Ground looping issue on my tablet?

[SEAT_problems_NorthEU]

Hello!

I'm new to this forum and have yet to purchase the HEX-V2 cable, I'm planning on buying it but there's only one issue that concerns me. I plan on using my Windows 10 tablet (http://support.linxtablets.com/Products/Linx1020/) with a USB-port and also have it connected to the cars cigarette outlet incase the battery dies while I'm diagnosing the car.

Apparently, charging a laptop/tablet while being plugged into the car can cause something known as a "ground loop", I've researched this and as far as I understand it ground loop is essentially when two ground points with different potentials make a connection allowing voltage and current to flow between them, this seems to really only be an issue that pertains to audio systems as far as I can tell. Reading Ross-techs website they don't advice using universal laptop 12V laptop chargers for this reason but I'm planing on just using a micro-USB cable connected to the cigarette outlet with a normal charger like this

https://imgur.com/a/n8glM5a

.

The question is, can charging my tablet from the cigarette outlet of the car with a micro-USB cable cause a ground loop if I'm also using VCDS? If yes, how do I prevent this? do I buy a different charger or do I simply charge and work on the car at different times? I suppose I could just bring with me a bunch of battery powerbanks but I'd rather just use the cars outlet if possible.

Just for reference, the car I'm working on is my personal vehicle which is a SEAT Ibiza 6l model year 2004 with a 1,4 petrol engine. I live in an apartment and don't really have access to a wall outlet nearby as my car is parked in a local garage.

 

[Bruce]

The question is, can charging my tablet from the cigarette outlet of the car with a micro-USB cable cause a ground loop if I'm also using VCDS?

The answer is yes.

What I would do is plug the interface into the tablet with the tablet on the charger. Now measure the voltage on pin 4 of the interface to pin 4 of the vehicle DLC. If the voltage is more than 1v, you have a problem for sure. Do repeat this on pin 5....

The ground connection in the car is on pins 4&5. The problem with the chargers is that they may not properly reference ground. One would think they would but they may not. So, be safe and check.

If you have the voltage difference, you have a couple of choices: try a different charger, or use a USB 3.0 isolator.

If you go the isolator, you have to test it to see if the latency induced by the isolation circuitry messes with communications.

The ground loop problem is real. It can smoke the interface and it can smoke the tablet. Safe is better than sorry.

 

[SEAT_problems_NorthEU]

How e

The answer is yes.

What I would do is plug the interface into the tablet with the tablet on the charger. Now measure the voltage on pin 4 of the interface to pin 4 of the vehicle DLC. If the voltage is more than 1v, you have a problem for sure. Do repeat this on pin 5....

The ground connection in the car is on pins 4&5. The problem with the chargers is that they may not properly reference ground. One would think they would but they may not. So, be safe and check.

If you have the voltage difference, you have a couple of choices: try a different charger, or use a USB 3.0 isolator.

If you go the isolator, you have to test it to see if the latency induced by the isolation circuitry messes with communications.

The ground loop problem is real. It can smoke the interface and it can smoke the tablet. Safe is better than sorry.

Thanks, so 1v would be the maximum allowed voltage between the ground points of the car OBDII port and the HEX-V2 interace before damage could occur? What should the voltage be ideally? (zero I'm guessing)

 

[Uwe]

1v would be the maximum allowed voltage between the ground points of the car OBDII port and the HEX-V2 interace before damage could occur?

Voltage is just a quick check. What really causes damage is current flow. Current will only flow if there is a voltage difference, but a big voltage difference measured with a high impedance device like a volt meter might still disappear without much current flowing when a low impedance connection is made. It really depends on the impedance of the device causing the voltage difference.

This is really the same principle as load testing a wire, except in that case you want the voltage to stay high (typically +12V) when you apply a load, but in this case, if the voltage stays high when you put a load between the computer's ground and the car's ground, and the voltage remains, then that's indicative of a problem.

Is the battery in your tablet so shot that you can't use it on battery power alone?

-Uwe-

 

[SEAT_problems_NorthEU]

Voltage is just a quick check. What really causes damage is current flow. Current will only flow if there is a voltage difference, but a big voltage difference measured with a high impedance device like a volt meter might still disappear without much current flowing when a low impedance connection is made. It really depends on the impedance of the device causing the voltage difference.

This is really the same principle as load testing a wire, except in that case you want the voltage to stay high (typically +12V) when you apply a load, but in this case, if the voltage stays high when you put a load between the computer's ground and the car's ground, and the voltage remains, then that's indicative of a problem.

Is the battery in your tablet so shot that you can't use it on battery power alone?

-Uwe-

I'm assuming that the test should be carried out without actually inserting the HEX into the car DLC, instead the multi meter leads should be put on either ground point when they're not connected. As long as there's negligible voltage difference between the ground points then current wouldn't be flowing either way so I should be fine right?

The battery isn't completely shot, in fact I could use it on battery power alone for maybe an hour before it needs a charge, most of the time I don't I'll be using it for that long but the scenario I have in mind is if I'm trying to diagnose a problematic sensor or a wheel bearing and I need to data log while driving, then I'd need a way to keep the tablet charged somehow.

I'm really just asking in anticipation before buying since I want to know what I'm getting into, I don't feel like splurging a couple hundred dollars and then bricking both my tablet and the interface.

 

[Bruce]

I'm assuming that the test should be carried out without actually inserting the HEX into the car DLC, instead the multi meter leads should be put on either ground point when they're not connected. As long as there's negligible voltage difference between the ground points then current wouldn't be flowing either way so I should be fine right?

Your assumption is correct and the theory is also equal to my thinking. No voltage, there should be no current.

If there is a voltage, then we cross that next bridge. We use a fuse to connect the grounds and we use an ammeter to see how much current flows. A couple milliamps? Likely not a problem. An amp? We're gonna let the magic electronic smoke out of something.

 

[Dana]

In addition to the above, the earlier suggestions via email that involved using a 12V Car Battery Jump Starter (i.e. boost box) with a USB port -or- portable DC USB "power bank" could be used for the very rare occasion that you needed more than 1 hour run-time. I think the 12V Jump Starter (as long as it has a USB port) would be the better option so it would have dual functions. Additionally, they tend to be higher quality than some of USB power banks out there.

 

[stefdds]

How about using one of these:

https://www.amazon.com/CableMax-Isolator-5000-Vrms-Dongle/dp/B00QU232UM/ref=sr_1_10?crid=2MX9EAWOXOADP&keywords=pc+usb+ground+loop+isolator&qid=1695158630&sprefix=pc+usb+ground+loop+isolator%2Caps%2C146&sr=8-10

 

[Uwe]

How about using one of these:

https://www.amazon.com/dp/B00QU232UM/

There are others, some of which are substantially less expensive:

Amazon.com : usb isolator

www.amazon.com

But if I were in the OP's shoes, I'd rather use a USB charger that doesn't cause a ground-loop problem to begin with.

-Uwe-