LED resistor circuit

Thread Starter

sprinkles

Joined Apr 8, 2010
1
Hi, just a bit of confusion on this lab:
http://blog.lib.umn.edu/arriaga/chem8153/Lab9-Digital%20electronics.pdf

pg 3, part 3 a).

I have a circuit with an LED and a resistor. The V_in was +5V and we varied the resistance of the resistor, recorded the voltage difference over the resistor, and observed the light output of the LED.

questions:
a) Approximately how much current is necessary to make the LED appear brightly?
b) Approximately what is the voltage drop across the LED when it is lit?
c) Is the voltage drop essentially constant as long as the LED is lit?

We found around 1000 Ω and lower was where the light was still considered bright.

attempt @ solution:

a)
Am i to use ohm's law so that current is I = V_R/R and ignore any resistance due to the LED? Then, I would have a list of currents for each R? The current for the R = 1000 Ω would then be the current needed for the LED to appear brightly?

b)
I was going to assume V_in - V_R = V_LED to find the voltage drop across the LED for different R.

c)
I would infer from b)

Thanks a bunch!
 

gizmoman0

Joined Jan 21, 2010
26
hope this helps

Hi, just a bit of confusion on this lab:

questions:
a) Approximately how much current is necessary to make the LED appear brightly?
b) Approximately what is the voltage drop across the LED when it is lit?
c) Is the voltage drop essentially constant as long as the LED is lit?

We found around 1000 Ω and lower was where the light was still considered bright.

attempt @ solution:

a)
Am i to use ohm's law so that current is I = V_R/R and ignore any resistance due to the LED? Then, I would have a list of currents for each R? The current for the R = 1000 Ω would then be the current needed for the LED to appear brightly?


Yes due to Kirchoffs Current Law, any current entering a node must equal the sum of currents leaving the node. Since the resistor and LED are in series, the current through them is the same. Simply calculate I = V_R/R which is the current through the resistor as well as the LED.

b)
I was going to assume V_in - V_R = V_LED to find the voltage drop across the LED for different R.

Yes V_led = Vin - V_R


c)
I would infer from b)

The voltage drop would change according to the size of resistor. infer from part b

Thanks a bunch!
 
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