Newbie LED question

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I am trying to put together a decorative project that will have four LEDs in
it
(http://www.mouser.com/ProductDetail/Seoul-Semiconductor/LW514/?qs=gLyyx31KZaA5z9f8mONvwA%3d%3d).
The LEDs are 3.2 volts each. I have a "wall wart" transformerless AC adapter
that is 12V.

Can I just connect up the LEDs in parallel and attach them to the PS? Do I
need a dropping resistor (what value)?

Thanks all



Re: Newbie LED question

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The LEDs will want a nominal 20 mA of current and will drop a nominal
3.2V at that drive. You could put two LEDs in series with a 330 ohm
resistor in series with each pair. That would give you about 17 mA
through the LEDs (should be plenty) and you'll drop about 100 mW in the
resistor, so a common 1/4 W 5% 330 ohm resistor (available at Radio
Shack) will be fine.


    .------o------------.
    |      |            |
    |     .-.          .-.
    o     | | 330      | | 330
 .-----.  | |          | |
 |     |  '-'          '-'
 |  +  |   |            |
 |     |   |            |
 |12 V |   |            |
 |     |   V ->         V ->
 |  -  |   -            -
 |     |   |            |
 |     |   |            |
 '-----'   |            |
    o      V ->         V ->
    |      -            -
    |      |            |
    '------o------------'
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)


If the LEDs are separated enough that you'll want them wired
individually, then use each LED in series with a 470 ohm resistor and
drive the four sets in parallel from the battery.

--
Rich Webb     Norfolk, VA

Re: Newbie LED question
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AACircuit... interesting... Thanks!

--
   @~@   You have the right to remain silent.
  / v \  Simplicity is Beauty! May the Force and farces be with you!
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Re: Newbie LED question

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---
No.
---

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Yes; 300 ohms +/- 5%, 1/4 watt.
---
            
        WALL-WART
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Re: Newbie LED question

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Thanks for your reply.

Not really sure how to read your diagram. I think it is saying:
 +12 to 300ohm  to LED1 to LED2 to gnd
 +12 to 300ohm to LED3  to LED4 to gnd
(basically group1 in series through resistor to, group2 in parallel through
second resistor)
the vertical lines (2nd, 4th row) are confusing me.

And yes, I would love to know why?

Thanks again.



Re: Newbie LED question

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If you mean the ones directly under the W and the T in "wall wart,"
they're just the sides of the box that the 12V supply is in. All the
rest are wires.
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LEDs need current limiting, because their voltage/current curve
is ridiculously exponential - a very small change in voltage can
cause a dramatic change in current. If the current is regulated
(by, in this case, the 300 ohm resistors), then the voltage will
be whatever it needs to be at that forward current.

The reason he's got two LEDs in series is so that there's less
power dissipated in the series resistors.

Hope This Helps!
Rich


Re: Newbie LED question

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---
My pleasure. :-)

sorry about the garbled diagram; here it is corrected:

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Re: Newbie LED question

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Got it (some anyway)



Re: Newbie LED question

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---
I probably clouded the issue with too much irrelevant data; sorry
about that.

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Re: Newbie LED question

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Assuming the transformerless 12 volt adapter is actually 12 volts, then your
solution is good. Common 12 volt transformer types will almost certainly be
under loaded, in that case the actual voltage will be 14 volts plus. That
permits all 4 LEDs in series and a single 100 ohm resistor will likely work
fine.
For the OP, each parallel LED needs one resistor to limit the current. 12
volts would cause too much current for the LED, and it would burn out. So
you add the dropping resistor. Since the LED drops 3.2 volts, each resistor
will drop 8.8 volts. (12 minus 3.2)
Since voltage divided by current equals the desired resistance, then 8.8
divided by 10 milliamps equals 880 ohms. 10 milliamps will give good
brightness without risking the LED. Each LED can be made a different
brightness by varying the resistance to each LED. You could try 1000 ohm
resistors (or higher) to see how changes in resistance change the
brightness. Don't go below 680 ohms if you use one resistor for each LED.




Re: Newbie LED question

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Thank you



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