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Newbe 8 leds connected to 120 volt ac

Discussion in 'Electronic Basics' started by [email protected], Apr 12, 2005.

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  1. Guest

    Hi, Let me start by saying that I know very little about
    electronics,but I would like to know how to hook up 8 white leds to 120
    volt ac. Is as simple as going to Radio Shack and buying a transformer?
    If so what size transformer will I need to get? Thank You,Randy
  2. aman

    aman Guest

    Get a transformer with appropriate DC output voltage (5/9/12 anything).
    Then calculate resistor you have to hook up in series depending on DC
    output voltage and forward drop of LED which is around 1.3V. Then you
    are all set.
  3. GotCoffee

    GotCoffee Guest

    You probably already have an old transformer laying around... look at
    some of your old wall adapters, maybe an external modem supply,
    speakers, tear apart an old radio or some kid toy.

    High intensity white LEDs are usually around 3.5V, check the package.
    Most important thing, make sure you limit the current going thru the
  4. Lord Garth

    Lord Garth Guest

    LEDs are not the best of diodes...when the AC reverses polarity, it will be
    bad news. You will need to connect the LEDs in inverse parallel which
    means + to - and - to + so that at least one LED is lit on each half cycle
    of the AC line or you need to install a protection diode that essentially
    lops off one half cycle.

    The way you connect the LED is important as well. They can all be in series
    or in parallel or some combination. The forward voltage drop of the LEDs
    all add if the LEDs are all in series. Lets say each LED drops an average
    3 volts when lit. 8 lamps in series would drop 24 volts plus the minimal
    ..6 volt drop of the protection diode plus the drop across your limit
    You wouldn't have enough voltage.

    The answer is to make several smaller series strings, each with their own
    limit resistor and connect these in series with the protection diode.

    I suggest you move this question to alt.binaries.schematics.electronic
    that is where people can post a picture with the wiring.

    What tools do you have for electronic work?
  5. Chris

    Chris Guest

    Hi, Randy. Quick answer -- white LEDs usually have a V(f) of 3 to 3.6V
    across them. Here's the Radio Shack Parts you need:

    5mm White LED $5.29 Catalog #: 276-320 (8 req'd)
    9V/300mA AC-to-DC Power Adapter $13.99 Catalog #: 273-1767 (1 req'd)
    330 ohm 1/4W 5% Carbon Film Resistor pk/5 $0.99 Catalog #: 271-1315 (2
    pk req'd)

    Connect them up like this (view in fixed font or M$ Notepad):

    | | | | | | | | |
    | | | | | | | | |
    | D| D| D| D| D| D| D| D|
    | V~~ V~~ V~~ V~~ V~~ V~~ V~~ V~~
    | - - - - - - - -
    +|9VDC | | | | | | | |
    --- | | | | | | | |
    - .-. .-. .-. .-. .-. .-. .-. .-.
    | | | | | | | | | | | | | | | | |
    | | | | | | | | | | | | | | | | |
    | '-' '-' '-' '-' '-' '-' '-' '-'
    | R| R| R| R| R| R| R| R|
    | | | | | | | | |
    | | | | | | | | |

    D = White LED
    R = 330 ohm, 1/4W
    created by Andy´s ASCII-Circuit v1.24.140803 Beta

    Have fun, and good luck
  6. PT

    PT Guest


    Do you have the LEDs picked out yet? If so, do you know any of the
    electrical specs? Specifically you want to know the current thru the
    LED and the forward voltage drop of the LED.

    Typically a data sheet will list a maximum current (say 60mA) and a
    nominal forward current (say 10-20mA) or one that they used to list
    other specs like brightness. LEDs are current devices and you control
    their brightness by how much current you allow thru them. Current
    control is usually done by simply selecting a proper sized resistor
    inline with the LED to limit current to whatever you need and below
    what would damage the LED. If you hook up an LED to a power source
    without a current limiting resistor, you will likely blow it.

