Calculating The Minimum Resistor Value For An LED

LED Schematic
Written by John Woolsey

Have you ever wondered why we typically see 330 Ω resistors used when interfacing light emitting diodes (LEDs) to digital electronics? Let’s figure out why that is.

Why does the minimum resistor size matter? Because if the resistance is too small, we could damage the LED, or worse, the surrounding circuitry. This would cause us to either waste time troubleshooting why something is not working quite right, or worse, cause a little smoke cloud on our workbench forcing us to purchase new components or entire development boards.

First, we need to understand the electrical environment where the resistor will be used. This involves knowing the maximum and minimum values for the various voltages, currents, etc. that the resistor will encounter.

Digital electronics typically have supply voltages, Vs, that fall within the 4.5 to 5.5 V range.

The forward voltage, Vf, of an LED often falls somewhere between 1.2 V and 4.0 V depending on the type of LED.

Again, depending on the type of LED, the maximum recommended forward current, If, typically has a range of 15-80 mA.

And finally, resistor tolerances, Tr, typically range between 1% and 10%. This means that a resistor with a specified value of 330 Ω and tolerance of 10% can actually have a resistance as low as 297 Ω (330 – 10%) or as high as 363 (330 + 10%) Ω.

To summarize:

  • Supply Voltage: Vs = 5 ± 0.5 V
  • LED Forward Voltage: Vf = 1.2-4.0 V
  • Maximum Recommended LED Forward Current: If = 15-80 mA
  • Resistor Tolerance: Tr = 1-10%

To calculate the resistance value needed for a resistor placed in series with an LED, we will use Ohms Law

\(\large V=IR\)

which can be rewritten as

\(\large R=\frac{V}{I}\)

and entering the voltage across the resistor and the current through the resistor for the V and I values respectively.

To determine the minimum resistance value required, we need to know the largest possible voltage drop across the resistor along with the smallest maximum current through the resistor. In addition, we want to account for the smallest resistance value possible based on its tolerance specification. Taking all these into account, the previous equation becomes

\(\large R=\frac{V_{s(max)}-V_{f(min)}}{I_{f(min)}(1-T_{r(max)})}\)

Entering actual values into the equation, we get

\(\large R=\frac{5.5-1.2}{0.015(1-0.1)}\approx318.5\hspace{0.25em}\Omega\)

Since this is not a standard resistor size, we want to round up to the next highest standard value, which gives you 330 Ω.

This is the smallest resistor value that can safely be used with almost all LEDs without damaging the LED or the digital circuitry to which it is connected. The resistor value, however, may need to be lower to sufficiently supply power to some of the more exotic types of LEDs. Additionally, you may want a smaller resistor value in order to have a brighter LED, but be careful as many digital electronics have a maximum current of 20 mA.

Let’s see a couple of real world examples:

For a Standard Red 5mm LED:

\(\large R=\frac{5.5-1.7}{0.018(1-0.1)}\approx234.6\hspace{0.25em}(240)\hspace{0.25em}\Omega\)

For a Super Bright White 10mm LED:

\(\large R=\frac{5.5-3.0}{0.07(1-0.1)}\approx39.7\hspace{0.25em}(43)\hspace{0.25em}\Omega\)

These values are for specific LEDs and may be too small for some LEDs. Also note that the super bright white LED requires a current much larger than the 20mA maximum current mentioned earlier.

Hence, this is why we typically see 330 Ω resistors used when interfacing LEDs across a wide variety of circuits.

As always, please consult the datasheets for the components used in your particular application to determine the best resistor values needed in your circuit.

About the author

John Woolsey

John is an electrical engineer who loves science, math, and technology and teaching it to others even more.
He knew he wanted to work with technology from an early age, building his first robot when he was in 8th grade. His first computer was a Timex/Sinclair 2068 followed by the Tandy 1000 TL (aka really old stuff).
He put himself through college (University of Texas at Austin) by working at Motorola where he worked for many years after that in Research and Development.
John started developing mobile app software in 2010 for himself and for other companies. He has also taught programming to kids for summer school and enjoyed years of judging kids science projects at the Austin Energy Regional Science Festival.
Electronics, software, and teaching all culminate in his new venture to learn, make, and teach others via the Woolsey Workshop website.


    • Forward voltage is the voltage drop across a diode when its anode (positive terminal) is at a higher voltage than its cathode (negative terminal). A diode connected in this way is referred to as being forward biased. A diode connected in the opposite direction has a reverse voltage and is reverse biased. LEDs only “turn on” when they are forward biased which is why you typically see specifications for Vf and If.

    • Oh. I guess I always assumed that the negative part had a zero voltage. Is forward voltage just another way of saying positive end of, in this case the LED?

    • It is not really the “end” of something as much as across something. Typically you put a resistor in series with an LED to limit the current through the LED. The resistor could be connected from the positive terminal of a battery to the positive terminal (anode) of the LED or the resistor could be connected from the negative terminal (cathode) of the LED to ground. So the forward voltage is the difference in the voltage across the LED regardless of how it is connected.

  • Put another way, is it safe to say the voltage after the diode is lower than prior to the diode? Sorry for all these dumb questions but it isn’t like that many people know this stuff!

    Also, does a resistor thought to block amps, volts or both? I’ve never known that either.

  • Instead of forward voltage why not call it soaked up voltage?

    Just so I understand, is it ok to say v5-forward voltage number=output voltage?

    Thanks again, I don’t know if I have trouble with these concepts because of the words used or for some other reason. Often if the words used don’t match my understanding of those words I can’t quite get the concept. I just realized that now of all things and I am close to social security age!

    • Forward voltage is not referring to whether the device produces or consumes, but whether the device is connected in a forward or reverse orientation. The device could still have a voltage drop in both orientations.

      Forward voltage is not equivalent to output voltage. Output voltage is used for devices that produce or supply a voltage, such as a battery or AC/DC converter/adapter.

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