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Hi, 9 leds connected in parallel to one resistor on 12 V, 40 ohm resistor started to smoke!

Discussion in 'Electronic Basics' started by David Harris, Oct 2, 2003.

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  1. David Harris

    David Harris Guest

    Hi, I have 9 leds connected in parallel to one resistor on 12 V in my car,
    the resistor is 47 ohms, and the resistor started to get hot and smoke!

    I take it that I will need a higher wattage resistor, any suggestions?

    The leds are bright blue and they will be running at approx 25ma, and the
    total current draw is 9*25 = approx 225ma.

    Thankyou!

    Scott
     
  2. Andrew Kirby

    Andrew Kirby Guest

    Hi, I have 9 leds connected in parallel to one resistor on 12 V in
    Firstly, I'd bear in mind that when the engine is running, you are likely
    to see more than +12v on the supply - it's more likely to be 13.5-13.8v.
    I don't know what the forward voltage of the particular LED's you have
    is, but bear this in mind. In any case, at 225mA, a 47R resistor will
    dissipate 2.38 watts.

    A secondary suggestion is, given the unstable nature of supply voltages
    in cars, it might be possible to protect the LEDs by including a PTC
    (positive temperature coefficient) thermistor in the series resistor, so
    that if the supply voltage increases, and the current tends to goes up,
    so too does the series resistance, thus hopefully preventing damage.

    HTH
    Andy
     


  3. You were probably using a 1/2 Watt resistor...

    P = I*I*R = 0.225 * 0.225 * 47 = 2.4 Watts. I'd use a 5 Watt resistor to
    help dissipate the heat due to its larger body.

    But there's another thing to consider with your setup. If one LED burns and
    stops drawing current, the remaining 8 LEDs will draw more current, which in
    turn may cause another LED to pop, etc., until all LEDs are destroyed. You
    could use 9 separate 430 Ohm 1/2 Watt resistors for each LED (space
    permitting of course). There was another thread a few months back that dealt
    with these issues, do a Google search for it.

    cheers,
    Costas
     


  4. To add to this, your LEDs must have *identical* V/I characteristics for your
    setup to work (i.e., if one of the LEDs has a slightly lower Vf than the
    rest (which is very likely), it will draw most of the current and will
    eventually burn (followed by the others shortly after).

    Just use separate resistors. Or put them in series like the other poster
    suggested (in which case if one burns every other LED in the chain will turn
    off).

    BTW, when LEDs burn, do they always open? Or can they also short?

    cheers,
    Costas
     
  5. John Fields

    John Fields Guest

    ---
    Using a single resistor like that is very bad practice because all of
    the LED's won't have the same forward voltage and the one with the
    lowest forward voltage will hog the current until the voltage drop
    across it rises to the point (The temperature coefficient of the forwrd
    voltage is something like -3.1mV/°K, so it may never get there!) where
    another LED starts to conduct some current, so it might blow up.
    ---

    If you're using LED's with a maximum continuous forward current of 25mA
    and your car voltage rises to 13.6V, then for a 4.5V maximum forward
    voltage you could put two in series with a current limiting resistor,
    like this:


    +13.6V----[180 ohms]---[LED>]---[LED]----GND


    If you _had_ to run 9 LEDs, then you could use four strings in parallel
    and a single LED, like this:


    +13.6V--+--[180 ohms]---[LED>]---[LED]--+
    | |
    +--[180 ohms]---[LED>]---[LED]--+
    | |
    +--[180 ohms]---[LED>]---[LED]--+
    | |
    +--[180 ohms]---[LED>]---[LED]--+
    | |
    +--[360 ohms]---[LED>]----------+
    |
    GND>------------------------------------+

    The 180 ohm resistors will dissipate (13.6V - 9V)*25mA = 115mW, so you
    could use 1/4 watt resistors, but the 360 ohm one will
    dissipate(13.6V - 4.5V)*25mA = 227mW, so you'd be well advised to use a
    1/2 watt resistor there.


    If you're worried about gigunta spikes on the 13.6V line(and you should
    be!) you could do something like this, where CR1 is a 1N5352B, a 15V 5W
    Zener diode.


