My CarPC shocked me with 20V AC at USB ground! Where's it from?

**parksgm** · 04-01-2006, 02:51 AM

Hi all,

I've been working toward the final installation of my CarPC. Currently, I have a 1.3GHZ Athlon running off a normal ATX PSU that is powered by a Jensen 300 watt 12V to 120V AC inverter. The inverter is powered and grounded at the same site as my main amplifier.

Here's the weird part...while installing my system, with the computer and inverter on, but the amplifer and headunit off, I brushed up against my metal-handled gearshift (a direct chassis ground, more or less) while also touching an exposed USB ground (the metal part that forms the plug) and noticed a distinct tingling feeling that I unfortunately recognized as a significant amount of voltage passing through my body...not a good thing, but fortunately not high enough current to hurt me.

So, I got out my trusty multimeter and measured the voltage drop between the USB ground and the gearshift...it was 20V AC (!!!!! not DC) and present in all of what I would consider to be computer "grounds" (the ATX PSU case, the monitor cable shielding, the audio out cable ground, etc...). The odd thing is that the computer posts, boots, and runs without issue, and the system otherwise works perfectly...despite buzzing badly when the computer audio is connected to the AUX input of my head unit, which I now suspect is directly related to the 20V AC signal that was apparently present whenever I connected the headunit and the carpc.

I figure that the current/voltage MUST be coming from either the inverter or some strange short in the PSU. To my knowledge, 20V is not a common computer power supply voltage, and the inverter should be producing somewhere between 115 to 130V AC.

Any questions/comments/advice are welcome. I can provide further info if needed.

Gregory

**xdjxklusivex** · 04-01-2006, 07:56 PM

try to reground or try to swap out the psu / inverter

**zootjeff** · 04-02-2006, 04:08 AM

Inverters some times don't ground their outputs. Measure the voltage between the ground of the - input (12 volt battery) and the ground terminal of the AC plug, or the neutral line. If your inverter doesn't suck, you'll measure no voltage there. If it sucks, you'll get a voltage. I'm guessing it sucks.

Some inverters that suck, won't work if you try to short the input ground to the output ground. Try it with the 10 amp setting of your meter and see if it draws any AC or DC current. Then you will get the idea of how much suckness your up against.

Cheers,

Jeff

**parksgm** · 04-02-2006, 02:00 PM

Thanks...that was an useful answer, and without sarcasm, an unusual thing for this board. But, I have a question...since the inverter is outputting AC current and the ground is 12V DC, what setting should I use on the multimeter to measure the voltage AC or DC? I ask this so I can get an accurate measurement, not because I'm worried about shorting stuff out...I'm well past that worry now.

Gregory

**zootjeff** · 04-02-2006, 02:31 PM

Originally Posted by parksgm

Thanks...that was an useful answer, and without sarcasm, an unusual thing for this board. But, I have a question...since the inverter is outputting AC current and the ground is 12V DC, what setting should I use on the multimeter to measure the voltage AC or DC? I ask this so I can get an accurate measurement, not because I'm worried about shorting stuff out...I'm well past that worry now.

Gregory

Well mos tlikley you'll be seeing AC current and voltage because the inverter is producing a AC output. I'd do all the measurements on the AC setting. It's also useful to know the DC component for reference. For instance, you could have a 20 volt AC signal riding on a 10 volt DC signal. In otherwords, if your meter is measuring 10 volts DC and 20 volts AC, it is possible that you have a signal that is centered around 12 volts and not ground. IE going from 0 volts, up to 20 volts, back to zero.. Rather than going from -10 to +10..

-Jeff

**parksgm** · 04-02-2006, 11:26 PM

Gotcha...wouldn't have considered both AC and DC voltage riding together, but certainly a possiblity. Haven't had a chance to check it out yet, but I will soon.

On the other hand, I'm tired of the inverter, as I need greater control over startup and shutdown...I think I'm might take the plunge and buy a DC-DC PSU (M2-ATX). Hopefully that will be enough to run my 1.3GHZ Athlon, fan, 512MB, and 20GB laptop HDD, and two USB devices without trouble.

Thanks for the help.

Gregory