Voltage across LED

Ol' Geo

Hi,

I'm studying What's a Microprocessor? course to refresh my ol' brain.

One thing in the text bothers me. On page 75 (84 on pdf), it says the voltage across a LED is ALWAYS 1.6 volts if Vdd is applied to 5 volts. But, my DMV measured the voltage to 2.0 volts. Why different? Is it because of the newer LED's on market?

Thanks.

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Ol' Geo
Retired Software Engineer

An oscilloscope is a window of unseen electronic world. - GM

Mike Green

I don't know the specifics of your setup, but the forward voltage of an LED (Vf) depends on the materials involved which usually·determines the color.· You can look at an LED datasheet for this data, but red LEDs usually have a 1.7V forward voltage.· Some others have a Vf = 2.1 (like green? yellow?).· Blues and whites have the highest Vf ... 3 something.

Ol' Geo

Thanks, Mike

Steph Lindsay

Thanks for pointing this out, Ol' Geo.· We have put·this·on our revision list for the next edition of "What's a Microcontroller?".· I hope you are otherwise enjoying it.· Please feel free to email me directly with any errata you find:editor@parallax.com.

Just FYI, there is an errata sheet of known issues available for download here:

http://www.parallax.com/dl/docs/books/edu/WAMv2.2Errata-v1.4.pdf

Stephanie Lindsay
Editor, Parallax Inc.

Post Edited (Steph Lindsay (Parallax)) : 8/14/2007 7:32:50 PM GMT

Ol' Geo

Glad to help, Stephanie. I think I found one earlier. Will report via email.

I don't see any errata sheet for WAM 2.2 in the link you posted. I already have WAM 2.2 text in pdf format.

Thanks.

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Ol' Geo
Retired Software Engineer

An oscilloscope is a window of unseen electronic world. - GM

Ol' Geo

Now I remember. In my old days, I knew the forward voltage across LED is ALWAYS fixed no matter how much current is fed thru LED. A LED is a diode which acts like a voltage regulator. Me, right?

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Ol' Geo
Retired Software Engineer

An oscilloscope is a window of unseen electronic world. - GM

Steph Lindsay

Oops - now that's ironic - an error posting the link to the errata file...I have corrected it.

-Stephanie

pwillard

Ol Geo said...
Now I remember. In my old days, I knew the forward voltage across LED is ALWAYS fixed no matter how much current is fed thru LED. A LED is a diode which acts like a voltage regulator. Me, right?

I'm not so sure I'd agree.··An LED is a diode that just happens to excite some material it contains·enough so·that it lights up.· The·behavior electrically is still as a diode.· You'd need more than just a diode to regulate voltage.· I'd use the term voltage·"adjust" or maybe say that a diode can be used in a voltage reference (crudely, unless it was actually a reverse biased zener).·· I'd ·a say diode reference is only valid if the supply voltage you are referencing a drop against is known and stable.


·The forward voltage drop of a diode is predictable and can sometimes be used to alter the "relative" voltage levels in a circuit.· You can see this commonly used in Voltage Level Shifting circuits for hobbyist flash device programmers where the designer used the voltage drop of a diode to alter the ground reference of a 3-pin regulator (making it regulate a different voltage than marked on the package).

I almost lost my self there... hope I didn't make that come out clear as mud.
·


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There's nothing like a new idea and a warm soldering iron.

Post Edited (pwillard) : 8/14/2007 10:51:01 PM GMT

Mike Green

The forward voltage of a diode is sometimes used as a voltage reference since it's dependent primarily on the material used to make the diode. It's not highly dependent on the forward current, but it is somewhat dependent on the current and the temperature. It's also not identical from diode to diode, so it's not a great voltage reference, but can be used as a rough low voltage reference.