Electronics Forums > Re: Calculating LED voltage drop

# Re: Calculating LED voltage drop

John G
Guest
Posts: n/a

 08-09-2003, 07:00 AM
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
--

John G

"bob cannetti" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed) om...
> I am having trouble understanding how to calculate the specific
> voltage drop of an LED. Can anyone clarify this for me? Thanks so
> much.

bob cannetti
Guest
Posts: n/a

 08-10-2003, 04:12 AM
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.

"Kevin Aylward" <(E-Mail Removed)> wrote in message news:<Ap1Za.12776\$(E-Mail Removed)>...
> John G wrote:
> > I don't think you can calculate the voltage drop of a LED.

>
> You can, but its a bit tricky. There is a formular for the Vsup.D.R.
> circuit here
> http://www.anasoft.co.uk/EE/widlarla...arlambert.html
>
> > 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.

>
> Yes. But its one generic formula.
>
> Id = Is.exp(Vd/Vt.N)
>
> Vd is the diode voltage, N a constant (~1-5 for leds), Vt is the thermal
> voltage.
>
> Kevin Aylward
> (E-Mail Removed)
> http://www.anasoft.co.uk
> SuperSpice, a very affordable Mixed-Mode
> Windows Simulator with Schematic Capture,
> Waveform Display, FFT's and Filter Design.

Kevin Aylward
Guest
Posts: n/a

 08-10-2003, 05:11 AM

>
> "Kevin Aylward" <(E-Mail Removed)> wrote in message
> news:<Ap1Za.12776\$(E-Mail Removed)>...
>> John G wrote:
>>> I don't think you can calculate the voltage drop of a LED.

>>
>> You can, but its a bit tricky. There is a formular for the Vsup.D.R.
>> circuit here
>> http://www.anasoft.co.uk/EE/widlarla...arlambert.html
>>
>>> 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.

>>
>> Yes. But its one generic formula.
>>
>> Id = Is.exp(Vd/Vt.N)
>>
>> Vd is the diode voltage, N a constant (~1-5 for leds), Vt is the
>> thermal voltage.
>>

bob cannetti wrote:
> Does the "Voltage Drop" of an LED vary directly with the amount of
> current going thru it?

Well, yes, thats what the above equation means.

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

If not, what are the common voltage drops for
> different LED's?

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
(E-Mail Removed)
http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

cpemma
Guest
Posts: n/a

 08-10-2003, 08:20 PM
bob cannetti wrote:
> 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.
>

You'll find Vf : If curves in the Kingbright datasheets
http://www.kingbright.com/product/ht...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.

 Thread Tools Display Modes Linear Mode

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is OffTrackbacks are On Pingbacks are On Refbacks are Off Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post Archimedes Electronic Basics 10 03-31-2008 05:56 AM sinoleds@gmail.com Beginner Electronics 0 12-04-2005 11:08 AM sinoleds@gmail.com Electronic Components 0 12-04-2005 11:06 AM sinoleds@gmail.com Electronic Basics 0 12-04-2005 11:04 AM Robert Murphy Hobby Electronics 2 03-17-2005 04:44 PM