3W-9W RGB LED + Resistor(s) ???

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Stapler
 
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3W-9W RGB LED + Resistor(s) ???

Post by Stapler »

Hello All,

I'm just getting into electronics + 3D printing and having a great time with it all. Apologize upfront if this has been answered elsewhere, but I've yet to find the answer. I just received this (https://www.electromaker.io/project/vie ... ight-sabre) kit. The instructions note that the two resistors that should be connected in series are 4.7 ohms. The parts list in the kit has them at 10 ohms, and the ones I received and tested are 12 ohms. Should I still connect two of them in series to get max brightness from my 9 watt LED? Thank you!

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adafruit_support_bill
 
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Re: 3W-9W RGB LED + Resistor(s) ???

Post by adafruit_support_bill »

Hmmm. Not the best circuit design. Instead of putting all the resistance on the common anode, there should be separate resistor for each LED - especially since the forward voltage for the red, green and blue will all be very different, so the current will not be shared equally between them.

Do you have a spec sheet for the 9W RGB LED?

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dastels
 
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Re: 3W-9W RGB LED + Resistor(s) ???

Post by dastels »

I was thinking the same thing as Bill. The different colors will be different brightnesses. But since it's a 3W LED that's well diffused it might not be noticeable.

It's really annoying that I can't see the schematic without signing up, which I have no reason to do. BUT, the picture does show 4.7 ohm resistors. If we go by that then putting them in series would give you 9.4 ohms. So just use a single 12 ohm resistor.

Dave

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adafruit_support_bill
 
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Re: 3W-9W RGB LED + Resistor(s) ???

Post by adafruit_support_bill »

Yea, the 12 ohm resistor should keep things at a 'safe' level. Although it is a 9W LED, you would need a heat sink and probably some ventilation to run it at that level for more than a few seconds. With everything enclosed in an unventilated thermoplastic enclosure, you will need to keep the heat dissipation to a reasonable level.

Assuming that you only run one color at a time - and assuming typical forward voltages for the R, G & B, you will be running the red channel (Vf ~2.5v) at about 1W. And the blue and green (Vf ~3.6v) at about 0.72W.

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Stapler
 
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Re: 3W-9W RGB LED + Resistor(s) ???

Post by Stapler »

Wow, thanks everyone for such quick and helpful responses! I'm still learning, so I appreciate your patience. I've attached the schematic as referenced in this project's youtube and written instructions. Also, here https://cdn-shop.adafruit.com/product-f ... RGB-Y2.pdf is the LED spec sheet.

Yes, I'll only have one color LED on at once, and powering it with 4 AA rechargeable batteries. I'm modifying an existing 3D printed lightsaber handle to hold the batteries, LED, and rotary switch, so i can 1) be sure there's some ventilation, and 2) make sure the back of the LED chip and resistor(s) are not packed in too tightly. I would like to aim for max brightness, so happy to put the best resistor on each LED color's wire back to ground. I'm seeing that some resistors have a watt rating in addtion to their typical ohm rating? Does that matter for this project? So if I'm on the right track here, what would be the right ohm level for the resistors on the departing wires (from the LED chip) that are headed back to the ground (via the rotary switch)? Thank you all so much! My nephews are going to love these!

Stapler
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adafruit_support_bill
 
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Re: 3W-9W RGB LED + Resistor(s) ???

Post by adafruit_support_bill »

Assuming that it is adequately ventilated, it should be possible to run the LED at 3W. Although the device will have a longer lifetime if you provide some additional heatsinking. https://www.mouser.com/c/?q=led%20heatsink

For 9W, you would definitely need a serious heat-sink. However, since you are only running one of the colors at a time, the maximum power per channel is 3W.

To get 3W from the red channel, you would need a 4 ohm resistor.
To get 3W from the green or blue channels, you would need 2 ohm resistors.

These would need to be separate resistors wire in series with the R, G and B cathodes - not the common anode as in the schematic shown.

And to handle the current levels involved, you would need to use resistors rated for at least 3W. And you would need to make sure that they are also well ventilated and/or heat-sinked.

Some LED heat-sinks are pretty cool looking. You might be able to work it into your design:

Image

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