Potential Divider Circuits (resistor + LED)

[Jimmy87]

Homework Statement

Attached to this thread is the question and mark scheme.

Homework Equations

The Attempt at a Solution

I got most of the question right but I have no idea about the explanation part of why to include the 100 ohm resistor which is required for the higher level. It says in the mark scheme about the fact that the LED gets 2V across it when points A-B get 6V. I have no idea how we know this? To know the LED gets 2V requires knowing its resistance which we don't know? Unless its related to the graph in the first part of the question but it says that is for a silicon diode. Also if it did relate to question in first part then 2V according to the graph would have much less than 50 ohms resistance

 

[gneill]

While LED's are diodes and do have characteristic curves that are very similar in overall shape to those of a typical silicon diode, their structure and chemistry is tweaked in order to enhance and tune their light emitting properties. This has the effect of altering the location of the "knee" in the curve. Whereas your basic silicon diode typically has this knee at about 0.7 V of forward bias, an LED's knee can be anywhere from 1 V for infrared LEDs to perhaps 4 V or so for blue LEDs. Do a web search for "typical LED characteristic curve" and see what you find.

 

[Jimmy87]

While LED's are diodes and do have characteristic curves that are very similar in overall shape to those of a typical silicon diode, their structure and chemistry is tweaked in order to enhance and tune their light emitting properties. This has the effect of altering the location of the "knee" in the curve. Whereas your basic silicon diode typically has this knee at about 0.7 V of forward bias, an LED's knee can be anywhere from 1 V for infrared LEDs to perhaps 4 V or so for blue LEDs. Do a web search for "typical LED characteristic curve" and see what you find.

Thanks. I still don't understand how they get the answer of 2V across the LED in the question. How do they know this?

 

[Jimmy87]

are we supposed to know that an LED is roughly 50 ohms. This is not listed in the textbook which this exam paper is based around and we haven't been taught it. Or is it a mistake?

 

[Jimmy87]

Apologies - please ignore this post. I found another post on another forum saying this question is incorrect. It is supposed to have the characteristic of an LED not silicon diode and the resistance of the LED is indeed 50 ohms. How annoying! Sorry!

 

[berkeman]

the resistance of the LED is indeed 50 ohms.

No, that is absolutely false. Please disregard whatever that post on the different forum is saying.

I still don't understand how they get the answer of 2V across the LED in the question. How do they know this?

By reading the datasheet for the LED. Depending on the color and efficiency of the LED, the forward voltage Vf of the LED will be shown in the 2V to 3V range. That's how you design the current limiting resistor for LED circuits that are driven by simple voltage sources.

Quiz Question -- Can you find a typical Red LED datasheet, and show us the min/typ/max specs for Vf for that LED?

 

[CWatters]

I got most of the question right but I have no idea about the explanation part of why to include the 100 ohm resistor which is required for the higher level. It says in the mark scheme about the fact that the LED gets 2V across it when points A-B get 6V. I have no idea how we know this? To know the LED gets 2V requires knowing its resistance which we don't know?

The fact that the LED forward voltage is 2V (rather than 0.7V) isn't really critical to the question. Suppose the question said that the circuit was used to plot the IV characteristic of a regular Si diode instead of an LED. Could you answer the bit about the 100 Ohm resistor then? Could you justify why its 100 Ohms and not say 1, 10 or 1KOhms?