Grounding 12V Power Supply With LEDs and Beaglebone

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mrmu
 
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Grounding 12V Power Supply With LEDs and Beaglebone

Post by mrmu »

I want to setup the following wiring of the LED strip and N channel MOSFETs but with a separate 12V power supply and BeagleBone Black (obviously powered by separate 5V power supply): https://learn.adafruit.com/rgb-led-strips/usage

I would still ground the Source pin (pin 3) on the MOSFETS to the the Beaglebone, right? And then ground the power supply into that same line?

What would happen if I didn't ground the setup into the Beaglebone, only connecting the 3 MOSFET Gate pins to the I/O pins on the Bone?

Thanks
Last edited by mrmu on Thu Mar 05, 2015 5:06 pm, edited 1 time in total.

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Re: Grounding 12V Power Supply With LEDs

Post by adafruit_support_bill »

You need to connect the grounds together so that you have a common signal reference. Otherwise, the signal from the Bone would be floating relative to the MOSFET gate and operation would be unpredictable.

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mrmu
 
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Re: Grounding 12V Power Supply With LEDs

Post by mrmu »

So I did the setup I outlined before submitting the message (not connecting to the Beaglebone ground, only the 3 R/G/B connections to the I/O pins).

And now my Beaglebone won't turn on. When I give it power, the blue power light flashes quickly and that's it.

Is it fried? Is it possible by not connecting to the Beaglebone ground that 12V somehow ran through the gate pins on the MOSFETs and into the Bone?

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Re: Grounding 12V Power Supply With LEDs

Post by adafruit_support_bill »

Post a photo of how you had everything connected. That kind of damage is not common. But with no common ground all bets are off.

A good general rule of thumb with working with separately powered circuits is that the ground connection should be the first one you make.

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Re: Grounding 12V Power Supply With LEDs

Post by mrmu »

Here's a picture.

I tested with a multimeter, and the voltage coming out of the MOSFETs' gate pins is nominal (~200mV)

Any idea why my Beaglebone won't turn on? Seems almost impossible that this fried it, right?
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Re: Grounding 12V Power Supply With LEDs

Post by adafruit_support_bill »

It may not work, but I don't believe that configuration would be capable of frying the Bone.

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mrmu
 
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Re: Grounding 12V Power Supply With LEDs

Post by mrmu »

Right, it actually did it to 2 Beaglebones. Both of which won't power on now.

Any idea? I've done some searching and couldn't find anything.

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Re: Grounding 12V Power Supply With LEDs and Beaglebone

Post by adafruit_support_bill »

I run it by some of the other engineers and see if they have any ideas.

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Re: Grounding 12V Power Supply With LEDs and Beaglebone

Post by adafruit2 »

oof not sure. it would be surprising if it really did damage it. but I'm also a little scared to try it on my 'bone. you could try looking for any console output? also check your polarity on the 12V adapter, make sure you didnt mix + and -!

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mrmu
 
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Re: Grounding 12V Power Supply With LEDs and Beaglebone

Post by mrmu »

Yeah, I checked the polarity on the power supply. It's good.

Previously I wired it up with the ground connection to the Bone and it didn't hurt the board. Only when I didn't connect the circuit to the Bone GND, did it apparently hurt my boards.

Another thing is that the circuit was very unstable, and when I touched the MOSFETs with my finger the lights would flickr or randomly change colors.

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Re: Grounding 12V Power Supply With LEDs and Beaglebone

Post by adafruit_support_bill »

Previously I wired it up with the ground connection to the Bone and it didn't hurt the board. Only when I didn't connect the circuit to the Bone GND, did it apparently hurt my boards.
I wouldn't expect that any damaging current could conduct back through the gate of the MOSFET. You might want to contact the BBB folks to see if they have any ideas.
Another thing is that the circuit was very unstable, and when I touched the MOSFETs with my finger the lights would flickr or randomly change colors.
With no common signal ground, the MOSFET inputs are effectively floating and would be easily influenced by things like that.

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