# 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
--

John G

Wot's Your Real Problem?

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

John G, Aug 9, 2003

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.

"Kevin Aylward" <> wrote in message news:<Ap1Za.12776\$>...
> 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/widlarlambert/widlarlambert.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
>
> http://www.anasoft.co.uk
> SuperSpice, a very affordable Mixed-Mode
> Windows Simulator with Schematic Capture,
> Waveform Display, FFT's and Filter Design.

bob cannetti, Aug 10, 2003

3. ### Kevin AylwardGuest

>
> "Kevin Aylward" <> wrote in message
> news:<Ap1Za.12776\$>...
>> 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/widlarlambert/widlarlambert.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

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, Aug 10, 2003
4. ### cpemmaGuest

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/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.

cpemma, Aug 10, 2003

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