    +13.6V--+--[180 ohms]---[LED>]---[LED>]--+
    | |
    +--[180 ohms]---[LED>]---[LED>]--+
    | |
    +--[180 ohms]---[LED>]---[LED>]--+
    | |
    +--[180 ohms]---[LED>]---[LED>]--+
    | |
    +--[360 ohms]---[LED>]-----------+
    |K |
    [CR1] |
    | |
    GND>----+--------------------------------+
     
  6. Terry

    Terry Guest

    Scott:

    Ohms Law; Current squared multiplied by the resistance = watts.
    (I x I x R = W)

    i.e. (0.225 x 0.225) x 47 = 0.051 x 47 = 2.4 watts.
    So you should use 'at least' a 3 watt resistor. (And it should
    mounted in a manner suitable to its type so that cooling can
    occur). Three watts being dissipated by small resistor can get
    hot enough to take skin off your fingertips. As you have
    discovered!

    Another way; suppose that 10.5 volts is being dropped or 'wasted'
    across the resistor.
    (In order to have, say, 1.5 volts across the LEDs).

    Ohms Law again; Voltage squared divided by the resistance =
    watts. (V x V)/R = W

    i.e. (10.5 x 10.5)/47 = 111/47 = 2.4 watts.
    Same result.

    There are better ways of 'Dropping the voltage'. However a
    current draw, with most of the electricity being wasted as heat,
    in this case, of less than one quarter amp, is probably not very
    significant in a typical motor vehicle (car).
    By the way, if those blue LEDs are outside the vehicle or can be
    seen from outside the vehicle, you might want to check what light
    displays are legal in your police jurisdictions. Here, blue
    lights signify some kind of emergency or road maintenance vehicle
    such as snow plough etc.
    At one time (back in the 1950s!) there was a custom here, I'm
    showing my age by this I know!, to have little blue lamps in the
    corners of ones front windshield. They were declared illegal and
    then disappeared. But there can be fines for 'illegal' light
    displays, especially if they are on while vehicle is driving. The
    fewer things you can give the authorities to 'pick on' the
    better! have fun.

    Couple of thoughts anyway. Good luck.

    PS. All the LEDs together are using approximately;
    (Using another version of Ohms Law!); Current multiplied by
    voltage = Watts (I x V = W)
    0.225 x 1.5 = 0.34 watts. However LEDs are much more efficient at
    turning electricity into light than lamps.
    At the same time 2.4 watts is being dissipated by the resistor!
     
  7. Terry

    Terry Guest

    Baph: I was amazed too, with that many diodes (of any kind in
    parallel) but am not that familiar with how consistent or
    inconsistent such diodes are!
    Seemed though, that the original poster needed to have some basic
    understanding of V x I = W
    And that a resistor is not a resistor is not just a resistor
    etc.!
    BTW How are those grouped LED rear (brake) light assemblies, on
    cars, arranged! Series parallel maybe
    Terry.
    PS. Maybe of interest to sci.electronics.basics posters is a
    story about an almost brand new Volvo Tractor trailer truck that
    broke down three times and then it and its loaded trailer had to
    be towed due to an electrical problem, eventually discovered,
    caused by a temporary replacement driver. Hint! Fuses are
    sometimes a better indicator of what and where a fault is
    occurring than circuit breakers?
     
  8. John Fields

    John Fields Guest

    ---
    Good luck!

    If the LEDs are Silicon Carbide, they've got a typical Vf of about 3.8V,
    and a maximum of about 4.5V at 25mA, so even at nominal, four LED's
    would drop 15.2V and five would drop about 19V, so they wouldn't be very
    bright with 13.6V from the car!^)
     
  9. Baphomet

    Baphomet Guest

    I'm amazed that all of the led's work at all. If you want to run them in
    parallel, you should seriously consider using a separate current limiting
    resistor for each led. Alternatively, you could run 1 string of 5 led's in
    series and 1 string 4 led's (each string with its own current limiting
    resistor) to save on parts count.
     
  10. Baphomet

    Baphomet Guest

    Mea culpa John -

    I didn't realize they dropped such a large voltage. I guess it's back to
    4 strings of 2 series led's and 1 string of 1 led.
     
  11. David Harris

    David Harris Guest

    Thanks all, nice for all the replies!

    I've chosen the 47 ohm resistor based on the 14.4V, this value is slightly
    above whats needed according to the equation, and at 12V the brightness
    shouldnt be too much less. 25ma will be the approx current per led.

    Sounds like this could be a potential led death chain reaction waiting to
    happen if one led dies! This is a prototype I put together in my bedroom! I
    can't access the led legs now because they are set in epoxy!! lol

    I'll redesign the next one, what was the details about using a zener diode?

    Thankyou all!!

    Scott


    PS the leds were nice and bright, and that was in daylight, and no heat was
    being thrown out either!
    Decided on a WELL overspecced 47 ohm 7W wire wound resistor, or maybe a 10W.
     
