# Re: Calculating LED voltage drop

Discussion in 'Electronic Basics' started by John G, Aug 9, 2003.

1. ### John GGuest

I don't think you can calculate the voltage drop of a LED.
It depends on the construction and comes from the LED manufacturer's data
sheet and is determined by the construction theniques. Super bright LEDS
have a higher forward voltage than others.
You only need to calculate a resisance to go with a particular LED and
supply voltage.

Try here http://www3.telus.net/chemelec/Calculators/LED.htm

2. ### bob cannettiGuest

Does the "Voltage Drop" of an LED vary directly with the amount of
current going thru it? If not, what are the common voltage drops for
different LED's?
thanks, bob.

3. ### Kevin AylwardGuest

Well, yes, thats what the above equation means.

Vd = vt.N.ln(Id/Is)

If not, what are the common voltage drops for
However, because of the log variation, it doesn't vary very much. If the
current changes by a factor of 10:1, the voltage only changes by 60mv.
Typical led voltages are 2-3 volts at their specified operating current.

Kevin Aylward

http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
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4. ### cpemmaGuest

You'll find Vf : If curves in the Kingbright datasheets
http://www.kingbright.com/product/h...on=english&file=detailcategory&PriCategory=13,
or at any other led maker's.

A typical blue will drop about 3.4v @ 2.5mA, 3.7v @ 10mA and 4.2v @ 30mA.

White and some pure green are in the same ballpark, red, orange, yellow,
amber and other greens are around 1.8v @ 5mA up to 2.2v @30mA.

It does vary with the exact chemistry, so check the datasheet for accuracy.