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Ground Loops and why you should always measure twice.

Gryd3

Jun 25, 2014
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Sorry, no pictures, although you wouldn't really see much anyway.

I had a space set of Bluetooth Headphones kicking around the house from 4-5 years ago, which worked great, but not for very long... Tiny used batteries will do that to you.
So I figured I would mate this thing to a 3.5mm headphone jack and a small cigarette lighter 5V adaptor.
Simple, plug it in the car, and instant bluetooth music :)

So the project begins as I remove the housing and the small board from the headphones. I clip the power wire from the battery, and use a small 5V usb adaptor I have as a test subject. I have success powering the unit directly to the battery terminals with a 5V usb adaptor. Great, next step.
I then hack apart an old 3.5mm plug and solder that in place of the headphone speaker. Unfortunately, I need to hook it up to the left channel only, as the speakers do not actually have a common wire, and blindly joining a wire to use as common can release magic smoke.
Once the USB adaptor and 3.5mm plug were secure, I proceeded to test out the unit, so I plugged the 3.5mm plug into my computer, and then plugged in power, and as I reach to push the power button on the bluetooth module, I realize the components are having a bit of a party. As the smoke begins to spew from the little hole I made for the wire, I realize that although I have measure for a common between headphone speakers... I failed to measure for a common or very low resistance path between the speaker terminals and the battery terminals...
I created an alternative current path between the power supply and computer...
So... moral of the story is to measure everything, be observant, and use decoupling caps if you are unsure if there is any isolation between the power supply and audio system... oops.

I'm off to go buy a cheap bluetooth module now. I should have completed the project before promising results ;)
 

Gryd3

Jun 25, 2014
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Measure twice, cut once!!
Good info Gryd3

Martin
It wasn't the cutting that did me in... it was the soldering ;)
I'm a little bummed out, but am incredibly grateful it didn't damage my computer. It would have been interesting to see the current being pulled. I know the 5V adaptor was only capable of 500mA, but when there is no isolation, those numbers could potentially be a lot higher.
 

Martaine2005

May 12, 2015
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I see the 9 lives are playing their part!.
You are very lucky not to knock out the USB on the motherbooard.
Would have been painfull.
Martin
 

Gryd3

Jun 25, 2014
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I see the 9 lives are playing their part!.
You are very lucky not to knock out the USB on the motherbooard.
Would have been painfull.
Martin
I can live with a burnt out USB port. They usually have protection on em...
I had an AC-DC 12V adaptor powering the small cigarette USB adaptor, which was hard-wired to the Bluetooth.
From there the bluetooth speaker wires were fed connected to a 3.5mm jack which I plugged into the lie-in on my computer. ;) The line-in is where the damage would have potentially happened.

Glad I figured it out there instead of plugging it into the wife's car though... a new CPU would have been cheaper than a new car stereo head-unit.
 

HellasTechn

Apr 14, 2013
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USB are overcurrent protected from their controller. i would worry more about burning the audio output of the PC.
 
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