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Potentiometer characteristics

Discussion in 'General Electronics Discussion' started by NZA, Dec 31, 2014.

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  1. NZA

    NZA

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    Dec 31, 2014
    Hello,

    I am beginner in electronics and i would like to build variable intensity LEDs with a potentiometer :

    Input voltage : 5V
    Leds : 5 parallel LEDs (maxi 30mA) with 180 Ohms resistors.

    I would like to variably dim these LEDs intensity, which potentiometer can I use (characteristics)?

    Thanks in advance,

    Nassim
     
    Last edited by a moderator: Jan 2, 2015
  2. chopnhack

    chopnhack

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    Apr 28, 2014
    Welcome to the forum Nassim, you may want to read this resource before proceeding on LED's. Assuming that you have your 5 led's in parallel and they EACH have a resistor, each LED would receive about 27mA of power. Are you certain your LED's can sustain 30mA? Three more mA and you are at your LED's working limit - to keep things safe you may want to consider using a 220Ω resistor to give yourself more headroom. (Remember that resistor tolerances can range quite a bit and if the resistor is not quite up to its spec you may overdrive the LED and burn it out).

    At 220Ω, we are now at ~23mA, I doubt their would be an perceivable difference in brightness from 27mA. Try using a 1k pot with a 220Ω resistor. You would need one pot and resistor for each LED. Another method would be to use pulse width modulation.
     
    Arouse1973 likes this.
  3. Colin Mitchell

    Colin Mitchell

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    Aug 31, 2014
    "I doubt their would be an perceivable difference in brightness "

    Can you rite in Emglish?

    "Remember that resistor tolerances can range quite a bit"

    When was the last time you bought a resistor???????

    "each LED would receive about 27mA of power."

    Where did you learn your electronics from ????

    Firstly, LEDs are designed for 17mA Max
    If you use any colour other than white, put 2 LEDs in series.
    Use 180R resistor and 1k pot in series
     
  4. chopnhack

    chopnhack

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    Apr 28, 2014
    I am sorry you can't understand my "Emglish" :D - what part did you not understand? Can you tell the difference in how bright an LED is when there is only a difference of 3 mA while at the top of its range? I doubt it and thus explained it so. I recall seeing a video of you and you are a native English speaker, so why are you attacking my statement? I am sure you can figure out the difference between there and their, they sound the same don't they Colin? Ain't that the cat calling the kettle black! Have you read some of your posts?? Here is one to refresh your memory, Mr. Perfect :p

    [​IMG]
    Colin Mitchell
    Joined:
    Aug 31, 2014
    Messages:
    238
    When you got the larges electronics wensite on the net, of course it is hard to navigate.
    That why I have an alphabetical index with

    Start here with PIC12F629
    http://www.talkingelectronics.com/FrameworkPages/cont13-Pic629.html

    Colin Mitchell, Yesterday at 10:06 PM Report
    #4 Like Reply

    I bought a series of resistors about 6 months ago - I happened to buy 1% tolerance type. They do still make 5% tolerance resistors and FYI the charts still show 10% tolerance as well. Therefore my warning is valid - to prevent over-driving the LED. Nothing wrong with building in a little protection for the circuit, right? You are assuming the original poster has access to the best resistors available, and as he has not posted his country of origin, nor do I believe English is his native language, that may not be the case.

    Ohm's Law from the original poster's given figures. 5v and 180Ω given, I neglected the 0.7v drop across the diode, so it would be closer to 23mA - Good catch, thanks!

    Self taught, as I believe you are, hopefully not as cranky though.

    LOL, really Colin? There are tons of LED's out there that have different specs for operating current. Most 3mm and 5mm common LED's have a max of ~20mA


    And let me guess, if one of the LED's is dimmer than the other, just swap it out, right?:p
     
    davenn, Arouse1973 and Supercap2F like this.
  5. Colin Mitchell

    Colin Mitchell

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    Aug 31, 2014
    Firstly you say 3mA change in current will be hardly noticeable and then you talk about 10% tolerance resistors can be a problem.

    Where can you buy a 10% tolerance resistor ???

    Yes, believe it or not, LEDs are designed for 17mA and say 50,000 hours. When at 20mA, the life drops to 20,000 hours.

    "And let me guess, if one of the LED's is dimmer than the other, just swap it out, right?"

    What do you mean ??????
     
  6. chopnhack

    chopnhack

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    Apr 28, 2014
    Colin, do you love to argue? Where you one of those children that just had to be right all the time? LOL :D

    Let me spell this out for you:

    Visually with our own eyes, I don't think we can see the difference that 3mA will make at the top of the range of the LED.
    On the other hand electrical circuits will have an obviously major impact. In this case a 10% variance in the load limiting resistor when the OP is close to the limit could blow the LED.

    I don't know, the 1% are cheap enough in my area to source, but the original poster's situation can be different - I thought we discussed this already. Review post 4, second response for more information.

    Are you now speaking for every LED in production Colin? I thought not. Have a look at the LED's offered by any of the major houses, the specifications can vary quite a bit. The O.P. did not give any other information other than the LED has presumably a 30mA maximum.

    I got it from your website under 1-WATT LED - a very good design

    I know you shouldn't connect LEDs in parallel, but the concept works very well in this case. If some of the LEDs have a characteristic high voltage and do not illuminate very brightly, simply replace them and use them later for another strip.
     
    davenn likes this.
  7. Colin Mitchell

    Colin Mitchell

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    Aug 31, 2014
    "I got it from your website under 1-WATT LED - a very good design"
    You are getting mixed up with series and parallel connection.

    "The O.P. did not give any other information other than the LED has presumably a 30mA maximum."
    I have never heard of any 3mm and 5mm LEDs with a current rating of 30mA.
     
  8. chopnhack

    chopnhack

    1,573
    352
    Apr 28, 2014


    How so, in the above referenced design, you wired a string of LED's on a two sided pcb, with legs on each side, that is a parallel circuit is it not?

    He didn't say that he was using a 3 or a 5mm LED, did he? Here is a link for some high current LED's from Digikey.
     
  9. Arouse1973

    Arouse1973 Adam

    5,164
    1,081
    Dec 18, 2013
    Colin, please don't be so rude. The fact that someone cant spell should not be held against them. It really pisses me off when people are like this,because a lot of great people cant or couldnt spell either. Like these. Actually, Da vinci said "You should prefer a scientist without literary skills than a literary person with no scientific skills"

    Albert Einstein
    Agatha Christie
    Jane Austen
    Winston Churchill
    John F Kennedy
    leonardo Da Vinci
    Benjamin Franklin
     
  10. davenn

    davenn Moderator

    13,669
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    Sep 5, 2009
    Agreed, guys

    I have given Colin a little holiday for a couple of weeks

    Dave
     
    Arouse1973 likes this.
  11. chopnhack

    chopnhack

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    Apr 28, 2014
    Please don't on my account, I have a thick skin anyways :)
     
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