A reader writes:
Thanks for your Embarrassingly Easy Case Mod page.
Sorry to be such a techno-dummy, but: You said that because each color-changing LED was 3V, you could connect 4 of them in series to a 12V source. So the LEDs divide the voltage between them? If that's true, how can you connect multiple AC devices to an exension cord and have each of them receive 117v?
Anyway, I'm converting an electronic theater organ to MIDI, and would like to add 20 color-changing LEDs to the console. (Thought you'd appreciate the details, eye-candy-wise.) How do you suggest I do that? If I wire them in series, what sort of DC power would I need?
I know you're a busy guy, so thanks very much for giving me a clue about this. I promise I'll do my best not to blow myself up.
Andy (Vancouver, BC)
Yes, you can run a string of four RGB LEDs from a 12V power supply. They're odd little critters, though, and it's important to understand why this works as well as just the fact that it does. You can make electronic things that work by just blundering around with no understanding of what's really going on, but it really does pay to spend some time learning the basics of at least DC electronics before you start on any electronic project. Hence, this lecture.
In the four-RGB-LEDs-from-12V situation, the LEDs can be regarded like ordinary passive DC-circuit components, like resistors or batteries. But LEDs can't usually be treated that way. Two-leg 5mm RGB LEDs may look like the usual kind of LED, but they're actually three LED dies with a tiny controller circuit, all in a normal 5mm LED package.
If you make a string of simple resistors that all have the same value - let's say, five two-ohm resistors - and hook one end up to the positive terminal of your DC power supply and the other to the negative, a current will flow that's determined by the total resistance and the voltage, according to Ohm's Law: Voltage in volts equals current in amps multiplied by resistance in ohms, or V=IR.
(Ohm's Law is usually written with "I" as the symbol for current, rather than A-for-amps, because when Georg Ohm came up with the Law nobody really knew what current was, and it was referred to as "Intensity". Feel free to write it with an A if you like.)
If the power supply is outputting, let's say, 12 volts, a string of five two-ohm resistors in series will work out as follows:
12 = I*2*5
12 = I*10
12/10 = I
I = 1.2
So the current in this circuit would be 1.2 amps. Because the resistors all have the same resistance, each one "drops" the same voltage. If you measure the voltage "across" the central resistive element of one of the five resistors in this circuit, it'll be 2.4 (12/5) volts. Measure across two resistors and you'll see 4.8V, three will be 7.2V, et cetera. (If the resistors in the chain have different values, they'll drop different amounts of voltage, and dissipate different amounts of power, making you use a polynomial equation if you want to figure out which resistor's doing what.)
To visualise this, think of the current as a flow of water in a hose and each resistor as a narrowing, or kink, in the hose. The higher the resistance, the narrower the path for water flow, and the more pressure (voltage) you'd need to achieve a given flow rate (current). (I've got water analogies for capacitors and inductors, too!)
To really get a grip on all this, I highly recommend that you get a little "breadboard" that you can plug components into without soldering (this sort of thing), and a selection of jumper wires (like this, or you can of course make your own), alligator-clip leads, resistors, capacitors, inductors, LEDs, battery holders etc to play with. And destroy - blowing up resistors, caps and LEDs can be very educational. Wear eye protection, especially when playing with electrolytic capacitors:
A proper adjustable bench power supply would also be nice, but would cost way more than all of the rest of this stuff put together. A lantern battery or hacked-up plugpack or PC power supply would be an adequate substitute, for this elementary stuff.
OK, back to LEDs. Ordinary LEDs do not behave like simple DC components; they don't just have a "resistance" where hooking them up to a given voltage will cause a given amount of current to pass. A blue or white LED might be specified "3.6V, 20mA", but if you connect it directly to a 3.6-volt power supply, it'll get warmer and warmer and pass more and more current - "thermal runaway" - until, if the power supply's internal resistance is low enough, the LED burns up. This will happen for series strings of LEDs as well; if you make a string of ten "1.8V 20mA" red LEDs and connect it to an 18V power supply, it will probably not last long.
(Power supplies that have high internal resistance are a special case; you can connect LEDs directly across such a power supply and they'll work fine. This is why Photon lights and "LED throwies" work; they connect an LED directly across a lithium coin-cell watch battery, but the battery's internal resistance keeps the LED safe.)
The simple solution to this, as I explain in my old piece about building a caselight, is to put a resistor in series with your LED or LEDs. It's easy to figure out what resistor values to use for a single LED or even an array, but again, doing this without understanding what you're doing is not a great idea.
Series and parallel are bedrock concepts, here, with direct application to a number of everyday situations. Take your question about the powerboard that delivers full mains voltage - in your case 117V, a nominal 230V where I live - to everything plugged into it. It does that because the powerboard's outlets, like the wall outlets in your house, are all in parallel. (There are some tricky things about household power wiring in some countries, but they need not detain us now.)
Now, consider the old-fashioned kind of Christmas lights, with a long string of little bulbs that all go out if one bulb blows, so you have to replace every bulb in turn with a fresh bulb until you find the one that's actually died. That sort of behaviour is a dead giveaway that you're dealing with devices wired in series. In the Christmas-lights case, they're a string of low-voltage bulbs whose total voltage adds up to mains voltage, and they "share" the voltage between them just like a string of resistors. If mains is 240V, twenty 12V bulbs in series will run from it happily.
(Mains power is, of course, alternating current, not direct current. The two are very different, but incandescent light-bulbs don't care.)
OK, now let's finally get to your specific application, adding trippiness to an electronic organ. If the organ is at all modern, it'll run from low-voltage DC inside, and contain a power supply that converts the AC mains to whatever voltages it requires, just like a computer PSU. This doesn't mean it's safe to go fiddling around in there while the organ is turned on, but it does mean that there's probably some supply rail you could easily use to power plenty of LEDs, since they don't draw much power.
You will probably have to fiddle with the organ's guts while it's powered up to find a suitable power rail (unless you've got a schematic or service manual, or the innards of the organ are unusually well-labelled), so all usual safety disclaimers go here, along with my traditional link to the Sci.Electronics.Repair FAQ. But I wouldn't be surprised if you could easily find a 12V-ish rail across which you could connect a string of RGB LEDs, or even multiple strings in parallel.
That last bit is a "series-parallel" array. If you've got 12V and want to run more than four 3V RGB LEDs, you make up multiple strings of four and connect them all in parallel. People often seem to find this concept a bit slippery, but it's another of the things that it's important to grasp if you're to know what you're doing.
Here's how I wired that LED caselight:
Those are 18 2-LED strings - and just one current-limiting resistor for the whole thing - all connected in parallel with each other. The little piece of "strip board" I used to make the caselight curls all of the copper traces around to make a rectangle and so is a bit confusing-looking, but electrically it's the same as two long wires, one positive and one negative, connected by 18 two-LED strings like the rungs of a ladder. (Rob Arnold's above-linked LED array wizard is very handy for figuring out LED array configurations, but remember that two-leg RGB LEDs aren't normal LEDs, so you really can just treat them as 3V DC components and not worry about resistors.)
If the organ doesn't turn out to have any tappable power rails, or if you just don't want to fool with them, the LEDs could less elegantly be run from a separate power supply, like a 12V DC plugpack. There's unexpected complexity waiting to ambush you here as well, though; if this page hasn't already turned you off electronics for life, try my essay on Humankind's Endless Quest for a Substitute Plugpack!