Blowing my LEDs

by Jerware on Mon Nov 12, 2012 9:11 pm

For the record, I'm a loyal Adafruit customer but this question involves a part purchased from other "other guys."

I purchased a few of these 1.5" OLED screens:
https://www.sparkfun.com/products/8537

They're great. I use them in a mod for a pinball machine, and they're powered by a 5V source on that machine. I also attach three LEDs in parallel to the 3.3V output on the OLED screen. The LEDs have glowed brightly for months but, over time, the LEDs appear to suddenly fail. This has happened on multiple kits I've made. They do not light anymore, as if they've experienced a surge of voltage, though the OLED screen is fine.

What can I include in the parallel LED circuit to protect them? Is there some kind of simple fuse that would stop voltage from going above 3.3V? I've read a little about Zener diodes but I don't know if that's what I need, or if some kind of real fuse would be better. I'd rather not have to replace components -- so a self-resetting or robust component would be ideal.

Thanks for any suggestions!
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Re: Blowing my LEDs

by Zener on Mon Nov 12, 2012 11:29 pm

I am guessing the Vf of your LED's is less than 3.3V. You need a current limiting resistor. Ideally each LED would have its own resistor. I am guessing in the neighborhood of 50 ohms but it's hard to say without knowing the LED you are using.
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Re: Blowing my LEDs

by Jerware on Tue Nov 13, 2012 3:46 am

Thanks, Zener. Here are the LEDs I use:

http://www.ebay.com/itm/170684291218
http://www.ebay.com/itm/110972973522

I'm using two white and one UV. They're both comfortable with 3.2V. I figured 3.3 was close enough, but perhaps not. Since they're all on the same parallel circuit, does it matter if they each have a resistor or if they all share one? Resistor calculators for parallel aren't as common, but I found one that recommends a single 30 Ohm. That's pretty close to what you suggested so maybe I'll try that first and see how it looks.

Thanks again.
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Re: Blowing my LEDs

by adafruit_support_bill on Tue Nov 13, 2012 5:46 am

Each led in a parallel circuit should have its own current limiting resistor. Otherwise, the led with the lowest Vf will pull more than its share of the current.
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Re: Blowing my LEDs

by Zener on Tue Nov 13, 2012 11:21 pm

The purple ones are min 2.8V and rated at 20mA. The white ones are min 3V and rated at 25mA. I would put around 20 ohms in series with teh white ones and 30 ohms in series with the purple and then measure the current (or measure the voltaged drop across the resistors and use Ohm's law. Then adjust the resistors if necessary. For the record, UV LED's are extremely dangerous to eyes, but those might be OK since they are "barely" UV. However, I would not personally use them unless I was sure they were safe.
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