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Series Resister or not

Discussion in 'Electronic Basics' started by GrimReaper, Oct 21, 2003.

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  1. GrimReaper

    GrimReaper Guest

    Would I be right in assuming that if the supply voltage was the same as the
    forward voltage of the LED then there is no need to fit a series resister or
    do you have to have a higher supply voltage and always fit a series resister
    to drop the supply voltage back to the forward voltage of the LED to limit
    the current?

    GrimReaper
     
  2. GrimReaper

    GrimReaper Guest

    Thank you my friend, I now know what to do

    Regards
    GrimReaper
     
  3. Baphomet

    Baphomet Guest

    You should always use a higher voltage supply and limit the current to the
    led with a series resistor, unless you're using a current limited power
    supply.
     
  4. John G

    John G Guest

    Very simplicticly.
    You could imagine a LED as a constant voltage device and given that, the
    current will rise to infinity or until the source limits it or the diode
    turns to smoking. The resistor on the other hand, drops more volts as the
    current rises and soon there is equilibrium.
     
  5. dB

    dB Guest


    Yes, use a higher voltage and a series resistor.

    This link may help

    http://pub40.ezboard.com/fbasicelectronicsfrm5.showMessage?topicID=16.topic
     
  6. While I agree with the sentiment, diodes do actually have a 'resistance' of
    sorts; its simply nonlinear, ie, it doesn't obey ohms 'law', so its a bit
    more difficult to figure out. The forward voltage is related to the natural
    log of the current through it, so the more current, the more voltage across
    it, just like a resistor. IIRC, one rule of thumb is that if you increase
    the current by 10 times (a decade), the forward voltage will go up something
    like 60mV, at room temperature (at least this works for bipolar transistor
    diode junctions).

    I think this means that the OP can omit the resistor without explosions...

    Regards
    Bob Monsen
     
  7. As a general rule, you should use a series resistor and a slightly higher
    (+0.5-1V) voltage. However a LED is not a constant voltage device and most
    LEDs have noticeable resistances that allows them to operate quite well
    ballanced at a constant voltage. Note nevertheless that the difference in
    the forward voltage rises very slowly on substantial current increase due to
    a LED's nonlinearity. This means that your power supply voltage should be
    _really_ constant, small changes may become lethal for the LED. Never rely
    on a 'trafo, diode and cap' style of power supply for a voltage stability.
    Note also that an LED is a temperature-sensitive device. When operating at a
    constant voltage it should be kept away from high temperatures and mounted
    in a case that can provide slight heatsink capabilities. If you plan to
    operate the LED with batteries, their internal resistance can help making a
    series resistor unnecessary, but batteries change condition easier than one
    can sometimes think, so I would classify a device of this sort as 'not
    reliable'.
     
  8. GrimReaper

    GrimReaper Guest

    Thanks for all your info. I want to connect 21 Blue LED's as part of a Coral
    Tank lighting system. The LED's have a forward voltage of 3.4v. A current of
    20ma. Based on your comments I was going to buy a 12 volt stabilised power
    supply and put 3 LED's in series with a 91 ohm resistor and repeat this
    series configuration 7 times. Does that make sense?
    No good at drawing that here. I am a marine biologist and what I know about
    electronics I have gained from here and Google searches - that's it.


    Regards
    GrimReaper
     
  9. Lord Garth

    Lord Garth Guest

    That'll do nicely GR... now using P=I²R, calulate the power rating of the
    resistor you
    need....Oh, use the nearest standard value resistor, it isn't at all
    critical. Don't forget
    to add a little extra power rating for slop margin.

    You have 11.2 volts across the LEDs, leaving .8 volts across the resistor,
    when supplied
    with 12 volts.
     
  10. John Fields

    John Fields Guest

    ---
    Yes.

    The difference between the supply voltage and the sum of the LED forward
    voltages divided by the LED current is 90 ohms and you have chosen the
    nearest standard 5% value, 91 ohms. Well done.

    All that's left to do is determine the wattage required for the resistor
    to dissipate, and that'll be the difference between the supply voltage
    and the sum of the LED forward voltages _multiplied_ by the LED
    current,

    (12V-10.2V)*0.02A = 0.036 watts

    so a 1/4 watt resistor will be fine.
    ---
     
  11. Lord Garth

    Lord Garth Guest

    Oops John caught me !

