supply.

Another way to do it:
Get this supply:
http://www.allelectronics.com/cgi-bin/category.cgi?category=480&item=PS-926&type=store
And wire four parallel strings of three LEDs in series, with no resistors at all. You'll drive the LEDs at 3.0V, they'll light up pretty well, you won't need any resistors, and since it's a regulated supply, you need not worry about a voltage spike frying your diodes.

More reading on the subject:
http://linear1.org/gm/archives/00000166.php
and of course I prefer this calculator:
http://linear1.org/ckts/led.php