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!

 

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

 

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.

 

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

 

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

 

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

 

Hello no12thward,

I found this in my mail:

you appear to be pretty knowledgeable of diode circuits. I am trying to find the voltage out of a diode circuit and seem to be having trouble. i have found the current passing through the diode, but need a method to find the voltage out?

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.

More info on diodes can be found in the links of this page from the EDUCYPEDIA:
http://www.educypedia.be/electronics/composemiconductorspassif.htm

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

Bertus

 

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

 

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!

 

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!

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.