I decided to make a post about the use of LEDs, due to the large number of comments about using them.

Using Light Emitting Diodes (LEDs):
LED’s are polarized. If you hook them up backwards they won’t work. To determine polarity: if the package is round, look for a flat spot on the edge of the device. The lead wire nearest the flat spot is probably the Cathode or negative lead. If the leads of the LED haven’t been cut, the longest lead is usually the Anode or positive lead.



Current limiting resistor:
LEDs can be operated on almost any voltage as long as they are used with the proper current limiting resistor. Most LEDs require a forward bias voltage of around 2 –3 volts and consume a current of about 20 mA. Using LEDs on voltages above 3 volts without an appropriate resistor will probably cause them to burn out quickly, if not immediately. If you don’t know the exact specs for the LED you are using, you can usually use 2 volts @ 20 mA as a starting point for calculating the required resistor. Higher brightness LEDs and specialty LEDs may require higher voltages and currents. If the LED appears too dim, use a slightly lower resistance; if it is too bright raise the resistance. To calculate the resistance use Ohms Law



Multiple LEDs:
You can put LEDs in series with one resistor for the whole string. Add up the voltages of all the LEDs in the string. This should not exceed 80 percent of the supply voltage. To calculate the resistance required, the dropped voltage will be the supply voltage minus the total voltage of the LEDs in the string. Series strings can be paralleled if each string has its own dropping resistor.

Wow, fantastic tech writeup. Thanks very much, this is really helpful as I'm building some headlights right now.

Can you clarify something for me... I use 4 Leds that draw 2v each, and run them all off a 9v battery, does that eliminate the need for a resistor, since I'd be drawing about 8v of current? Or is it not quite as simple as that?

So, yes and no as far as the simplicity. LED's are often viewed as fixed resistance, classical devices with completely static power draw, or at least only proportional to the square of the input voltage. The reality is that since they are in fact diodes, the amount of power they used is governed by a nasty, sorta nonlinear relationship to the input voltage. (...damn semiconductor physics)

Where this applies to your application is the following:
You've quoted your LED's as being two volts each; because of the reasoning above, LED's like most other semiconductor devices are really rated by current. As a user of LED's your goal in life is to make sure the current flowing through the LED's is what they should be, which when used individually should be two volts. However, because of the nature of variable resistance devices, placing them in series and hoping for a fixed-resistance aggregate effect is not always guaranteed. Variances in the LED's themselves, both from manufacturing inconsistencies and the usage of different but equally labeled parts can account for several problems; problems symptomatic of unequal luminous intensity and possibly the burning out of the part itself.

Without a current meter, I present you with the following solutions:

1) The practical route: All the dangers shouldn't be a problem, and if they're really rated for 2v each, an additional .2v shouldn't be a big deal, but who knows. If you burn down your house and soil your pants, I warned you.

2) The practical safe route: Assume the principles of option 1, stack them in series, and use a variety of power supplies to ease it up to 9v. Start low (not too low, because there is a minimum voltage (Vt) that you have to exceed for electrons to flow through the semiconductor), and make sure that each LED is about equally bright and no brighter than they would be when used alone. The scientific version of this approach involves using a multimeter, and that when driven in a series chain, the voltage across any one part is no greater (or not much greater) than 2 volts each (say, + or - .3 volts)

3) The safe route: Give each his own resistor, so, four LED's in series with his own resistor; with each LED-resistor pair in parallel with each other against the 9 volt source.

...my pick, #2. You'll learn more, and undoubtedly have better battery life (less wasted in resistors).

Thanks for the great technical response. You know your stuff. Are you an engineer of some type?

Can I ask a few more questions, so that I can really wrap my mind around this concept:

1. So is there a difference in the brightness of an led when powered by different voltages?

2. You said this: "The practical route: All the dangers shouldn't be a problem, and if they're really rated for 2v each, an additional .2v shouldn't be a big deal, but who knows. If you burn down your house and soil your pants, I warned you."

Where does the .2v number come from? Is that the resistance of an led?

Thanks again!
James

googooatlarge nice explanation

Used red LED's high intensity thoroughout my sailboat (25 yrs. ago) for nite vision . A Mini-Mag lite (AA batereries) will take a LED in place of the bulb. Epoxy them into 12V bulb sockets for real qick and dirty.

1) Yes there is a difference, the higher the voltage the brighter the LED gets. However, the higher above the actual run voltage the shorter the LED will last because it will burn out. You can buy LEDs that are made for the higher voltages and they are amazingly bright, however they tend to cost a lot and tend to need their own heat sinks because they got hot enough to melt lexan without them.

2) .2 v is just a number that i use as a rule of thumb for plus or minus errors in making cheap electronics. Its a number in this case that won't really make a difference if it is between the plus or minus .2 v.