    Seems I flubbed that 3.4 times three ....back to second grade I go!!!
     
  12. GrimReaper

    GrimReaper Guest

    John / Lord Garth
    Thank you for that. I had not thought about the power dissipated in the
    resistor


    Kind Regards
    GrimReaper
     
  13. Baphomet

    Baphomet Guest

    That's certainly one of the things that makes this such a powerful medium. A
    final solution becomes a collaborative effort, each one seeing a possible
    solution from a slightly different perspective.
     
  14. "Just like a resistor"? You're saying that the resistor, which obeys
    ohm's law, obeys the natural log relatonship? I don't think so.

    And there is a resistive component to the current/voltage
    relationship, just that it's not a predominant component. There is
    the bond wire resistance, the internal resistance of the chip itself,
    etc. But as the LED gets hotter, the V drop gets lower, so putting
    the LED on a constant voltage supply means that it will draw excessive
    current as it warms up. This could let the smoke out.

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  15. No, I'm not saying that. You apparently misread my post...

    Regards,
    Bob Monsen
     
  16. It looks like you have never measured the voltage across a LED while
    it's operating. If you had, you would know that it has a negative
    tempco. As the LED gets warmer, it drops less voltage. This is
    really pronounced on big LEDs such as the Luxeon Star. If you connect
    a LED to a constant voltage, it draws more current as it gets hotter,
    which causes it to draw more current and get even hotter, in a vicious
    circle which will destroy the LED if it doesn't shorten its life
    considerably. So I'd say your advice is bogus.

    [snip]

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    goes directly to the trash unless you add NOSPAM in the
    Subject: line with other stuff. alondra101 <at> hotmail.com
    Don't be ripped off by the big book dealers. Go to the URL
    that will give you a choice and save you money(up to half).
    http://www.everybookstore.com You'll be glad you did!
    Just when you thought you had all this figured out, the gov't
    changed it: http://physics.nist.gov/cuu/Units/binary.html
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  17. Eh? 3.4V times 3 is 10.2V. That leaves 1.8V across the res. There
    will be some variation in the voltage drop, but it shouldn't be too
    much. That can be measured with a DMM. If it's too much, change the
    LED.

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    goes directly to the trash unless you add NOSPAM in the
    Subject: line with other stuff. alondra101 <at> hotmail.com
    Don't be ripped off by the big book dealers. Go to the URL
    that will give you a choice and save you money(up to half).
    http://www.everybookstore.com You'll be glad you did!
    Just when you thought you had all this figured out, the gov't
    changed it: http://physics.nist.gov/cuu/Units/binary.html
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  18. If you can't get that relatively hard-to-find value, use two 47 ohms
    (much more common) in series, or 100 ohms will work okay with no
    perceptible difference in brightness.
    I bought a couple dozen blue LEDs and I've found that every one of
    them that I've taken out of the package is intermittent. And it's
    really strange, too. I crank the PS up to 1.5V and the LED starts to
    draw current, maybe a few mA, but the current is really erratic, I
    can't get a stable reading. If I go up to 3.5V at 25 mA, sometimes
    the LED will light steady, sometimes it will blink, maybe a few times
    a minute, maye less, once every few minutes I've checked at least 6,
    probably more, and they're all like that. Bad news. Sucks.



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    ###Got a Question about ELECTRONICS? Check HERE First:###
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    My email address is whitelisted. *All* email sent to it
    goes directly to the trash unless you add NOSPAM in the
    Subject: line with other stuff. alondra101 <at> hotmail.com
    Don't be ripped off by the big book dealers. Go to the URL
    that will give you a choice and save you money(up to half).
    http://www.everybookstore.com You'll be glad you did!
    Just when you thought you had all this figured out, the gov't
    changed it: http://physics.nist.gov/cuu/Units/binary.html
    @@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@@[email protected]@[email protected]@@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@@
     
  19. Maybe it's static damage. Most blue LEDs are static-sensitive.

    - Don Klipstein ()
     
  20. Dan Dunphy

    Dan Dunphy Guest

    In an LED VI Curve, the independant variable is current, and the
    dependant variable is voltage.
    Use the resistor. You can series a number of leds with a higher
    voltage supply and one resistor, or use a current scource, if you want
    to get sophisticated.
    Dan

    Colorado Springs, CO
    My advice may be worth what you paid for it.
     
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