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Old 08-08-2012, 08:38 PM   #1
SlickWRX2010
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Member#: 318836
Join Date: May 2012
Default USB Cell Phone Charger + Add-a-Circuit

Hey guys!

I've searched google and these forums, but I cannot seem to figure out my issue. I'm not sure it's exactly an issue I'm having with regards to the vehicle, but I'm sure some people on here have attempted to hardwire a USB wire at some point and may be able to advise me as to where I went wrong.

Basically, because my commute to work takes me across several highways with plenty of options, I like to track the traffic with my cell. I don't want to be looking down at the cell in my hand, so I went and bought a mount with a built in charging system (Galaxy Nexus is the phone).

I hate wires hanging all over my dash, so I've found the add-a-circuit which I figured would be a grand idea. So, I grabbed a typical 12v cigarette lighter charger (which works and charges my phone), clipped off the 12v part, and connected the add-a-circuit to the red positive feed from the USB. Then, I ran the black negative line to a metal screw on a nearby frame.

However, when I plug in the add-a-circuit and connect it to the phone or mount, nothing. The fuses are plugged in, and I think correctly. I've tried plugging everything in in different directions, trying to determine if there is a hot/cold side, nothing. Does anyone have any idea as to where I'm going wrong? Why there is no current running through the system?

Thanks!
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Old 08-09-2012, 12:32 AM   #2
Bluefoton
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Member#: 141373
Join Date: Feb 2007
Chapter/Region: E. Canada
Location: Quebec, QC
Vehicle:
2008 2.5i-hatch -
now with a HID retrofit

Default

Likely because USB is +5V and your car provides +12V? You seem to have bypassed the part that drops down the voltage before it goes to the USB - hope you didn't fry the phone at the same time.

If you want to have a dedicated "stealth" charging wire, do the same as you did but connect the +12V car add-a-circuit source to the tip of the cigarette plug. Alternatively, disassemble the plug, and connect to whatever wire goes to the central pin - but DO NOT bypass the plug adapter, as it contains voltage-reducing circuitry. A voltmeter would help you
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Old 08-09-2012, 08:27 PM   #3
SlickWRX2010
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Member#: 318836
Join Date: May 2012
Default

Thanks for the response Bluefoton. I haven't fried anything at this point so far lol.

Now, I understand the idea behind restrictors reducing the voltage to the appropriate 5v. However, what I don't understand is why I'm not getting a charge (or seemingly any power at all) without reducing it? I would think it would just overcharge and fry out the phone. I figured there were restrictors within the mount itself as it has a ton of circuitry, but I guess we know what assuming does.

Any ideas? I think I'm going to grab one of those usb hardwiring kits with a built in adapter that turns the 12v to 5v, you think that would work if I used that along with the add-a-circuit? They are only like 15 bucks.

Thanks.
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Old 08-09-2012, 11:26 PM   #4
Bluefoton
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Member#: 141373
Join Date: Feb 2007
Chapter/Region: E. Canada
Location: Quebec, QC
Vehicle:
2008 2.5i-hatch -
now with a HID retrofit

Default

Most likely the phone circuitry protected it and cut the supply off. Battery charging circuits became fairly complex, it's not a dumb DC source anymore.

Yes, the 12-to-USB kit should work fine. Make sure though the adapter works up to 14.5V - the actual voltage in a car varies in between 12 (on battery), to over 14 (engine running, no load).
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Old 08-10-2012, 09:53 AM   #5
bassman12350
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Join Date: Nov 2009
Location: Twain Harte, CA
Vehicle:
2011 Impreza WRX 5dr
WR Blue Pearl

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You might want to check and see if the screw you are using is really grounded to the car's frame. There is a possibility that some of the metal parts in the interior of the car are not solidly grounded or at ground potential - - no good ground = no 12 volts to your device.
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