    LEDs have a forward voltage drop that tends to be related to color. For
    example, typical RED LEDs have a forward voltage drop of about 2V or
    so. I think White LEDs have a much larger forward voltage drop, like
    3V-4.5V or there abouts. This is important because your voltage source
    needs to be a bit higher than this to work with the LED and will help
    you determine if you can hook up the 8 in series, parallel, or a
    combination of both. As an example, let's say your White LEDs have a
    forward voltage drop of 3.3V. If you took 8 and connected them in
    series, this would be a total forward voltage drop of 8 x 3.3V = 26.4V,
    so you would need a power supply of at least that, 28V being the
    typical common size or maybe 32V.

    So instead you might hook them up in a combination of 2 or more
    parallel strings or a couple/few LEDs. As an example, if you used two
    strings in parallel of 4 LEDs, this would be a forward voltage drop of
    13.2V, so a 24V power supply would work. Or 4 strings of 2 LEDs so you
    need a power supply of over 6.6, so a typical 9V or 12V supply would
    work or even a nine volt battery could be used.

    Anyway, back to your original question, yes, you would be best off
    buying a small wall-wart type transformer from Radio Shack or somewhere
    like that. Something about 9Vdc or larger would probably do the trick
    and have a current rating of maybe 200mA or larger would be plenty. Or
    you might look in your junk drawer at old chargers and transformers you
    may already have for cell phones, radio controlled toy chargers, etc.
    The specs (voltage and current) should be listed on the case.

    Then the other thing(s) you will need are current limiting resistors,
    but you first have to know what current(s) you are going to supply to
    the LEDs so you can calculate what values and wattages you need.

    To give you an idea, again using the 3.3V drop LED as an example and
    say the LED is best run at 20mA and say you have a 12Vdc source.

    Ohm's law is V=IR (voltage = current x resistance).
    Power is P=VI (power = voltage times current).

    For one LED off the 12V source at 20mA you have...

    R = V/I = (12-3.3) / 20x10-3 = 435 ohms. Then you just pick a standard
    resistor value that is in that ballpark (430, etc.).

    Of course any errors I might have made above will quickly be corrected
    by the rest of the group. :)

  7. GotCoffee

    GotCoffee Guest

  8. Bill Bowden

    Bill Bowden Guest

    You could use a 2.7K, 5 watt resistor to
    connect the LEDs directly to the 120VAC line.
    Put 4 LEDs in series, and the other 4 LEDs
    in series the opposite way in parallel with the first.
    Like the drawing below if it shows up right.

    (+) (-)
    |----LED---LED----LED---LED----| 2.7K 5 watt
    120VAC-| |-----/\/\/\-------120VAC
    (-) (+)

  9. Lord Garth

    Lord Garth Guest

    The kids going to hurt himself like to let him use a transformer.
  10. Guest

    Thank You all for writing to help me out,sorry I didnt get to check
    back and reply but my dad caught me surfing porn on his computer and he
    grounded me :) LOL

    I already have my leds here are the spec that came with them

    Reverse Voltage (VR)5V
    Reverse Current ( at VR=5V)<=10uA
    Operating Temperature-40C ~ 85C
    Storage Temperature-40C ~ 100C
    Lead Soldering Temperature 260C for 5 seconds (Max)
    Power Dissipation 5.5 mW
    Emitted Color White
    Chromaticity Coordinates (20mA) X : 0.32Y : 0.31
    Lens Appearance Water Clear
    Absolute max Rating Pd : 120 mW IF : 20 mA
    Peak IF : 120 mA
    Electro-optical Date (20mA)
    VF : 3.2V (Max 3.4V)
    IV : 15,000 mcd
    (14,000/16,000 mcd Min/Max)
    Viewing Angle 15 +- 5
    Life (normal usage) 80,000 - 100,000 hours
    my other leds I ordered are:

    20 x 18000mcd SUPER WHITE 10mm LED Lamp
    Product Description :
    Emitted Colour : WHITE
    Size (mm) : 10mm
    Lens Colour : Water Clear
    Peak Wave Length (nm) : N/A
    Forward Voltage (V) : 3.2 ~ 3.8
    Reverse Current (uA) : <=30
    Luminous Intensity Typ Iv (mcd) : Average in 18000
    Life Rating : 100,000 Hours
    Viewing Angle : ?10?
    Absolute Maximum Ratings (Ta=25?C)
    Max Power Dissipation : 80mw
    Max Continuous Forward Current : 30mA
    Max Peak Forward Current : 75mA
    Reverse Voltage : 5~6V
    Lead Soldering Temperature : 240?C (<5Sec)
    Operating Temperature Range : -25?C ~ +85?C
    Preservative Temperature Range : -30?C ~ +100?C

    I would like to make two differant circuits,one using the 5mm leds and
    another using the 10mm leds to see which leds are whiter. I prefer to
    use a transformer instead of 120 volt.

