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Topic: stumped by this LED array (Read 1 time) previous topic - next topic

treebykooba

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

majenko

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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afremont

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.
Experience, it's what you get when you were expecting something else.

fungus


That seems like wayyyyy too much current for those LEDs


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

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fungus


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 less 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"...)
No, I don't answer questions sent in private messages (but I do accept thank-you notes...)

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