LEDs with Resistors on Short Notice

Quidam · Post by **Quidam** » Tue Aug 21, 2007 3:55 pm

I received a package of LEDs from a company whose web site led me to believe that they'd include the resistors already attached. Now I'm a good number of LEDs short, don't have the time to add resistors and am hoping for a suggestion from the hivemind.

Does anyone know of a company that sells white LEDs with the resistors already wired to them? Preferably in the LA area, as there's precious little time for shipping at this point, but overnight could work, too.

Barring that, does anyone have a suggestion for a compact light source that could run off of a 12v?

Thanks in advance.

Quidam

Prometheatrics (Esp. & 3:00)

MikeVDS · Post by **MikeVDS** » Tue Aug 21, 2007 4:21 pm

I don't know what you mean about LEDs having resistors attached. I think you may have a misunderstanding about LEDs. Light Emitting Diodes are diodes with the difference that they consume some of the energy and turn it into light. This essentially makes them act like a diode plus a resistor in a circuit. On a circuit diagram if you wanted to represent a LED as both diodes and resistors it would just be a diode plus a resistor in series.

I assume this will open up more questions. Post all the details you have about what exactly you have and what you want for the final outcome and we'll give you some pointers.

Quidam · Post by **Quidam** » Tue Aug 21, 2007 4:27 pm

Here's a link to the site I got my LEDs from. It goes into the need for resistors and has links to sites with more info on it.

http://www.superbrightleds.com/led_info.htm

Basically, I have these acrylic cubes that I want to dangle a cluster of LEDs from a hole in the ceiling, wiring them all up to a 12v battery.

Dork · Post by **Dork** » Tue Aug 21, 2007 4:38 pm

Mike - all the raw LEDs I've dealt with required a resistor in series to limit current.

Quidam - do you not have the resistors already, or is it just that they aren't attached? They're pretty easy to solder on. Fry's should have the resistors in stock, and they have LEDs but they may be more expensive than what you got online and may not be pre-wired.

If you really need pre-wired, I'd suggest checking in automotive stores and the auto section of big stores. They usually have a few goodies for the ricers to light up their stuff.

Valkyrie · Post by **Valkyrie** » Tue Aug 21, 2007 4:50 pm

Yeah, for some reason that's a common misconception about LEDs. A diode is a device that pretty much just electrically just looks like a wire one way and an open circuit the other, whether they emit light or not. If you don't current-limit them with a resistor, then either you get really lucky or you burn them out. I think most folks don't realize that a lot of the LEDs they buy have IC resistors inside the diode itself.

Quidam · Post by **Quidam** » Tue Aug 21, 2007 5:21 pm

I guess I'll see about picking up some resistors at Fry's. It's just that I know they're sold with the resistors attached, and it'd save me a hunk o' work. Ah well. It's not like I'll be the only one soldering en playa.

run_w_xcors · Post by **run_w_xcors** » Tue Aug 21, 2007 5:22 pm

If you're burning all white LEDs, there shouldn't be an issue. The only reason you'd really need to put resistors on your LEDs is if you have different colors in your circuit. You should only need on resistor for the whole circuit.

Now, that being said, you should check the voltage rating on your LEDs and run as many in serial as you need to eat up that voltage. Since White LEDs are usually around 3.15volts, 3 of them in series should be fine. You could then run as many in parallel as you want if you wanted to have more lights.

Check out a calculator like http://metku.net/index.html?sect=view&n ... /index_eng

If you're not sure about the concepts of wiring electricity, I'd skip it or enlist the help of a friend.

One a completely different note, look for videos on the Graffiti Research Lab and their LED throwies. I do this kind of thing all the time for quick light thingies and they last for days, sometimes weeks (albeit dimly).

Dork · Post by **Dork** » Tue Aug 21, 2007 5:26 pm

Quidam wrote:I guess I'll see about picking up some resistors at Fry's. It's just that I know they're sold with the resistors attached, and it'd save me a hunk o' work. Ah well. It's not like I'll be the only one soldering en playa.

With mine I just soldered one lead of the resistor directly to the LED lead. No extra wire, just a little extra soldering and something that needs to be covered up.

MikeVDS · Post by **MikeVDS** » Tue Aug 21, 2007 10:35 pm

The only reason you'd need a resistor, is if you don't design your circuit to match your voltage without resistors. You could do each 3v LED in series with a resistor or you could just put 4, 3v LEDs in series, for a 12v circuit. If you don't want those extra LEDs, you can always just use a resistor, but most people are using multiple LEDs anyway. LEDs are "resistors" so you can use them in place of resistors, and instead of making heat, you make more light.

