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post #1 of 11 (permalink) Old 07-08-2008, 03:28 PM Thread Starter
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Electrical Help

So long story short i need to wire up an array of led's...
I have a 9v 500 mA power supply and the led's i am using are here...

whites: http://www.besthongkong.com/product_...products_id=12

blues: http://www.besthongkong.com/product_...&products_id=3

and reds: http://www.besthongkong.com/product_...roducts_id=442

i figure with my power supply i can run this series..

1 white - 1 blue - 1 red

that should equal about 8.8 volts...

okay now that i've shown you what i know, here's my question:
How many times can i parallel that series with the power supply i have?
also... do i need some sort of resistor in this circuit? if so, where?

thanks in advance for any help...


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post #2 of 11 (permalink) Old 07-08-2008, 03:42 PM
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Is that power supply regulated or unregulated?
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post #3 of 11 (permalink) Old 07-08-2008, 03:51 PM Thread Starter
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it's just a brick power supply i grabbed from an old sony cordless phone... i'll list everything thing it says on there

sony
ac power adapter
AC-T122

INPUT: AC 120V 60Hz 9W
OUTPUT: DC 9V 500mA
CLASS 2 POWER SUPPLY

PC-0950-DUL

then it has those UL logos on it


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post #4 of 11 (permalink) Old 07-08-2008, 06:46 PM
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Ok, this may be wrong--so, someone please correct it, if it is. But this is what I am seeing:

You can run the 3 leds as you have setup on that 9v.

The whites use 20-30mA--depending on brightness.
The blues 100uA looks like it uses 20mA
The reds use 20mA

So, 500mA/(30 + 20 + 20)= ~7 parallel lines.....

Again, this may be wrong.....

You only need a resistor to downgrade unneeded voltage. The Red=2v, blue=3.8v max, white=3.8v max=9.6v, so you shouldn't need any resistors since the leds can draw more power than you can supply, but will still function on less, ie., what you can supply. They just won't be working at their max or brightest capacity....


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post #5 of 11 (permalink) Old 07-08-2008, 08:49 PM Thread Starter
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seems right... can anyone with more understanding than me help us out and validate that?


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post #6 of 11 (permalink) Old 07-08-2008, 09:02 PM
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In that case, we'll assume it's unregulated (most are unregulated). The output is only approximiately 9 volts. If the current draw is lower than the 500 mA specified, or the AC line voltage is higher than 120V, the output will be more than 9 volts. Conversely, the output will be lower if you load the transformer with a greater than 500 mA, or the Ac input is lower.

LED's operate with a approximately constant voltage drop, as you see from the specification - nominally 3.3 V for the blues and whites (whites are just blue LED's with an extra coating), and nominally 2.0 V for the reds. The brightness and the operating point for the LED's is determined by how much current is pumped through them; again from the specs, 20 mA is a good figure. Lower than that and the LED's will be dimmer than they would be otherwise; much over that and they'll run hot and their life will be shortened considerably.

The way to power an LED is with a constant-current supply, set at the desired operating point. A true constant-current supply is spendy, so most of the time a quick 'n dirty alternative is used instead. Namely, a resister is used between the LED and a conventional constant-voltage, or near-constant-voltage, supply. Since the votage across the LED is approximately constant, and the output from even an unregulated power supply is approximately constant, the voltage drop across the resistor will be constant, and thus the current will be constant. The "gotcha" is to ensure that the voltage of the power supply is high enough above the voltage across the LED, so that the voltage across the resister stays approximately the same, and thus so does the current.

In this case, you want to limit the number of LED's in series to keep them below 6 or 7 volts or so. For example, 3 reds would be nominally 6.0 volts, and 2 blues or whites would be 6.6 volts. For a 20 mA current draw, the resistor should be (9-6)/0.02 = 150 ohms for each string of 3 reds, or (9-6.6)/.02 = 120 ohms for each string of 2 blues/whites.
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post #7 of 11 (permalink) Old 07-08-2008, 09:40 PM Thread Starter
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okay... i think i've wrapped my head around that... but how many different series can i run on 500mA?


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post #8 of 11 (permalink) Old 07-08-2008, 09:57 PM
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This site may help. It helped me. I used it to build my LED array.

IMO it is easier to use a bunch of LEDs that are exactly the same..

http://led.linear1.org/led.wiz

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post #9 of 11 (permalink) Old 07-08-2008, 10:19 PM
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Quote:
Originally Posted by stev0205 View Post
okay... i think i've wrapped my head around that... but how many different series can i run on 500mA?
500/20 = 25 strings, of 2 or 3 LEDS in each string. Use a resistor with each string.
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post #10 of 11 (permalink) Old 07-08-2008, 10:28 PM Thread Starter
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25 strings? really? i figured that if each LED was 20mA each and there was 2 or 3 LED's per string... doesn't that mean that there are 40 to 60 mA of resistance through each string? and if that's true... couldn't i only have like 9 or 10 strings?..

either way.. i was only planning on using 21 led's due to the shape i need to arrange them in... is there anything i need in the circuit besides the resistors and the led's?


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post #11 of 11 (permalink) Old 07-09-2008, 12:53 AM
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Nope. 25 strings. That's a good size supply, and LEDs are very efficient.

Look at it this way: your power supply puts out 9 x 500 = 4500 milliwatts. Each LED uses 3.3 x 20 = 66 milliwatts. 66 x 25 x 2 = 3300 milliwatts for 25 strings of 2 blue/white LEDs each (50 LEDS). That's less than the 4500 milliwatts put out by the power supply, with the resistors using the rest.

just the LED's, and one resistor per string, is all that's needed.
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