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Author Topic: stumped by this LED array  (Read 1713 times)
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hello,
I am controlling 8 LEDs in an array (these: https://www.sparkfun.com/products/8860) through a TIP120 and an arduino...doing PWM on them. They are powered by a seperate 9V wall wort. There are two for each resistor, 33ohm 1/2W...so 4 resistors in all. I used an LED calculator to make the array. (this one: http://led.linear1.org/led.wiz)

it goes: 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 controlled by TIP120. repeat 4 times.

It's working fine except for two of the eight are not working and I'm not getting any multimeter reading on their wires. However, when I mix the wires, use the power from one and gnd from other, I get light. But I don't get any power the other way around. I redid the whole thing and still getting the same.  this is so weird and I'm very stumped. Why would just 2 not be working even though they are wired exactly the same as the other 6? any ideas? thanks!

also: these two lights in the array WILL work if I wire them up like this:
both + sides are in the 9V rail, both - sides are in the resistor that goes to the GND controlled by tip120

could I do this or this wouldn't be good in the long run, right?
« Last Edit: March 21, 2013, 10:54:36 am by treebykooba » Logged

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Broken wire?
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no broken wire as the wires work fine when configured differently...also i tested the wire connections with a multimeter...everything seems normal.
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One of the transistors is faulty?

Try swapping transistors between the faulty string and a working string of LEDs.
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i'm just using one transistor...it works fine for the other 6 LEDs. currently everything is working it's just I have those two weird LEDs wired up wrong. Both + sides are in 9v rail and both - sides are in the 33ohm resistor, into the transistor's GND rail. Does anyone know why/if this is bad? it's been working fine for the past 30 minutes, pulsing away. thanks for the help!

edit: both LEDs just fried. what is going on????????
« Last Edit: March 21, 2013, 12:42:38 pm by treebykooba » Logged

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

You wired the LEDs in parallel.
Two 80mA devices in parallel draw 160mA
160mA through a 33 Ohm resistor will drop the voltage by 5.28 volts. (V=IR)
9 - 5.28 = 4.72 volts, applied to 3 - 3.4 volt LEDs = Magic smoke!
That's what has been going on. Your LEDs, however, will not be going on again. smiley-sad

If you're going to wire them in parallel, you'll need a resistor for each LED.
9 volts - 3 volts = 6 volts
To drop 6 volts at 80mA you'll need 75 Ohms (R=V/I) on each of them. I'd use 82 Ohms, just to be on the safe side.
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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.

I don't know what's going on though, unless the wiring isn't what he thinks it is (eg. bad breadboard...)
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My guess is that you had a wiring fault that you haven't found yet. The other possibility is that both of those LEDs require a higher than normal forward voltage, but that is unlikely.
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Please show your schematic - the description you give is not precise enough for us engineers.  We need pretty pictures.
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My guess is that you had a wiring fault that you haven't found yet. The other possibility is that both of those LEDs require a higher than normal forward voltage, but that is unlikely.

If I understand him, could have just been one bad one, but he kept it with the other as a set.  Outside that, maybe one of the resistors is wrong by a factor of 10 or 100.  With him putting the LEDs in parallel, I would have expected the OP to be mentioning that the brightness was off on those; he didn't. 
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thanks so much for all the replies...here is a png of what i had wired up:


it's controlled by a tip120 with a diode and a 1k resistor going to the arduino pwm pin.

now what i have is the last two LEDs aren't in series...they're in parallel...with one 120ohm resistor for both, 1/2 watt, which is definitely overkill but i don't want to burn them out again. they are just slightly dimmer than the others that are in series. It's been working fine for a few hours but i do need this to work for around 7 hours a day for a month. you think it'll be okay?

thanks so much for the help
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Quote
they're in parallel...with one 120ohm resistor for both
That is bad.  Every individual LED, or series chain of LEDs should have its own resistor.  You should never use one resistor for multiple LEDs in parallel.
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That seems like wayyyyy too much current for those LEDs, but it depends upon the LED and the maximum duty cycle (and pulse rate) you use.  Are you sure that all four resistors are the same value?  Check them with a meter.
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That seems like wayyyyy too much current for those LEDs

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

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thanks so much for all the replies...here is a png of what i had wired up:

Your problem might be that you typed in "3.4V" for the voltage when the page says "3.0-3.4V". Putting 3.4V through a LED which came out of the machine as a 3.0V LED is bad. Maybe what happened is that you got two 3.0V LEDs on the same string, if that happens it's easy to get far too much current going through them.

Never run LEDs at their max rating. You gain very little and you run in to problems like this. The numbers on the sheet are maximums, not recommended working values for 24/7 use.

With designs like this you need to measure the current going through each chain with a multimeter. Make sure it's <i>less</i> than 80mA. If it's more than 80mA you need a bigger resistor.

It's been working fine for a few hours but i do need this to work for around 7 hours a day for a month. you think it'll be okay?

If you want it reliable, aim at at 75% (ie. 60mA), not 100%. LED output is a curve, not a straight line. Those last few mA gain you a lot more heat than light.

(PS: The datasheet says "Suggestion Using Current 65-75mA"...)
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