# Another LED question



## BillBrakeman (Jan 3, 2008)

Is there a simple way (Table or Chart) to determine the "Forward Voltage Drop" and "Forward Current" of LED?

I have LEDs of different colors and sizes, but the packages do not specify either of the above values needed to calculate the resistor.

Any suggestions?

Bill


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## Greg Elmassian (Jan 3, 2008)

all LEDs vary somewhat, but normally in your calculations you can assume 3.1 or so for white, and 1.7 for red... the only time this is critical is if you are using a very low voltage supply, close to that of the LED... (thus small changes in the dropping resistor value make large changes in the current) 

You are even more out in the cold with the current... unless the absolute max brightness is paramount, I just do 20 milliamps for normal leds... the small surface mounts are good at 6... high power ones can be anywhere from 30 on up. 

If max brightness is again critical, you have to experiment and blow a few up... maybe pick half the current that blows them up.. but that is still really not good, different qualities will have wildly varying results with over current. 

Just use the "nominal" values I suggested above and you will be fine. 

Greg


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## jimtyp (Jan 2, 2008)

If I don't know the specs on the LED, I use similar numbers as Greg posted above for calculating a resistor. I've even gone a little higher: 3.5 volts and 30 milliamps, and have never had a problem. As a disclaimer, I use exclusively a warm white LED, so I don't have any experience with colored LEDs. I stock piled a bunch of LEDs from a Walmart xmas sale a couple of years back. There are several calculators online. Assuming you have a single LED being powered this is the one I like the best:  LED calculator link


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## BillBrakeman (Jan 3, 2008)

Thank you Greg & Jim. Your posts are something I can work with.

Bill


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## Greg Elmassian (Jan 3, 2008)

Bill, go to the calculator Jim recommended and hit the ? under the forward voltage and under the current and you will see the author's "ballpark" figures too... he's a bit higher on red leds than my experience, but great recommendations nontheless. 

Greg


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## Treeman (Jan 6, 2008)

How is this for a real general rule? For track power I use a 1K resistor.


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## Greg Elmassian (Jan 3, 2008)

Poor... what is the current at 24 volts? 

if you use a red led, the current is 50 ma! Boom! 

Also, you would need a 2 watt resistor. 

So, not very good advice when you have an easy to use calculator for free... in my opinion. 

Greg


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## Gary Armitstead (Jan 2, 2008)

I was having the same "problem" last week with LED's. Paul Burch passed this calculator down to me. http://www.hebeiltd.com.cn/?p=zz.le...calculator

Also if you have Stan Silverman's software (Handy Converter V16), it contains a nice little calculator for led's. Source voltage, diode forward voltage, diode forward current (mA) and number of led's in the circuit. Very handy.


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## Dan Pierce (Jan 2, 2008)

Greg, please check your math, I get 24 volts divided by 1000 to be 24ma. However if allowing for a 3 volt led, it would be less, but not down to 20ma. 
So 24 volt track power through a decoder and then to any led would be around 20 ma for the led!! 

BUT, there are 10 ma leds and a larger resistor is needed, and there are decoders with led drivers rated for 10ma, so one must be real careful with dropping resistors.


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## Greg Elmassian (Jan 3, 2008)

I would swear his first post said 470 ohms... 

Seems to say 1k now....

Since I did not quote him, and there is no sign when a post is edited, I'll stand by what I said.... 

Greg


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## Gary Armitstead (Jan 2, 2008)

Posted By Greg Elmassian on 10 Mar 2013 11:47 AM 
I would swear his first post said 470 ohms... 

Seems to say 1k now....

Since I did not quote him, and there is no sign when a post is edited, I'll stand by what I said.... 

Greg 
When I read Mike's post around 5:40pm and made a reply, I saw 1K, not 470.


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## Greg Elmassian (Jan 3, 2008)

whatever... strange I would pull 470 ohms out of the thin air... (check my calculations for 470)

Greg


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## Treeman (Jan 6, 2008)

I agree it is best to use a calculator. My post did say 1K from the start. Just for the heck of it I just connected a red 3mm LED to a LGB 1 AMP throttle with a 1K resistor in series and had 2.16V across the LED. My Fluke shows 24.24V output.


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## Greg Elmassian (Jan 3, 2008)

OK, I guess I was hallucinating.... let's forget it. 

Yeah, 2.16 is pretty common, the led calculator recommend:

*color* *voltage (Volts)* *IR* 1.5 *red* 2.0 *orange* 2.0 *yellow* 2.1 *green* 2.2 *true green* 3.3 *blue* 3.3 *white* 3.3 *UV* 3.3 *blue* (430 nm) 4.6 
I was used to 1.7 in the real early days of LEDs, before high brightness / efficiency... in the late 70's we got a LED at HP labs, and we soldered it to a D cell... ran for months... of course was not running at max current because supply voltage was lower than typical forward voltage of the led..


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## Treeman (Jan 6, 2008)

The meter connected in series reads .020 AMP


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## Greg Elmassian (Jan 3, 2008)

Yes, yes, the numbers work... 

By the way, what wattage resistor were you using? 1/2 watt should be burning your fingers... 

back to the CL2's which I have been using for years... no more calculations... 

Greg


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## Treeman (Jan 6, 2008)

It is 1/2 watt, getting warm.


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## Greg Elmassian (Jan 3, 2008)

Yep .4 watts... leave it on and I'll bet you cannot hold it with your fingers continuously.... always double the wattage required... I have no idea why they rate resistors like this... a 1/2 watt resistor actually getting 1/2 watt will melt plastic. 

This is a warning to beginners, I know you already know this Mike. (the LED calculators adminishes you to use a 1 watt resistor, and issues a further warning about the voltage) 

Greg


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## BillBrakeman (Jan 3, 2008)

Meanwhile, back at the ranch. Do any of the posts coming after Greg’s and Jim’s (jimtyp) first three posts have any relevance to my original question? 
If not, does anyone have any advice that may be of use? 
Bill


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## Gary Armitstead (Jan 2, 2008)

Posted By BillBrakeman on 10 Mar 2013 07:44 PM 
Meanwhile, back at the ranch. Do any of the posts coming after Greg’s and Jim’s (jimtyp) first three posts have any relevance to my original question? 
If not, does anyone have any advice that may be of use? 
Bill 

Say what?!









Thank you. You're welcome.


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## Treeman (Jan 6, 2008)

The chart Greg posted gives the voltage for the different colors. Then figure from the calculators.


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## Dick Friedman (Aug 19, 2008)

Greg's mention of CL2 leads me to suggest that use of LED drivers negate the need for resistors and the calculation. One benefit of using the CL2 is that it appears that any number of LEDs (perhaps as many at 20) can be powered with them.


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## Greg Elmassian (Jan 3, 2008)

Just be very careful to connect them in the proper polarity or you can destroy them


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## Dick Friedman (Aug 19, 2008)

Yup! Been there, done that. It took me a while to figure out a from b from c. Then I had some pc boards made. After that, assembling the circuit was easy. Still got two made up, and boards for about three more, if anyone's interested.


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