Two leds in parallel reversed

[xcaldazar]

Well it is probably to late for me to get points for this last homework question but I still want to know how this would work out. I simply took a picture of my last homework question instead of attempting to redraw and retype the questions. I believe I have done part A correctly but I'm not even sure about that part. I wish I could just plug this into a breadboard and try it out. This is only our first homework assignment so I am aware this may be a bit simple for some of you, I got through the rest of the homework pretty easily so this one is bugging me. Although I was looking at them as in parallel not in a single element as the first question asks. I keep thinking myself in circle, your help is very appreciated.

**broken link removed**

 

[ericgibbs]

Well it is probably to late for me to get points for this last homework question but I still want to know how this would work out. I simply took a picture of my last homework question instead of attempting to redraw and retype the questions. I believe I have done part A correctly but I'm not even sure about that part. I wish I could just plug this into a breadboard and try it out. This is only our first homework assignment so I am aware this may be a bit simple for some of you, I got through the rest of the homework pretty easily so this one is bugging me. Although I was looking at them as in parallel not in a single element as the first question asks. I keep thinking myself in circle, your help is very appreciated.

hi,
Consider what the voltage is across a conducting LED.
You have a forward LED voltage of 2.4V across a conducting LED and a reverse breakdown of -20V .....???

 

[xcaldazar]

^ There are two LEDs not just a single one. They are facing separate directions. The teacher actually posts the answers after we turn in our homework which I wasn't aware of beforehand.
I should probably rewrite out his answers instead of posting them here since that may be some sort of copyright/plagiarism deal I would think.

for part A the graph as I thought showed the LEDs would turn on at 2.4 and -2.4 volts. It also had 20 and -20 volts but it was supposed to be assumed that the circuit would not reach +20 or -20 volts because it is already turn on at 2.4 and therefor would not have to worry about the reverse breakdown of either one. Which was the answer to part B.

That is what threw me through a loop, I figured the LED would burn out from too much voltage since the amperage would get way too high based on the graph. I wasn't sure what would happen once it did burn out, and once I asked the teacher he said I was thinking on the right lines but since I was not given a max wattage I was going beyond what he wanted us to think about. I guess we were supposed to assume it would not burn out.

From there part C was simple enough that light would be emitted whenever the voltage was less than -2.4 and more than +2.4.

 

[ericgibbs]

^ There are two LEDs not just a single one. They are facing separate directions.

hi,
I know, thats why I suggested you consider what the voltage is across a conducting LED, thats the REVERSE voltage thats applied across the non conducting LED.
So across the non conducting LED there is only 2.4V reverse voltage.
The LED specification is -20V for the LED's reverse voltage, so its well within its specification.

You are correct regarding the limitation of the current flowing thru an LED, it must be limited by a series resistor or a Constant Current source.