# LED resistor circuit

Discussion in 'Homework Help' started by sprinkles, Apr 8, 2010.

1. ### sprinkles Thread Starter New Member

Apr 8, 2010
1
0
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!

2. ### gizmoman0 New Member

Jan 21, 2010
26
1
hope this helps

Mar 24, 2008
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