Calculating R1 for LED Circuit Question with 12V Supply Voltage

[cissey]

In the circuit the LED's rated current is 20 milliamphers. With a voltage drop across the LED of 1.5 volts, calculate the value of the series resistance R1 necessary to be used if the supply voltage is 12 volts DC.
Here is how I answered this, I am not sure if I am right.
R1=Er1/Ir1
R1= 10.5/.020
R1=525 ohm
Can someone help me please?
This problem has me confused

 

[Ouabache]

In the circuit the LED's rated current is 20 milliamphers. With a voltage drop across the LED of 1.5 volts, calculate the value of the series resistance R1 necessary to be used if the supply voltage is 12 volts DC.

Nice job, describing your question and showing what you've worked out.

The next small learning steps are: (1) start this as a new thread (it is a different question from the LC one). (2) Notice we had your LC question moved over to the homework> introductory physics subtopic. If I were asking your LED question, I would probably post under: homework>engineering.

You're method is correct, using Kirchhoffs Voltage Law around circuit path.
Vs-V_led - (I R_lim) = 0
12-1.5 - (0.20)R_lim = 0
R_lim= (12-1.5)/0.20 = 525$\Omega$

Depending on the extent of your assignment, you may be able to stop there. But if you were asked to choose a resistor, they don't make 525$\Omega$ resistors. To choose a practical one, you can find standard values on a http://ece-www.colorado.edu/~mcclurel/resistorsandcaps.pdf" . You don't want to exceed the current rating spec, so your limiting resistor needs to be $\ge$ 525$\Omega$ Using the table of std. values, the closest would be 560$\Omega$

For practical applications, you wouldn't want to run the LED continuously at 20mA, as it can shorten the life of your LED, causing the junction to break down. One http://www.etcs.ipfw.edu/~linm/2005Spring/cpet190/suppl/LED_Rs/LED_Rs.html" , for choosing a minimum limiting resistor value, is one that operates at 80% of the rated current. (20mA x 80%) = 16mA. R= (12-1.5)/0.016 = 656$\Omega$. Again choose a standard resistor greater than or equal to the calculated value, from the table. Other practical considerations to consider are; choosing tolerance and wattage rating for your resistor. If you haven't covered those concepts yet, you probably will soon.

 

[Mene]

.

Your approach to solving this problem is correct. To determine the value of R1, we can use Ohm's Law, which states that the resistance (R) in a circuit is equal to the voltage (V) divided by the current (I). In this case, the voltage is the supply voltage (12V) minus the voltage drop across the LED (1.5V), which gives us a voltage of 10.5V. The current is given as 20 milliamperes, which needs to be converted to amps by dividing by 1000. This gives us a current of 0.020 amps.

Plugging these values into Ohm's Law, we get:

R1 = V/I
R1 = 10.5V / 0.020A
R1 = 525 ohms

Therefore, the value of R1 necessary to be used in this LED circuit with a 12V supply voltage is 525 ohms. This resistance will limit the current flowing through the LED to the rated current of 20 milliamperes, ensuring that the LED operates safely and efficiently.