not blowing up LEDs!

[24vdc]

I've set up LED lights before but I've never done this: How can i run my LED lighting system off of my main power (it happens to be 24v for this project) and not off the receiver power without just exploding the LEDs?
So basically i want to run an LED light setup on a 24v battery. Do I need resistors or something to reduce the voltage to the LEDs? Thanks!

[Fluffy Monster]

LED's are a current device so you want to restrict the amount going to the LED.
They drop around 1.2V so you will need to drop 22.8V across the resistor.
If you only want 10mA through you LED (medium strength) then use V/I = R
22.8V/0.01A = 2,280R
So a 2.2K should be about right.

Check your data sheets for the max current and voltage drop of LED

[Darth_Elevator]

If you know the LED's forward voltage and its forward current, here's a handy little calculator (http://led.linear1.org/1led.wiz) to help you know exactly which resistor you need. There's also a link (http://led.linear1.org/led.wiz) there for calculating what size resistor for a series of LEDs versus just one.

Here are a couple examples using the values for 5mm 18000mcd white LEDs that I commonly use, which is 3.3 volts forward voltage and 20mA forward current.

[24vdc]

Awesome. Thanks for the great info and links! BTW, why is it called forward voltage. what does the forward mean? is it because it is referring to a diode? also what does mcd stand for? (i know what it means, just wondering what the letters stand for.) Thanks again.

[rick.benjamin]

millicandela (mcd)

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

[Brandano]

20 to 30 ma is a ballpark figure that works for most LED's. The forward voltage figure refers to the voltage drop that the LED will cause internally, in simple terms the minimum voltage it needs in order to conduct current at all. Any less and current just won't flow through it. Sometime this feature is used to drop a voltage for a small circuit or as a form of crude voltage regulation. plain red LED's usually drop 1.2 volts, white LED's up to 4 volts. The white LED's you can find in cheap keychain lamps that are driven directly by 3 button cells rely on the internal resistance of the button cells to limit the current, as you can imagine the internal resistance of rechargeable batteries, and expecially those used for RC purposes, is usually negligible when compared to alkaline cells or similar. The way I set up a proper load resistor for an unknown LED is to put in line the LED, a trimmer set up as a variable resistor that has about the double of the value I'd expect and an amp-meter. Starting with the trimmer set for max resistance I just trim it down until I get between 20 and 30 ma, and check if the LED brightness "seems right". then disconnect the lot, measure the value of the trimmer and find the nearest value resistor (by excess) to use as current limiting resistor.

[Darth_Elevator]

millicandela (mcd)Thanks, I always wondered what that stood for but never got around to googling it.

[24vdc]

OK, i'm a bit lost on these diagrams. what do the little circles on the ends mean on this example? where would the positive and negative battery leads go? sorry, this is all new to me.
http://www.hak5.org/releases/2x02/led-diagram.gif

**EDIT** NM. I just realized it's a combination series/parallel setup.... right?

[Darth_Elevator]

Yes, it's a series/parallel array and the circles represent the positive and negative leads. You realize, though, that it's only 9V input, right?

[24vdc]

Yes, it's a series/parallel array and the circles represent the positive and negative leads. You realize, though, that it's only 9V input, right?
yeah, that was just a quick picture i found. i didn't know how to post a pic of the diagram it made for me. anyway, thanks for the help guys and i'm officially proud of myself:
http://i174.photobucket.com/albums/w83/olvidame/lightssetup.jpg

[24vdc]

would it now be possible to put some sort of flasher module in my setup to get the leds to flash or even better, to chase? if there is something like that what voltage would i want it to be? the voltage that my circuit is now at, or 24v?

[Brandano]

you can get them all to flash, but to chase you need a more complex circuit, each LED on its own supply line, with a programmed IC driving them. In the simplest chase setup you can just run 3 lines set up as the one you have, and flash each line in sequence. By phisically placing the LED's in the right sequence you'll have a chase sequence every 3 LEDs. The easiest way to get them all to flash as a string of LED's is probably just to add a flashing LED in line with the others.

[Malc C]

you can get them all to flash, but to chase you need a more complex circuit, each LED on its own supply line, with a programmed IC driving them.

And as CMOS runs at 12V, and PICs need 5V, some form of regulation would be required to drop the 24v down to these levels.

If you simply wanted to make them all flash on and off at the same time, you could try a suitable automotive flasher, but it would need to be one that doesn't use a bi-metallic strip type as the current draw would not be high enough to make it function.

With a little bit of work you might be able to change the values of this oscillator to run off 24v rather than 9-15v

http://wild-bohemian.com/electronics/images/t-flash2.gif

[RonD]

With such a high voltage, you are wasting a lot of power in the series resistor. Could you string some of them in series to be more efficient? Maybe some type of series/parallel circuit. Or in the above multivibrator circuit, you could put half of the LED's in each side of the circuit and they would flash alternately.

[BushmanLA]

BTW, why is it called forward voltage. what does the forward mean? is it because it is referring to a diode?


Diodes are not linear devices. This means if you graph the current throughput vs a range of voltage it will not be a straight line.

Forward voltage refers to the minimum amount of voltage that the diode requires before it begins to conduct. Generally around .7V for your average diode.

If you ignore the whole forward voltage drop thing you will get the answer pretty much 90% right if you use R = V/I where V is your input voltage, and I is the current desired, about 20mA.

You might also see a reference to reverse breakdown voltage. This is the voltage required to make the diode conduct in the reverse direction. It is generally very high (10's or 100's of volts), which is why diodes are used to make sure current only goes in one direction. Some diodes called zener diodes have a much lower reverse breakdown and are used for special purposes.

[Dare_Devil]

if you would like i can draw you up some pcbs for a 555timer to make you leds flash together or a more complex one to make them flash in sequance