Series or Parallel? I have heard arguments against both and proponents of never doing it one way or the other (some say never serial, some say never in parallel).
Uh, so which one? Heh.
From what I gather, with Serial since I have LEDs that are all identical, I could just figure out what resistor to use for ONE of the LED lights and then shove that on the start of the chain and it would make everything work a-ok.
I have not been able to find any sort of Idiots Guide to which resistors to use when hooking LEDs up in parallel though.
Guides like those at [url) show hookups for one resistor one LED, but uh. . . hmm.
Thank you for your help ahead of time, this is going to drive me nuts! (that and I don’t want to fry anymore LEDs, already killed a few before I found out that I needed resistors. )
Are you wondering why you need a dropping resistor? Or how to figure out the value?
Figuring out the value is pretty easy. An example illustrates it best:
Let’s say your supply voltage is 24V, and you’re using a generic, red, “garden-variety” LED. Your typical red LED has a forward voltage drop of around 2V (1.7V to 2.4V depending on current). It also has a “safe” current range of 0 mA to approx. 20 mA. (For a real application, make sure you know the safe current range for the LED!)
Now let’s say you want to operate it at 10 mA, which will give you about medium intensity.
The voltage drop across the resistor will be approx. 24-2 = 22 V.
The current through the resistor will be same as through the LED, i.e. 10 mA.
Ohm’s Law says V=I*R. Therefore, we can calculate the resistance:
R = V/I = 22/10 mA = 22/0.01 = 2200 ohms.
The resistor must dissipate a power of IV = 0.0122 = 0.22 watts. Therefore, a ¼ watt resistor will work fine.
Note that this is an approximation, since I assumed the voltage drop across the LED was 2V. If you want a more exact calculation, you’ll need the I-V curve for the LED.
Com2Kid, it looks like you made a mistake with your url tag (it needs to be [square brackets], not <angle brakets>. This is what the OP was supposed to look like.
If you’re running LEDs at their intended voltage and current, they’ll each produce a given amount of power (power = voltage * current). For multiple LEDs all running at spec, this will just add up, so eight will produce eight times as much light as one. Note that, due to peculiarities of the human eye, it won’t look like eight times as much light… If you want something to look eight times brighter, the easiest way is just to experiment with different light levels until you get what you want.
Now, then, about series and parallel. If you can hook one set of components up to a battery, then you can hook any number of repetitions of the same thing to the battery, in parallel. For instance, you could have one branch consisting of a resistor and an LED hooked up to the battery, and another branch identical to it also hooked up to the same battery. This is never a problem unless you’re drawing more current from the battery than it can handle, which isn’t likely with LEDs.
You can hook LEDs up in series, as well, but only within limits, and you’ll have to re-calculate what resistance you need. You’ll have some voltage drop across each LED (which will pretty much depend only on the LED), and a voltage drop across your resistor, which will depend on the current (V = IR), and you want all of those voltage drops to add up to the voltage of the battery. Of course, series circuits also have the common problem of Christmas-tree lights: If something happens to any element in the circuit, the whole thing goes dead.
Yea. Com2Kid’s link is to the Radio Shack search engine. To get to the link he’s referring to, you need to cut-n-paste the text into your browser’s address bar. It goes to a page that performs LED & resistor calculations. Totally unneeded, IMO.
Even worse since I opened the tag correctly but did not close it properly. . . .
::sighs::
Thank you for your help though.
Strange, one site I read said that I need only put a resistor of the proper amount on the FIRST connection in a serial and then everything would work right. . . .
::sighs yet again::
The other part of my question was what the output of an array of eight LEDs at 120MCD is like. I have no previous experance to mentally compare those numbers against, so they are just numbers to me.
(kind of like telling a Customary Person some metric temperatures. )
Serial, parallel, serial, parallel… it all depends on the application. But if I’m making an LED array, I would be more apt to string as many LEDs in series as I can. In most (perhaps all?) cases, a series arrangement of LEDs is a more efficient arrangement than parallel (assuming the same source voltage).
Borrowing from my previous example, let’s say the source voltage is 24V and I want to power 10 LEDs. It will be assumed that each LED has a forward voltage drop of 2V, and that each LED will be excited at 10 mA.
If I connect a series resistor to each LED, then connect them all in parallel, all 10 resistors will be dissipating a total of 2.2 watts.
If I simply connect all the LEDs in parallel and use one resistor, the resistor will be dissipating 2.2 watts. Same as above.
If I connect all 10 LEDs in series, and use one (series) resistor, the resistor will be dissipating 0.04 watts.
Note that in all three cases, each LED is powered identically (20 mW). Yet the resistor in the series arrangement dissipates 55 times less heat than the resistor(s) in either parallel arrangement. Overall, the series circuit is 10 times more efficient than either parallel circuit in this example. And the more LEDs you have, the more efficient the series arrangement becomes vs. its parallel counterpart. (If we want to power 11 LEDs, for example, the series arrangement is 11 times more efficient than the parrallel arrangement.)
The main drawback to the series arrangement, of course, is that you need a fairly high DC supply voltage. But it is so much more efficient than a parallel arrangement that it might be “worth it” to pump up the voltage with a DC-to-DC converter. (It should be noted that another potential problem with a series arrangement is mismatched LEDs; if they’re not well matched, some will be brighter than others.)
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