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Author Topic: stumped by this LED array  (Read 1162 times)
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Also says 80mA Absolute Max.
With a voltage range of 3 to 3.4V.
Keep in mind the resistors will also have some variance.

I would design for worst case voltage of 3V as that would have highest current flow, and use the midpoint of the suggested current of 65-75mA:

(9-3-3)/70mA = 42ohm.
TIP120 also has some voltage drop across it, 2V typical for 3A of flow:
http://www.adafruit.com/datasheets/TIP120.pdf
So say it was only 1V even:
(9-3-3-1)/33 =  60mA
So the 33 ohm should have been fine, as long as each string had a resistor in series.

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That seems like wayyyyy too much current for those LEDs

The sparkfun page he linked to says "80mA"...



If you hadn't snipped off the rest of my comment, it would still be correct.  I said it depended upon the individual LED and duty cycle.  The datasheet says that 80mA is the ABSOLUTE MAX (their shouting not mine) and that 65-75mA is the suggested driving current.  Peak forward current is 120mA though so PWMing it allows for a little more current thru it, but the duty cycle is obviously limited to ~50% at that point.  Driving it 100% at 80mA is not allowed.

EDIT:  Corrected currents
« Last Edit: March 22, 2013, 10:26:10 am by afremont » Logged

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edit: both LEDs just fried. what is going on????????

You wired the LEDs in parallel.

If you're going to wire them in parallel, you'll need a resistor for each LED.

He had a resistor in each branch of the LEDs, he should be OK.


From the OP: "both + sides are in the 9V rail, both - sides are in the resistor that goes to the GND controlled by tip120"
Only one resistor there, with two LEDs in parallel.
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From the OP: "both + sides are in the 9V rail, both - sides are in the resistor that goes to the GND controlled by tip120"
Only one resistor there, with two LEDs in parallel.

"9V to positive end of LED, negative end of same LED to positive end of next LED, negative end of this LED through resistor, resistor goes into a ground ... "

Doesn't sound very parallel to me.
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6 of the LEDs are in series....2 are parallel right now sharing the same resistor so you're both right  smiley

so I'll put in separate resistors for the 2 that are in parallel...but this tutorial says you only need one...it's totally wrong?

http://www.quickar.com/ledbasics.htm
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"Wired In Parallel : where each led gets the same power at the same time

( not a good idea for more than a few leds because of minor differences between leds, some will get too much power, and some wont get enough)"

Certainly NOT a good idea for LEDs drawing 80mA.
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so I'll put in separate resistors for the 2 that are in parallel...but this tutorial says you only need one...it's totally wrong?

http://www.quickar.com/ledbasics.htm

Yes. As noted, your LEDs need between 3.0V and 3.4V. It's a range of values. Wiring them in parallel = bad because you might pick one 3.0 and one 3.4.

OTOH your first circuit should be OK ,in theory.
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