# Diode Circuit with LED?

Discussion in 'Homework Help' started by no12thward, Jan 10, 2011.

1. ### no12thward Thread Starter Member

Jan 10, 2011
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0
need help solving and selected a standard value resistor to provide a current through this LED of 14mA. Best i could come up with was 1.1k from the table shown, but i'm almost sure i worked this one wrong. {note}Diode is a silicon, 0.7V).

Thanks guys!

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2. ### retched AAC Fanatic!

Dec 5, 2009
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What a sec.. that appears to be a LED.. so being silicon isnt the only worry.

You need to know the forward voltage.. If your teacher told you to use .7 for Vf, then thats a little odd, but it should be simple ohms law

R=V*I

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3. ### thatoneguy AAC Fanatic!

Feb 19, 2009
6,357
718
Find Vf of the diode, if you were told it is 0.7V, then it is probably not an LED. At Vf, current increases exponentially with voltage (it looks like a short circuit, basically).

Find the voltage between supply and Vf, divide by maximum current, and you have the resistance value you need.

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4. ### Cerkit Active Member

Jan 4, 2009
275
3
Under normal circumstances seems like it should be 15-0.7=14.3V
The divide that by the current.

But your graph suggests a Vf of around 1.5V , so 15-1.5=13.5V
13.5/0.14= 964 Ohms

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Apr 5, 2008
15,809
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Hello,

Why is the graph on the paper?
It is probably to look up the forward voltage of the led.

Take a look at the image what I have done:

Bertus

• ###### LED_question.png
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6. ### no12thward Thread Starter Member

Jan 10, 2011
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It all makes sense now once Ive figured out the value of the LED. Thanks Guys!

Apr 5, 2008
15,809
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Hello no12thward,

I found this in my mail:

As I do not answer in emails I will try here.

When you look in the modified dwaring you posted, you will see two lines drawn in the graph .

If I want to lookup the voltage at 14 mA, I look at the vertical line and see where the 14 mA is.
Then I go to the left until I reach the diode curve.
At this point I go down to the base line, where the diode voltage is given.

http://www.educypedia.be/electronics/composemiconductorspassif.htm

There is also a special page for leds:
http://www.educypedia.be/electronics/composemiconductorsled.htm

Bertus

8. ### hgmjr Moderator

Jan 28, 2005
9,030
214
I would start my analysis in the same manner as Bertus has done in his graph. However, I would continue the graphical approach by drawing a line from the 15V point on the x-axis through the intersection of the 14ma horizontal line and the diode curve on until it intersected with the y-axis at what I estimate would be 19ma. The slope of that line would be the resistance. The estimated slope would be 15V/0.019A = 789.4 ohms. The nearest resistor from the given table that would yield the closest LED current to 14ma would be 820 ohms.

hgmjr

9. ### no12thward Thread Starter Member

Jan 10, 2011
30
0
Thanks bertus,

Im sorry i did not provide sufficient information. The voltage out that I am actually looking for is for the attached circuit, if you can offer any help. Thanks!

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10. ### thatoneguy AAC Fanatic!

Feb 19, 2009
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In that circuit, look at the voltage drops of the 2 diodes, they add up to 1V.

Now look at the formula you've written for the current through the 1k resistor. They aren't the same number.

You need to inspect to calculate if the diodes have sufficient forward bias to be on, if so, the voltage across the 1k resistor will be the sum of the 2 diode drops, if not, it will be the formula you've written down.