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You are here: Home / Basic Electronics / Current Limiting Resistor

Current Limiting Resistor

September 9, 2013 By Øyvind Nydal Dahl 47 Comments

A current limiting resistor is a resistor that is used to reduce the current in a circuit.

A simple example is a resistor in series with an LED.

fi-simplest-circuits

You would usually want to have a current limiting resistor in series with your LED so that you can control the amount of current through the LED.

If too much current is going through your LED, it will burn out too fast. If too little current is going through it, it might not be enough to lit the LED.

Calculating the necessary resistor value

A current limiting resistor in series with an LED

Check the datasheet of your component to find voltage drop and the appropriate current for your LED.

If you can’t find a datasheet, you can test it.

Connect the LED and a resistor in series to a variable voltage source. Start at 0 volts and increase the voltage gradually until the LED lit up.

Measure the voltage over the LED and the current going through.

Let’s say the LED needs 15 mA and has a voltage drop of 2 volts. You have a 5 V power source that you would like to power it with. Which resistor value do you need?

To find the resistor value, we start with finding the voltage drop over the resistor. Since there is a 2 volt drop over the LED, there will be a 3V drop over the resistor.

Ok, so we have 3V and we want to have 15 mA going through the resistor and the LED.

To find the necessary resistor value we use Ohm’s law.

V = R * I \Rightarrow R = \frac{V}{I}

this gives us

R = \frac{V}{I} = \frac{3V}{15mA} = 200 \Omega

So the necessary value for the current limiting resistor is 200 Ohms.

Choosing the right resistor

So you know that you need a 200 Ohm resistor.

But when you look for components you find that there are several different types of resistors.

Well, the only thing you need to be aware of is the wattage rating of the component. How much effect can the resistor handle?

So you need to figure out how much effect that will be dissipated in your resistor.

To find this you use the following formula for calculating power

P = I*V

It states that power equals the current multiplied with the voltage. So we get

P = 15 mA * 3V = 45mW

This means your resistor must be able to handle at least 45 mW.

Usually, most resistors handle from 250 mW and up, so it will be easy to find a suitable resistor.

Return from Current Limiting Resistor to Electronic Schematics

Filed Under: Basic Electronics

Reader Interactions

Comments


  1. Ken Murray says

    November 13, 2014

    This is great! Gives the detail we need in easy speak! Thanks!

    Reply

    • admin says

      November 13, 2014

      Great to hear Ken!

      Oyvind

      Reply

    • johnb says

      August 13, 2020

      yeeeeeeee

      Reply

  2. Rahul says

    December 17, 2014

    Thanks. Simple and great tutorial for a beginner.

    Reply

  3. faisal says

    July 1, 2015

    what a nice post. i love it. thanks

    Reply

    • Brendan says

      January 26, 2019

      Thanks for the explanation. You kept the explanation nice and simple which is good for the beginners.

      Reply

  4. Anurag says

    July 1, 2015

    Wow simple with a great value…….hats off….

    Reply

  5. Naik Javeed says

    September 27, 2015

    Very Nice! I’m really impressed with these topics!!

    Reply

  6. Prashnt says

    October 25, 2015

    very nice answer sir awesome which is really needy for learners.
    Sir, now in above you explained the conditions of requirement of resistor in circuit, i am very interested to know what are the conditions that makes to use capacitor & inductor in circuit & how to calculate the exact value of capacitor & inductor.
    please explain me the importance of EMI & EMC in the circuit. Please explain sir, waiting for your valuable reply.

    Regards,
    Prashant.

    Reply

  7. Prashnt says

    October 25, 2015

    Hi Sir,

    Please explain me what is the meaning of Equivalent Series Resistance(ESR) & Its importance in the circuit.

    Regards,
    Prashant.

    Reply

  8. Jum says

    December 28, 2015

    Nope. No can do

    Reply

  9. Jum says

    December 28, 2015

    It’s not important

    Reply

  10. Solomon says

    February 5, 2016

    What will happen if you use a higher value of resistor ?? .. I used a 9v battery 390ohm resistor to light up an led and it still works for 1k and even 10k .. Pls explain

    Reply

    • admin says

      February 8, 2016

      If you use a higher value resistor, you get less current through your LED and your LED will be less bright.

      Cheers!
      Oyvind

      Reply

      • Ranjith says

        January 8, 2020

        Sir
        In a 5watts Led drive circuit there are 4 tiny flat black resistors marked as R1, R2,R3,and R4′.on top of each resistor,400odds values are marked.
        I want to know i f that’s Ohm values or resistance power of each?
        2question
        Can I remove one of them from the circuit board so as to increase the brightness?
        Thanks sir

        Reply

  11. Mac Morvant says

    March 27, 2016

    It would be nice if Your teaching wouldn’t assume that we dommies new what these formulas were about , As You teach add notations to Your formulas !, to explain them , I no it takes up more space ! Pictures fill in the Q?????

    Reply

  12. Enoh Chuddy says

    June 24, 2016

    I really love this

    Reply

  13. Radek says

    September 1, 2016

    Thank you for this. Now I can order some resistors to test :)

    Reply

  14. Georgekid says

    October 13, 2016

    thank you so much!!!!

