PC resets when its USB port is connected to a portable device on charger

[jamesnguyen]

Hi,
My company makes a portable device (battery operated). If we connect the device's USB port to a PC while it sits on the charger, sometimes device resets, sometimes the PC reset, sometimes both reset.

I think the problem is caused by the voltage differential between the grounds of the two systems. As the charger is powered by a linear, 2-prong AC/DC adapter (transformer-based), its "ground" is actually floating with respect to the PC's USB ground. I got hundred of volts between the two grounds of the two systems! I think when the two system connects through USB, the voltage difference causes a large current transient, which causes the two systems to reset.

Is there a way around this?
James

 

[hydroman247]

Not an elec. engineer but isn't this what diodes are for?

 

[Skogsgurra]

hydroman, I think you do a better job as a mechanical engineer.

OP: This is something you have to sort out on your bench. There should definitely not be a large transient when you connect the device with charger to the PC. Does the charger have a UL or CE mark on it? Or been aproved by some other authority? What part of the world are you?

If you read hundred of volts wuth a DMM, then it may be capacitively coupled and nothing to worry about. If you can light a lamp with the voltage difference - then anything can happen. Including killing people.

Back to the drawing board, I would say.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[VEBill]

Beware the alternate "CE" (China Export) mark.

I think that the problem is probably with the AC Adapter. If the charger is powered by a DC input from an adapter, then the charger is probably innocent.

 

[OperaHouse]

I would place a 1,000 ohm resistor between the two system grounds and then measure that voltage. That "sometimes" indicates some sort of electrical problem.

A little related..... I have been playing around with a UNO and they can make a really cheap data logger. Unfortunantly, these reset and loose all the collected data when you pluga the USB port into a computer to read. The troubleshooting page on the web site says this now happens with newer operating systems. I have yet to try it with an old WIN98 system. Haven't reasearched it much but I gather that the new software now sends out a reset signal when plugged in to make it more compatable with later USB devices. You may also have a software issue in addition to an electrical problem.

 

[jamesnguyen]

When I connect 100 kohms between the two grounds, they are at the same potential. As soon as the 100k resistor is disconnected, the charger's "ground" voltage starts to drift.

Neither system resets when I earth grounded the charger's ground, but I don't think it's practical or safe.
James

 

[Skogsgurra]

Opera
I have also discovered the Uno. There, you've got bang for the buck. Love it.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[IRstuff]

"Neither system resets when I earth grounded the charger's ground, but I don't think it's practical or safe."

> Actually, it is the exact opposite, i.e., it's SAFER. This is why most high-power devices use a grounding plug. By grounding the charger's ground, you guarantee that the two grounds are at the same potential.
> This tells you that your charger is badly wired. It likely has a direct connection between the AC input and the ground of the of the USB connector, and the AC plug is not polarized or not correctly wired for a polarized plug.

TTFN
faq731-376

 

[jamesnguyen]

Is there such an AC/DC wall adapter with the DC ground earth grounded? Most wall adapters out there have 2 prongs. Some has 3 prongs but the DC ground is isolated from earth grounded.
James

 

[IRstuff]

But, most wall warts aren't plug polarized either. And, one has to wonder why tons of other USB devices dont' do the same as yours, even with nongrounding plugs.

TTFN
faq731-376

 

[VEBill]

Try a different AC adapter (a.k.a. wall wart) - one with actual safety markings. It's not worth much further investigation for a $5 adapter.