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Old 10-08-2017, 07:45 PM   #1
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Question for computer literate

I would like to break the + voltage circuit of a usb cable to control charging of my tablet. Does anyone see an issue outside of adding a resistor to keep the input voltage at 5 volts? Reason being, I would like to be able to stop the charge cycle when fully charged. It may not be necessary to implement but it's something that I'm interested in doing.
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Old 10-09-2017, 04:13 AM   #2
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You're going to have major issues with that. You need a proper voltage regulator if you're trying to output 5v over USB.

For your specific case, you'd probably be best off wiring a new 12v cigarette lighter socket and semi-permanently (tape!) shove a USB charger in there with a USB cord coming out of the dash somewhere.

Last edited by Reoze; 10-09-2017 at 04:15 AM.
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Old 10-09-2017, 07:05 AM   #3
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Good point,I can modify that answer to create the alternate power source. If I remove (cut) the + wire out of the usb cable, I still have a ground and the +,- for data so I should retain full functionality correct?
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Old 10-09-2017, 08:48 AM   #4
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What you want is a buck converter like this: http://a.co/b4t2WUZ
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Old 10-09-2017, 09:25 AM   #5
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I would NOT trust amazon for this specific type of thing. I have bought quite a few different DC-DC converters on amazon and about 90% of them were completely out of spec.

If you really need to buy one off amazon at least look for someone who's a reputable parts supplier.

Last thing i'll really say on this subject. Buying electronic components on amazon is a lot like buying car parts off ebay. You have absolutely no idea what you're actually getting.

Last edited by Reoze; 10-09-2017 at 09:36 AM.
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Old 10-09-2017, 10:46 AM   #6
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What my goal is... is to remove the charging duty from the grom interface all together. I may be trying to do something completely unnecessary but I don't want to turn the car on to charge the tablet. I'm also not in the car long enough to significantly charge while driving. This is in an 8 that I don't drive. I just start it once a week to keep the battery up. I have two others that I drive regularly. Once I get the kinks out of this design, I'm going to repeat it for my daily driver.
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Old 10-09-2017, 11:38 AM   #7
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This is probably a pretty bad idea. Your iPad can pull 2.5-3A continuously. Plus whatever losses you get from the buck converter.

If you forgot to unplug it at the wrong time your car won't start.

What some car manufacturers have done is add a relay to the switched power circuit that shuts itself off when the battery voltage reaches a threshold. This makes the project about 10 times more complicated though.
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Old 10-09-2017, 12:41 PM   #8
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This may be the reason it hasn't been posted here. I'm not smart enough to give up on the idea until I'm convinced it's not worth the effort.
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Old 10-09-2017, 01:22 PM   #9
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Quote:
Originally Posted by Reoze View Post
I would NOT trust amazon for this specific type of thing. I have bought quite a few different DC-DC converters on amazon and about 90% of them were completely out of spec.

If you really need to buy one off amazon at least look for someone who's a reputable parts supplier.

Last thing i'll really say on this subject. Buying electronic components on amazon is a lot like buying car parts off ebay. You have absolutely no idea what you're actually getting.
I've had the opposite experience.
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Old 10-09-2017, 01:39 PM   #10
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Honestly I've only had issues with hardware that was related to power regulation or conditioning.

I have at least a dozen breadboard power supplies (buck converters) that are outputting between 4 and 5v. The last one I bought was supposed to be dual 3.3 and 5v. The 5v line output 6v and the 3.3 was giving me 2.1.

None of the converters i bought from a well known company ever exhibited these issues.
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