MikeVDS · Post by **MikeVDS** » Tue Aug 21, 2007 10:46 pm

If you don't current-limit them with a resistor, then either you get really lucky or you burn them out. I think most folks don't realize that a lot of the LEDs they buy have IC resistors inside the diode itself.

All LEDs current limit themselves to some extent. The fact that they convert electricity into another form of energy means that they already have resistance without any other modifications. Aside from being a diode, the only difference between a resistor and a LED is that one creates resistance by converting electricity into heat (typically) and the other creates resistance by converting electricity into light. I don't know if many LEDs have conventional resistors built in, but there is no reason a LED would need to have a device to create resistance by creating heat instead of just making it match the needed voltage by making it so that it creates more light. To make it create heat would seem quite odd to me.

MikeVDS · Post by **MikeVDS** » Tue Aug 21, 2007 10:50 pm

If you're burning all white LEDs, there shouldn't be an issue. The only reason you'd really need to put resistors on your LEDs is if you have different colors in your circuit. You should only need on resistor for the whole circuit.

Whoa there!!! Color has nothing to do with it. The voltage is what matters. That site has their LEDs of different colors at different voltages, but that is not a rule that goes for all other LEDs.

Edit: The rest of the calculator looks fine. Test it out using 12V and series LEDs which the voltage drop * the number of LEDs is equal to 12v. You'll see what I'm talking about not needing and conventional resistor.

MikeVDS · Post by **MikeVDS** » Wed Aug 22, 2007 12:21 am

Here's a link to the site I got my LEDs from. It goes into the need for resistors and has links to sites with more info on it.

http://www.superbrightleds.com/led_info.htm

Basically, I have these acrylic cubes that I want to dangle a cluster of LEDs from a hole in the ceiling, wiring them all up to a 12v battery.

How many LEDs and of which colors?

Example of how to find your voltage drop based on their given information.
---------------------------------------------
Find the resistance of the LED (18ma)
V = I * R
12 = .018 * (R(LED) + 470)
12 / .018 = (R(LED) + 470)
12 / .018 - 470 = R(LED)
R(LED)= 197

Find the voltage drop across your LEDs
V = I * R
V = .018 * 197
V = 3.55

So the 18ma LEDs are 3.55 volts.
----------------------------------------------

For your particular LEDs:
18ma = 3.55 volts = white, blue green aqua
20ma = 2.6 volts = red, yellow
21ma = 2.13 volts = orange

Wire in series adding up to ~12 volts (Just over, rather than just under. Having their voltage drop higher than 12 will dim them slightly but will put less stress on your LEDs, though you should aim for exactly 12). Put as many in parallel as your wire and source can handle. You make a much more efficient circuit, save on parts and time by wiring this way.

run_w_xcors · Post by **run_w_xcors** » Wed Aug 22, 2007 9:50 am

[quote]Whoa there!!! Color has nothing to do with it. The voltage is what matters. That site has their LEDs of different colors at different voltages, but that is not a rule that goes for all other LEDs. [/quote]

Yeah, you're right as I didn't include all information. I was basically referring to having to "match" voltage by using different resistors for different colors as some voltages are different.

I thought it was best to try to have all your voltage drops consistent in the circuit when using LEDs, but have to admit I put all my colors on different circuits with one resistor when needed.[/quote]

run_w_xcors · Post by **run_w_xcors** » Wed Aug 22, 2007 9:51 am

[quote]Whoa there!!! Color has nothing to do with it. The voltage is what matters. That site has their LEDs of different colors at different voltages, but that is not a rule that goes for all other LEDs. [/quote]

Yeah, you're right as I didn't include all information. I was basically referring to having to "match" voltage by using different resistors for different colors as some voltages are different.

I thought it was best to try to have all your voltage drops consistent in the circuit when using LEDs, but have to admit I put all my colors on different circuits with one resistor when needed

MikeVDS · Post by **MikeVDS** » Wed Aug 22, 2007 1:40 pm

You will run into issues if you start putting things together that are very mismatched, but most of these small LEDs are so similar in their power consumption that it should not be a problem. In a lot of devices you need to look at a lot more than just V=IR, but these simple and forgiving circuits you can usually just look at and know they'll work out alright. All of the LEDs on the page he pointed out are so darned close you could probably treat them all as 3v devices and never have any problems.

Badger · Post by **Badger** » Wed Aug 22, 2007 5:22 pm

Mike - all the raw LEDs I've dealt with required a resistor in series to limit current.

Not exactly sure what you're looking for but we ended up having to solder our own resistors to our little contraptions.

Have a look here http://www.playaflies.com which has the type, the cost and vendor contact information.

LEDs with Resistors on Short Notice

LEDs with Resistors on Short Notice

LEDs are resistors!!!