    Now my questions are:

    1. Is the 9V/300mA AC-to-DC Power Adapter going to put out enough
    voltage for 8 leds in each of the two circuits I want to make?

    2. Should I still use the 330 ohm 1/4W 5% Carbon Film Resistor for all
    leds in each circuit?

    Thank you all again for helping a rookie like myself,Randy
  11. One day got dressed and committed to text
    The power adaptor should be OK if you run the LED's at 20mA. Since the
    forward voltage of the LED's id 3.2v 3 in series would not be good for the
    9v supply, so you will have to have 4 series sets of 2 LED's connected in
    parallel (if that makes sense ?). Each group would need its own resistor (=
    4 resistors).
    You should work out the values of the resistors and the current dissipation
    yourself for yourself as a start in your experience. Folk in this NG will
    help if you show some indication of effort.
  12. Guest

    Like I said I know very little about electronics but let me see if this
    is correct:

    9 voltage
    -6.4 (3.2 volt bulb x 2 since its in series)
    =2.6 volts

    2.6 volts/0.040 amps (.020 amps x 2 in series=.040)
    =65 ohms

    so the colors on my resistor would be Blue,Green,Black,Gold

    Is this correct and am I on the right path?

    And would the 65 ohm resistor work for the 10mm leds as well because I
    see that the Max Continuous Forward Current is 30mA. Would this make
    the leds dimmer?
    Thanks again for your help
  13. Lord Garth

    Lord Garth Guest

    In a series circuit, the current is the same through both lamps
    so do not multiply by 2.

    130 ohms is your target value.

    The power dissipated by the resistor is P=I*E
    ..02*2.6=.052 Watts so a 1/4 watt resistor is fine.
  14. No. When you wire LEDs in series, the voltage drops add up but the current
    is the same for all of them. 2.6/0.020 = 130 ohms.
    Yes, just dimmer.

    Notice that your LEDs actually specify a range of forward voltage drops,
    from 3.2 to 3.8 V for the 10mm LEDs. That means that the voltage drop might
    be anywhere in that range. If it is on the low end, 3.2 V, then the current
    will be perfect, because you used 3.2 V in the calculations. If it is on the
    high end then the current is (9 - 3.8 - 3.8)/130 = 1.4/130 = 0.011 A or 11
    mA, which is maybe a bit low. You probably don't care though.

  15. Rich Grise

    Rich Grise Guest

    NO. They're in series, so the current through the whole series string
    is still 20 mA. The current is the same through every point in a
    series circuit. Remember, volts are pressure, amps are rate of flow.
    Just do the math again for 20 mA.

    The total current through the 4 series strings (of two LEDS and R)
    will be the sum of the currents through the legs, or 80 mA.
    Other than the resistor value, (which you should calculate as above,
    using the forward V of the other LEDs), your setup should give equal
    currents through the LEDs, which is actually what you're looking for:
    The dimmer LEDs are dimmer LEDs.
    My Pleasure!

    P.S. Many people prefer bottom-posting, for continuity of the conversation
    for newcomers to the thread.
  16. Guest

    Okay now I have a better understanding what I am doing,I will give
    these two circuits a try,Thanks again for all of your help,Randy
  17. Guest

  18. Yes. The relay output is rated for 12 VDC, 10 A, and the current drawn by
    your LEDs is much less than that. Just wire the normally-open relay contacts
    in series with the power supply that you use to run your LEDs; it works just
    like a switch.

  19. Guest

    Okay I figured it would but I just wanted to make sure.Thanks a million
    to all of you,you all are a great bunch of people here in this

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