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Los Angeles, CA
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Hey guys,

Another question that I'm sure is very basic but I am having a hard time finding a concise answer.  How do you determine the forward voltage drop across an LED? Is this the same thing as the FW Supply listed in the LEDs tech specs? How can you measure a forward voltage drop with a multimeter?  Thanks so much in advance and I apologize once again for my extreme n00bness!  smiley-sweat
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Fennel Rye

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Hi,

Put 5Volts across the LED and a series current limiting resistor of around 470 or 680 omhs, the value of the resistor is not vital provided that its low enough for the LED to light and high enough to stop it from burning out, the suggested values are good for a 5 volts supply.

The values specified will give us around 10ma of current through the LED.

I = V/R

.010 = 5/500

Now measure the voltage across the LED with your volt meter, thats the forward voltage drop. It will vary slightly across a batch of LEDs, It will vary a lot between different colour LEDs.

Here is a great resource that will tell you much more about LEDs than I can -

http://www.thebox.myzen.co.uk/Tutorial/LEDs.html

Duane B

rcarduino.blogspot.com
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Thanks, Duane! One thing I just want to clarify, though.  Is the FW supply listed on a LEDs tech specs the same thing as the voltage drop?
« Last Edit: May 23, 2012, 02:19:22 am by Fennel Rye » Logged

Fennel Rye

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Is the FW supply listed on a LEDs tech specs the same thing as the voltage drop?
Roughly speaking: yes (IMO)
I'ts rather the maximum voltage drop. Current will increase until the led dies without rising the voltage drop significantly above that VF
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Is the FW supply listed on a LEDs tech specs the same thing as the voltage drop?
I have never seen a FW supply listed in any data sheet for an LED, such a thing does not make any sense. I have seen a forward voltage listed, this is normally a range of voltages, any one LED could be anywhere in that range. The forwards volts drop can varie with the led's age and temperature.
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I have never seen a FW supply listed in any data sheet for an LED, such a thing does not make any sense.

On the back of the packaging of an LED from Radioshack it lists a "FW supply: 3.3V (typical), 4.0V (max.)".  Does this mean you should plan for and use resistors for this max setting (in this case, 4.0V)? 

Or is the best method for figuring out an LEDs voltage drop what Duane explained above?
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Fennel Rye

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On the back of the packaging of an LED from Radioshack
It goes to show you what muppets inhabit Radio Shack.

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FW supply: 3.3V (typical), 4.0V (max.)
So that should read
FW voltage drop: 3.3V (typical), 4.0V (max.)

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Does this mean you should plan for and use resistors for this max setting (in this case, 4.0V)? 
No you should use the average for calculating the resistor and then go check the current if it were to be the minimum and maximum.

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Or is the best method for figuring out an LEDs voltage drop what Duane explained above?
Yes it works for that one diode but as they age this value will change.
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Or is the best method for figuring out an LEDs voltage drop what Duane explained above?

You can't reliably 'figure out' or 'predict' an LEDs voltage drop and you really don't usually have to since this value is a characteristic, not a rating.

If you are thinking along the lines of supplying 'voltage' to an LED you are doomed to failure.  This is because you do NOT apply some specific voltage voltage to an LED to get it to work.  You apply some arbitrary voltage that is greater than the expected forward voltage of a typical LED of the type that you propose to use.  You then use a series resistor to limit the current to a value that is less than the forward current rating of your LED.

It is in the calculation of the series resistance that you use the forward voltage characteristic.  Here is an example that I did for a previous post.

To deal with an LED you start with the forward current that you need, typically about half it's maximum rated value (lets use 20 mA). 

Next you estimate what the forward voltage drop will be with that current flowing through the LED.  If you have a datasheet for the LED you may be able to get a fairly accurate value, otherwise you take a guess based upon your experience or the experience of others (I usually use 1.7v for a red LED). 

Next you pick out a supply voltage which must be higher than the voltage you just determined (I'll use 5v). 

Now you can use Ohm's law to determine the required resistance.  The voltage across the resistor will be the difference between the supply voltage you decided to use and the voltage that you guessed would be across the LED (5v - 1.7v = 3.3v).  The current through the resistor will be the same as the current through the LED (20mA).   Ohm's law for the resistor says that R = V/I (R = 3.3/0.020 = 165 ohms). 

You then pick the closest value resistor that you happen to have and stick that in your circuit.  Most likely the current won't be exactly what you desired and the LED voltage won't be what you guessed would be there but you won't see any smoke either and you will see light from the LED.


Don
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as they age this value will change
Any real life experiece with that?
Data sheets don't tell anything in this direction, or am I just not able to interpret them properly?


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Any real life experiece with that?
Yes.

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Data sheets don't tell anything in this direction
Yes the do they give a maximum and minimum range for this parameter.
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An LED's brightness is determined by the CURRENT flowing through it. You can run it near its maximum current rating and minimise fluctuations by using a simple (one transistor) current regulator.

You can make it look even brighter by supplying it with a train of high current pulses, such that the average current does not exceed its maximum rating. The beauty of the Arduino is that you don't need to build a fancy pulse generator. You can do it in the firmware! Just make sure that the driver transistor can cope with the current pulse. For example, a ZTX650 with a 10 Ohm series resistance to limit the maximum current would be a starting point. Just make sure that it's impossible for the LED to remain on. It must always be pulsed. (Refer to the simple LED flasher example.)
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Re: Aging effect on voltage drop
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Data sheets don't tell anything in this direction
Yes the do they give a maximum and minimum range for this parameter.
I understand (now, thanks) that the "typ." and "max." values for VF are not only individually different,
but may change over time for a given led.
(Besides a temperature dependency of course)

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Any real life experiece with that?
Yes
It it usual that VF will grow with age ( making current decrease in a given resistor controlled circuit )?
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It may be worth noting that different color LEDs have reasonably predictable "typical" forward voltage characteristics. For example, an ultra-bright blue or white LED is likely to clamp at around 4 volts, whereas a red LED may be nearer 1.2 volts. Other colours will be somewhere in between.
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It it usual that VF will grow with age
Yes. That is also coupled with the fact that for any given current less of it goes into producing light.
Most life times of LEDs are quoted as half life, the time it takes for the brightness to drop by half.
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I understand (now, thanks) that the "typ." and "max." values for VF are not only individually different, but may change over time for a given led.
The 'typical' values are not only individually different, but may change over time for a given led.

The 'maximum' value will not be exceeded by any led at any time (unless you abuse it).


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It it usual that VF will grow with age ( making current decrease in a given resistor controlled circuit )?
In many applications this is of little consequence since the LED current can vary over a wide range without the amount of visible light, as perceived by the human eye, appearing to change.

Don
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