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Current Limiting Resistor

September 9, 2013 By Øyvind Nydal Dahl 31 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

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Reader Interactions

Comments

  1. Ken Murray says

    November 13, 2014 at 4:44 pm

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

    Reply
    • admin says

      November 13, 2014 at 5:03 pm

      Great to hear Ken!

      Oyvind

      Reply
  2. Rahul says

    December 17, 2014 at 5:38 am

    Thanks. Simple and great tutorial for a beginner.

    Reply
  3. faisal says

    July 1, 2015 at 1:04 pm

    what a nice post. i love it. thanks

    Reply
    • Brendan says

      January 26, 2019 at 1:08 am

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

      Reply
  4. Anurag says

    July 1, 2015 at 7:43 pm

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

    Reply
  5. Naik Javeed says

    September 27, 2015 at 5:04 pm

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

    Reply
  6. Prashnt says

    October 25, 2015 at 3:46 am

    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 at 3:49 am

    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 at 4:57 am

    Nope. No can do

    Reply
  9. Jum says

    December 28, 2015 at 4:58 am

    It’s not important

    Reply
  10. Solomon says

    February 5, 2016 at 2:50 pm

    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 at 3:10 pm

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

      Cheers!
      Oyvind

      Reply
  11. Mac Morvant says

    March 27, 2016 at 5:21 pm

    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 at 5:27 pm

    I really love this

    Reply
  13. Radek says

    September 1, 2016 at 10:59 pm

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

    Reply
  14. Georgekid says

    October 13, 2016 at 3:13 am

    thank you so much!!!!

    Reply
  15. Mark anthony says

    December 4, 2016 at 12:09 am

    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 at 3:30 am

    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 at 9:11 am

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

      Reply
  17. Jorge Taylor says

    April 10, 2017 at 2:43 am

    Very easy to understand,thank you.

    Reply
  18. Sarang Rajendra Thakre says

    April 13, 2017 at 4:25 am

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

    Reply
  19. Abdul Rasheed says

    May 11, 2017 at 4:52 pm

    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 at 5:58 pm

    Great

    Reply
  21. robert says

    July 27, 2017 at 3:32 pm

    i salute you sir thanks this webpage i learn much more

    Reply
  22. Yuv kishore says

    November 25, 2017 at 11:07 am

    Fantastic!Fantastic!Fantastic!

    I loved the way of explanation…Great work…

    So easy…

    Thanks a lot …

    Reply
  23. rosdy says

    January 12, 2018 at 10:15 am

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

    Reply
  24. Vikranth says

    January 23, 2018 at 11:13 am

    Beautiful explanation understood in my life…. thanx bro

    Reply
  25. Brendan says

    January 26, 2019 at 1:10 am

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

    Reply
  26. Bala says

    March 23, 2019 at 3:51 pm

    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 at 6:05 am

    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

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