1. Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.
    Dismiss Notice

LED and wall adapters

Discussion in 'General Electronics Chat' started by airbrush, Jun 11, 2005.

  1. airbrush

    airbrush New Member

    Joined:
    Dec 29, 2004
    Messages:
    94
    Likes:
    0
    okay thx...yah that helps...but you said the LEDs will still be dim...i want them to be totally off when you turn it down...that is why i thought we got rid of that one resistor? or should the resistor value just be different then?
     
  2. audioguru

    audioguru Well-Known Member Most Helpful Member

    Joined:
    Mar 16, 2004
    Messages:
    32,430
    Likes:
    933
    Location:
    Canada, of course!
    Whether your LEDs will be completely off when you turn down your pot depends on the spec's of your LEDs. My MV8191 LEDs will be off. Just reduce the value of the 180 ohm resistor a little if they aren't.

    If the 180 ohm resistor 's value is too low or shorted, then you must turn up the pot to more than half-way before the LEDs would begin to light. That's what I call a huge dead zone in the pot.

    Which 1.7V LEDs did you order? I'll look at the datasheet to see their spec's. :lol:
     
  3. airbrush

    airbrush New Member

    Joined:
    Dec 29, 2004
    Messages:
    94
    Likes:
    0
    here is the link to where i'm getting my leds from:
    http://www.lsdiodes.com/5mm/
    i am using the
    5mm sea green 4000mcd
    5mm orange - 4000mcd
    5mm white - 6000mcd

    right...i'll play around with the resistor if i have to, try and removed that dead area from being too much.
     
  4. dave

    Dave New Member

    Joined:
    Jan 12, 1997
    Messages:
    -
    Likes:
    0


     
  5. audioguru

    audioguru Well-Known Member Most Helpful Member

    Joined:
    Mar 16, 2004
    Messages:
    32,430
    Likes:
    933
    Location:
    Canada, of course!

    There aren't any curves on the datasheets for your LEDs so I am just guessing. The LM317 circuits for your Sea Green and White LEDs will be similar to this:
     

    Attached Files:

  6. airbrush

    airbrush New Member

    Joined:
    Dec 29, 2004
    Messages:
    94
    Likes:
    0
    so the resistors values for the 1.7 leds on the previous page are still correct then??

    thx again for your help
     
  7. audioguru

    audioguru Well-Known Member Most Helpful Member

    Joined:
    Mar 16, 2004
    Messages:
    32,430
    Likes:
    933
    Location:
    Canada, of course!
    Yes.

    You're welcome, I'm glad to help. :lol:
     
  8. airbrush

    airbrush New Member

    Joined:
    Dec 29, 2004
    Messages:
    94
    Likes:
    0
    well finally getting some time to finish up this project this week....been slowly chipping away at, gathering materials, cutting, building...now its finally time for the guts...the circuits and wiring. Will have it done soon...i'll post up some pics when its all done.
     
  9. airbrush

    airbrush New Member

    Joined:
    Dec 29, 2004
    Messages:
    94
    Likes:
    0
    okay...a slight problem...all the LEDS are still on when the pots are turned down....so i need to reduce the value of the 180 resistor for 1.7volt leds and the 910 resistor for the for the 3.3volt leds schematics accordingly??

    How much do I need to reduce it?? what do you suggest to try??

    The orange LEDs are the only ones that dim totally off with the pot....but turning it up to max, the LED's are no where near as bright as they should be...what should i check on these?? The resistor for the LED in a series of 2 seems to be correct at 390. So does the resistor on the pot that you are saying to change also affect the max brightness of it...as these are the only ones that dim totally off but are not nearly as bright??
     
  10. audioguru

    audioguru Well-Known Member Most Helpful Member

    Joined:
    Mar 16, 2004
    Messages:
    32,430
    Likes:
    933
    Location:
    Canada, of course!
    Did you build both circuits?
    When the pots are turned down, measure the voltage of the regulators' outputs and the voltage across each LED.
    Then in the 1st circuit, reduce the value of the 180 ohm resistor and in the 2nd circuit, reduce the value of the 910 ohm resistor.

    I guess your LEDs are lower than 1.7V and 3.3V, so calculate the values of new resistors using Ohm's Law.
     

    Attached Files:

  11. airbrush

    airbrush New Member

    Joined:
    Dec 29, 2004
    Messages:
    94
    Likes:
    0
    for the 1.7 leds i'm getting 2.77 where you have 2.75 and 1.53 where you have 1.25...i have no idea how to get a mA reading out of my meter..none of the settings i turn it to give me anything close the number you have there :? .

    for the 3.3 leds i'm getting 6.02 where you have 6.0 and 4.77 where you have 1.25!?

    like i said...the orange ones are the only ones that seem to work correctly..dim totally off and very little play in the pot before they start lighting up...but they dont seem very bright at all... :?
     
