# Simple question about LED's...



## Brakeman Jake (Mar 8, 2009)

I have installed a few LED's so far and always trusted the suppliers (NIMT and the local electronics store guys) to calculate the proper resistor for the LED's supplied.It always worked fine so I never really new what is the voltage a LED needs to work.Do all LED's run on the same voltage and what is it?If not,how can one know what's the specific voltage for any given LED?They pretty much look all similar.

I believe the formula to calculate the resistor is simple...I just don't know it.Then,pushing the question a little further...if LED's are used in series,does the resistance get halved?

I know this is basic for many.I know DC,understand voltage,amps,polarity,etc but electronics are still a new field to me.Thanks for baring with me.


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## Carl (Feb 19, 2012)

I do not want to step on the foot of the more experienced electrical folks on this site. I have found this site to be most informing for my needs with LEDs

http://www.kpsec.freeuk.com/components/led.htm


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## gunrunnerjohn (Nov 10, 2010)

The site Carl posted has some decent information.

Most white and blue LED's have an operating voltage of about 3 volts, most other colors have an operating voltage of about 1.5 volts. This is primarily due to the characteristics of the materials used to create the various colors.

The computation is simple. If you want to run an LED at 20ma (maximum for most) and you have 10 VDC, you need to drop the excess across the resistor. For example, a white LED would require you to drop 7 volts. That computes out to a 350 ohm resistor.

Note that I very rarely run LED's at the maximum ratings, typically they're much too bright for modeling. The exception might be for buildings and perhaps locomotive headlights, but even then I rarely get there.

I typically start out with a 1,000 ohm resistor and see how bright it is, then tune the resistor to the light output desired. As long as you don't go below the minimum resistance (and maximum LED current), you're fine. I almost never do the actual calculations, since I don't want maximum brightness anyway. Many LED's I use for markers and the like are running on a small fraction of the maximum current allowed.


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## waltr (Aug 15, 2011)

I do the calculation just the ensure I don't over current the LED and to get a starting value for the resistor. But since there are some big differences in the efficiency of different LEDs the brightness is not as easy to predict. So then the resistor value needs adjustment.
As other said (gunrunnerjohn) for most LEDs and 12V 1k Ohm is a good starting point.
This is about 10mA for the yellow & red LEDs and 9mA for the blue & white LEDs.

The equation if you're interested is:
(Vpower - V LED drop)/LED current
If you put two LEDs in series then the V LED drop is twice.

Unfortunately, most of the LED sources do not have a data sheet or even basic specs available. This means you must guess and pick a conservative current for a starting point (1K Ohm).
High quality LEDs from a know source would have a full data sheet that also includes the optical output and illumination angles. With this data the exact resistor values for a wanted brightness can be calculated.
Here is a link to such a data sheet:
http://www.cree.com/~/media/Files/C...HB/Data Sheets/C513A WSN WSS MSN MSS 1042.pdf


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## gunrunnerjohn (Nov 10, 2010)

Even with exact datasheets, computing the exact resistor value is somewhat of a fool's errand, because when you look fire it up, how do you know that you want some exact illumination value?


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## waltr (Aug 15, 2011)

For hobbyist this is completely true.
For optical engineers knowing the illumination is a requirement.


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## T-Man (May 16, 2008)

This is probably my worst thread. Search "Ohms Law" and you will find enough reading.


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## gunrunnerjohn (Nov 10, 2010)

waltr said:


> For hobbyist this is completely true.
> For optical engineers knowing the illumination is a requirement.


Not many optical engineers here.


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## manchesterjim (Dec 30, 2011)

Before I jump into the debate, I want to re-answer the original question:

As stated earlier, 1.5 volts DC (on average) for most LEDs, however blue and white can use up to 3 volts.......that said, you'll probably have some form wall wart or power supply on hand.....usually anything from 9 to 13 volts DC....just make sure you use the resistor in series as you've seen from LEDs that NIMT sent you.



waltr said:


> For hobbyist this is completely true.
> For optical engineers knowing the illumination is a requirement.


Kinda thought this was a hobbyist's forum




gunrunnerjohn said:


> Not many optical engineers here.


And even if you take the time to do all the calculations....you still don't know the exact "illumination" value until you measure with a light meter...then you'll need to know very specific things like, viewing distance, angle, etc......and then you still have variables like, power supply stability, connection methods to the power supply and inline resistors (not many of us have machine-like precision when it comes to soldering) (crimping and screw terminals will create higher voltage drops too)....

John's method is the more practical way to do it.....start with 1K resistor, look at the thing on the bench....if the light level and color suits you then fine, if not, try different values of resistors to get the level of light you want.....simple as that....:thumbsup:

Oh and by the way....if you happen to let the smoke out of an LED (I know I've done it).....no big deal, you're out the few cents you paid for it....and you can plug in another one! 

Jim


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## gunrunnerjohn (Nov 10, 2010)

The only thing I'd add is that it is nice to know what the minimum value of resistor is for the voltage you'll be supplying. A real simple rule of thumb is to have 50 ohms for each volt of the supply, that gives you 600 ohms for a 12V supply, 900 ohms for an 18V supply, etc. Going up from that value is not a problem, going down much will start to get into over current through the LED. For the truly anal among us, you can subtract the working voltage from the resistor value to squeeze the absolute maximum light from the LED, but the difference in light output will be minimal. Also, remember that if you're powering them on AC with a single diode, you divide the voltage by two, since you're powering them with half the cycle, hence half the effective voltage.


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