  12. Blue LEDs have about 3.6V aross them, so that leaves about 10V. At 21
    mA per LED, that means you should have a 470 ohm resistor for each
    LED. Quarter watt resistors would be okay, but it would be better to
    use half watters. Use one resistor in series for each LED, do not
    connect the LEDs in parallel.

    --
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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
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  13. That's really no help at all. You can't find a PTC in any store, it
    would have to be special ordered. And why bother when a simple
    circuit like the following would do the same thing.

    One way to do it would be to use three groups of three LEDs in series,
    each group with the following circuit. For 25 mA, change the 33 ohm
    resistor to 24 ohms, or easier, put two 47 ohm resistors in parallel.
    Three blue LEDs in series in the circuit need 11V to get current
    regulation. Below that, the current will drop off to a few mA at 9V.


    +--------------------+------- Positive
    | | Supply V.
    | |
    | --- LED
    10k \ \ / =====>
    to 47k / ===
    ohms \ |
    / |
    | / Q1
    | | / Gen'l
    | | Purp NPN
    +----------------| 2N3904 or
    | | 2N4401
    | | \ E
    Q2 \ \
    Gen'l \ | |
    Purp NPN | 470 ohms |
    2N3904 or |------/\/\/\----+
    2N4401 | |
    E / | |
    / \
    | 33 ohms /
    | for \
    | 20 mA /
    | |
    | |
    | |
    +---------------------+-------
    Negative Supply V.


    --
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    ###Got a Question about ELECTRONICS? Check HERE First:###
    http://users.pandora.be/educypedia/electronics/databank.htm
    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
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  14. Didja catch your boo-boo? Should be 9 separte resistors, one for each
    LED, instead of 9 separate resistors for each LED.

    And indeed, 81 resistors would definitely need some space. ;-)

    --
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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]@@
     


  15. Yeah, that's what I meant: 9 separate 430 Ohm resistors, *one* for each LED.
    It wouldn't work otherwise anyway. Thanks for pointing it out!

    cheers,
    Costas
     
  16. [snip]
    Blue lights signify that you bought the car at K-Mart, so take that
    Blue Light Special back and get your money back! :p
    Around here in So Cal, a mod that kids make to cars is to add blue
    LEDs to the windshield washer squirters on the hood. I've seen blue
    LEDs in other places too, sometimes flashing. It's very unwise as he
    said to have these on when on the streets, because the cops may think
    you're trying to pretend your car is an emergency vehicle.

    Another fad around here in La-La Land is to replace the headlight
    lamps with bluish-white ones. Light looks like it's coming from a
    Beemer, Boxster or Benz. Then you get closer and see it's just a
    little old Honda with a six inch exhaust pipe that sounds like it's
    farting continuously. :-O
    I can't figure out where you got the 1.5V from above. The V drop for
    red LEDs is about 2V, and for blue ones in this case, it's 3.6V.


    --
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    ###Got a Question about ELECTRONICS? Check HERE First:###
    http://users.pandora.be/educypedia/electronics/databank.htm
    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]@@
     
  17. No, no, NO. Blue LEDs drop at least 3.3V, so 5 LEDs would add up to
    more than 16V, and four would be 13.2V; both strings would barely
    light if at all.

    I put 3 blue LEDs in series, and it took over 10V to get the current
    up to 20 mA.

    --
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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]@@
     
  18. Andrew Kirby

    Andrew Kirby Guest

    A secondary suggestion is, given the unstable nature of supply
    Huh? You might not find PTC's in radio shack, but they are readily
    available, and offer protection using a simple series implementation, which
    the OP can definitely manage.
     
  19. Like I showed in the (missing) schematic, it only takes two ten cent
    transistors and 3 resistors to make a current limiter, and it does a
    better job. View with Courier font.



    +--------------------+------- Positive
    | | Supply V.
    | |
    | --- LED
    10k \ \ / =====>
    to 47k / ===
    ohms \ |
    / |
    | / Q1
    | | / Gen'l
    | | Purp NPN
    +----------------| 2N3904 or
    | | 2N2222A
    | | \ E
    Q2 \ \
    Gen'l \ | |
    Purp NPN | 470 ohms |
    2N3904 or |------/\/\/\----+
    2N2222A | |
    E / | |
    / \
    | 30 ohms /
    | for \
    | 20 mA /
    | |
    | |
    | |
    +---------------------+-------
    Negative Supply V.


    --
    @@[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]@
    ###Got a Question about ELECTRONICS? Check HERE First:###
    http://users.pandora.be/educypedia/electronics/databank.htm
    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]@@
     
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