Calculate LED Current Draw w/ Resistor & 5V Supply

In summary, to drive an LED using a 5V power source and a resistor in series, you can use Kirchoff's Voltage Law to determine the voltage across the resistor (supply voltage - forward voltage) and then use Ohm's Law to calculate the current through the resistor. Since it is a series circuit, the current through the resistor will also be the current through the LED. It is important to check the LED datasheet for the typical operating voltage at a specified current, and use this information to calculate the proper resistance to avoid burning up the resistor.
  • #1
tangodirt
54
1
Simple question for you guys. I am trying to drive an LED with a 5V power source and a resistor in series.

I understand how to calculate the minimum resistance needed to drive the LED with maximum current ((voltage supply - forward voltage) / maximum current) but I can't for the life of me figure out how to calculate the current through an LED with a given resistor.

Say, we have our 5V source --> 330 ohm resistor --> LED with forward voltage of 3.15V --> ground. What's the current flowing through the circuit?
 
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  • #2
By applying Kirchoff's Voltage Law, you know the voltage across the resistance (supply voltage - forward voltage).

With this, you can apply Ohm's Law to calculate the current through the resistor (you know the voltage across it and the resistance). Since it is a series circuit, the current through the resistor = the current through the LED.
 
  • #3
mdjensen22 said:
By applying Kirchoff's Voltage Law, you know the voltage across the resistance (supply voltage - forward voltage).

With this, you can apply Ohm's Law to calculate the current through the resistor (you know the voltage across it and the resistance). Since it is a series circuit, the current through the resistor = the current through the LED.

Ugh, of course. Too simple. I knew I had enough information to back that number out, I suppose it has just been a long day and my brain already went into weekend mode. Thanks!
 
  • #4
You'll want to check the LED datasheet for typical operating voltage @ specified current.

Assuming a standard 5mm LED: White @ 20mA

(Vin - Vf) / R = I

Vin = 5 V
Vf = 3.2V (found from LED datasheet @ 20mA)
I = 0.02A

5V - 3.2 / R = 0.02A

R = 90 Ohm

Dont burn up your resistor! Calculate power in R:
P = I^2 * R
P = (0.02 * .02) * 90
P = .036W
 

1. How do I calculate the current draw of an LED with a resistor and a 5V supply?

To calculate the current draw of an LED, you will need to know the forward voltage of the LED (typically between 2-3.6V) and the desired current (usually between 10-20mA). Subtract the forward voltage from the supply voltage (5V) and divide by the desired current. This will give you the resistance value needed for the resistor.

2. Can I use any resistor for an LED circuit?

It is important to use the correct resistor for an LED circuit to ensure the LED does not receive too much current and become damaged. A standard 1/4 or 1/2 watt resistor with a 5% tolerance is commonly used for LED circuits. It is recommended to use a resistor with a higher wattage if the LED will be receiving a high amount of current.

3. How does the supply voltage affect the LED current draw?

The supply voltage affects the LED current draw by determining the amount of voltage that is available for the LED. The current draw will decrease as the supply voltage increases, and vice versa. It is important to ensure that the supply voltage is within the voltage range of the LED to avoid damaging it.

4. What happens if the resistor value is too low for an LED circuit?

If the resistor value is too low for an LED circuit, the LED will receive too much current and may become damaged. This can also result in the LED appearing brighter than desired. It is important to calculate the correct resistor value to ensure the LED receives the appropriate amount of current.

5. Can I use a voltage regulator in place of a resistor for an LED circuit?

No, a voltage regulator cannot be used in place of a resistor for an LED circuit. A voltage regulator is designed to maintain a constant output voltage, whereas the purpose of a resistor in an LED circuit is to limit the current. Using a voltage regulator in place of a resistor can result in damaging the LED.

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