    Reply

  15. Mark anthony says

    December 4, 2016

    What if a have 12v input and needs to light up a 10v led.what resistor will i need and the type of transisitor to be use?

    Hoping you could help me.tnx..

    Reply

  16. Gene Bock says

    December 27, 2016

    Explaining scientifc knowledge (or fact) easy is not an easy thing to do. Great explanation and an example. What if I change + and – of the battery? Does an resistor still function what it has to do (does it reduce properly the voltage needed to the LED?)

    Reply

    • Gary says

      May 13, 2017

      Resistor are not polarity sensitive but LEDs are it will burn out the LED.

      Reply

  17. Jorge Taylor says

    April 10, 2017

    Very easy to understand,thank you.

    Reply

  18. Sarang Rajendra Thakre says

    April 13, 2017

    Nice sir give us information ike this????????????????☺????????☝????

    Reply

  19. Abdul Rasheed says

    May 11, 2017

    Great explanation and very easy to understand as a beginner like me.

    thanks for your great effort

    Reply

  20. DE APOGEE says

    July 24, 2017

    Great

    Reply

  21. robert says

    July 27, 2017

    i salute you sir thanks this webpage i learn much more

    Reply

  22. Yuv kishore says

    November 25, 2017

    Fantastic!Fantastic!Fantastic!

    I loved the way of explanation…Great work…

    So easy…

    Thanks a lot …

    Reply

  23. rosdy says

    January 12, 2018

    very easy to understand even im not very understand english…tq pal…

    Reply

  24. Vikranth says

    January 23, 2018

    Beautiful explanation understood in my life…. thanx bro

    Reply

  25. Brendan says

    January 26, 2019

    I liked the explanation as it was kept simple. Good for those beginners like myself

    Reply

  26. Bala says

    March 23, 2019

    Im having a circuit for 12 volt and 240 mA.
    Power source: 12 volt 5 amps transformer.

    What value of resistance should i place?

    Thanks.

    Reply

  27. James S Saltaformaggio says

    May 9, 2019

    This is exactly what I have been looking for since the age of 6(the first time I shocked myself, plugging in an Oreck vacuum for my grandma). It was then when my life’s mission to learn everything I can about this invisible monster…. Now I can plot my revenge…..⚡💣💥🤳🍻🍾

    Reply

  28. DNeale says

    April 8, 2020

    If we want the current through the entire circuit though shouldn’t we use 5V/15mA instead of 3V?

    Reply

    • admin says

      April 8, 2020

      5V is the voltage drop across both the resistor and the LED. But only 3V out of the 5V from the battery drops across the resistor (if we are using a 2V LED). So we need to use 3V.

      Reply

  29. Prince promise says

    July 7, 2020

    Thanks for the post. But i have a question that has been bothering me for long. Why don’t we put the power supply current into consideration?

    Reply

    • admin says

      July 7, 2020

      That’s because the current specified on the power supply is the maximum current, not the actual current. The actual current flowing is decided by the circuit, not the power supply.

      Reply

  30. Sungory says

    August 21, 2020

    Thanks for the lessons

    Reply

  31. Sunglory says

    August 21, 2020

    More strength to your elbow

    Reply

  32. Chris Augustine says

    September 4, 2020

    So a LED that has a Voltage drop V_drop = 1.8V / 20mA
    is a load on the circuit and thus has a resistance: in this case 90ohm
    this is independent of voltage as long as voltage is over 1.8V (right?)

    Seeing that a resistor is also a load and limits (and conducts) a current; is it even needed if we get the voltage to the LED right at V-drop (perfect world)?

    But for a given 5V voltage you have a spare 3.2V to get rid of. All voltage must be removed?
    So this is your equation to figure out a resistor (in a perfect world): (5.0 – 1.8V) / 0.020A
    or 160ohm.
    Resistors just waste heat so we should minimize their ohms, right? So we should minimize
    voltage every chance we have, right?

    Reply

    • admin says

      September 7, 2020

      Yes to all your questions =)

      Reply

  33. Levi Haskell says

    December 13, 2020

    What if my power source voltage matches the specified LED voltage, let’s say they are both 3V. Do I still need a resistor?

    Reply

    • admin says

      December 14, 2020

      Then it’s okay to skip the resistor.

      Reply

  34. Madalin Sava says

    December 21, 2020

    Wait, so how did you get to 3V across the resistor? You only measured the current and voltage for the LED. Is there a connection between the 2V in the LED and 3V in the resistor? I don’t get it.

    Reply

    • admin says

      January 4, 2021

      There is 5V in total. Since 2V drops across the LED, the remaining 3V must drop across the resistor, since there are no more components in series.

      Reply

  35. David says

    December 23, 2020

    Hi. Love the site. I suggest being more fastidious with your units. Make a note when using milliamps or milliwatts, because the formulas are in amps and watts. Conversions are easy, as you know, but people can get tripped up with that.

    Reply

    • admin says

      January 4, 2021

      Thanks, I’ll try to remember that!

      Reply

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