  12. audioguru

    audioguru Well-Known Member Most Helpful Member

    Joined:
    Mar 16, 2004
    Messages:
    32,430
    Likes:
    933
    Location:
    Canada, of course!
    Hi Airbrush,
    I goofed on my calculation of the voltage at the ADJ terminals, your measurements were very close to my corrected calculations.
    The current readings are determined by Ohm's Law, since you must break a connection to put a current meter in series. A current meter has a voltage drop so its reading would have error anyway.

    The 1.7 circuit puts a max of 26.7mA through two 1.7V LEDs. The max continuous current rating for most LEDs is 30mA. So your orange LEDs are operating near their max.
     
  13. airbrush

    airbrush New Member

    Joined:
    Dec 29, 2004
    Messages:
    94
    Likes:
    0
    hmm...thats very strange...the orange ones are no where near as bright as the green or white and they are actually rated at a higher brightness than the green LED are :? ...

    okay, so onto the next issue then...the green and white remain on all the time...what would you recommend I change the 910 resistor value to??
     
  14. audioguru

    audioguru Well-Known Member Most Helpful Member

    Joined:
    Mar 16, 2004
    Messages:
    32,430
    Likes:
    933
    Location:
    Canada, of course!
    How much is their current? Measure the voltage across the 390 ohm resistor and use Ohm's Law.
    You never said what is your voltage measurement of the LEDs so we are just guessing that they are 1.7V.

    You never said what is your voltage measurement of the LEDs so we are just guessing that they are 3.3V.
     
  15. airbrush

    airbrush New Member

    Joined:
    Dec 29, 2004
    Messages:
    94
    Likes:
    0
    the LEDs are 1.7 and 3.3 respectively...that is the specs from where i bought them from...www.lsdiodes.com

    orange:
    http://www.lsdiodes.com/shop/index.php?main_page=product_info&cPath=1&products_id=10

    green:
    http://www.lsdiodes.com/shop/index.php?main_page=product_info&cPath=1&products_id=11

    white:
    http://www.lsdiodes.com/shop/index.php?main_page=product_info&cPath=1&products_id=22

    ...again..i appreciate your patience and help on this...couldn't have done this project without this forum and the helpful people on it. :)
     
  16. audioguru

    audioguru Well-Known Member Most Helpful Member

    Joined:
    Mar 16, 2004
    Messages:
    32,430
    Likes:
    933
    Location:
    Canada, of course!
    Ok, if the green and white LEDs are rated at 3.3V each, then with two in series with a 220 ohm resistor, why do they light with the 6.02V that you measured? The only way to reduce their brightness is to measure their voltage then change the circuit to match.

    You can'simply reduce the value of the 910 ohm resistor to dim the LEDs at minimum pot setting, because then their max brightness will also be reduced. The resistance of the 240 ohm resistor also must be reduced but not as much as the reduction of the value of the 910 ohm resistor.

    What is the voltage across the orange LEDs' 390 ohm resistor when the circuit's pot is at the brightest? Also, double check and measure the 390 ohm resistance of the resistor.
     
  17. airbrush

    airbrush New Member

    Joined:
    Dec 29, 2004
    Messages:
    94
    Likes:
    0
    will check the voltage across the leds 390 tonite...
     
  18. audioguru

    audioguru Well-Known Member Most Helpful Member

    Joined:
    Mar 16, 2004
    Messages:
    32,430
    Likes:
    933
    Location:
    Canada, of course!
    [quote="airbrush]so how do i measure the LED individual voltage?[/quote]
    Turn up the brightness pot to max and measure the voltage across each LED. Also measure the voltage across the resistor that is in series with the LEDs.
    Then turn down the brightness pot to min and measure the voltage across each LED. Also measure the voltage across the resistor that is in series with the LEDs. Don't you have two LEDs in series with a resistor?

    Yes, but the resistors affect each other. We need to know the actual operating voltage of the LEDs and their series resistor to recalculate the resistors properly.
     
  19. airbrush

    airbrush New Member

    Joined:
    Dec 29, 2004
    Messages:
    94
    Likes:
    0
    yup i have 2leds in series with the resistor....I will do as you said tonite and check that and get back to you....thanks again for your help
     
  20. airbrush

    airbrush New Member

    Joined:
    Dec 29, 2004
    Messages:
    94
    Likes:
    0
    alright, here's what i have:

    orange pot min:
    2.77 vout
    2.77 at the resistor
    1.32 at the LEDs

    orange pot max:
    11.27 vout
    2.78 at the resistor
    1.88 at the LEDs


    green pot min:
    6.02 vout
    5.36 at the resistor
    2.62 at the LEDs

    green pot max:
    11.48 vout
    6.44 at the resistor
    3.18 at the LEDs
     
  21. audioguru

    audioguru Well-Known Member Most Helpful Member

    Joined:
    Mar 16, 2004
    Messages:
    32,430
    Likes:
    933
    Location:
    Canada, of course!
    The voltages don't add correctly so I don't know what you are measuring.
    To calculate the current of the LEDs, we need to know the voltage across the 390 ohm and 220 ohm resistors.
     

    Attached Files:

Share This Page