As the PCSU1000 is connected to the USB Port of the PC, it will have the same common ground as the PC, in most cases it will be connected to the “earth”.
It can happen tough that you create a shortcut over ground, because you think that the ground level of the measured device is at “earth” level, but it isn’t. On an legacy scope the ground is mostly done by high width connections, but on a PC the USB port is designed for 500mA.
My thoughts are: How can i protect the connected PC as well as the PCSU1000 scope against this kind of shortcuts?
Sure, intelligent measuring would be the best way to avoid any mistakes and shortcuts, but an additional protection would be interesting.
I was planning on buying that scope, and using it together with a notebook. In that case the Ground is not connected to the “earth” when it’s running on battery, but when its plugged to it’s power supply, I’m not sure.
You are correct, the PCSU1000 is not optically isolated from the PC.
Attention: If you measure on the AC power grid, all parts of the PC are connected to a phase of the grid, even when it is running on battery !
Therefore, its inputs are rated at 30Vp. You can either power the device under test from an isolation transformer or you could consider the PCS500, which is fully isolated from the PC. Please note that the PCS500 is a parallel port device. USB-> parallel adaptors are not compatible with the PCS500.
The only way to make the measurements save for the PC, would be then to build a small USB-adapter with an external power supply and optocouplers.
But that will not be as easy as it sounds. I will try to find a finished product.
The PCS500 is ended an alternative, but it isn’t that mobile.
I did some searching at Google too, and found also this hub, as well as some others, but all in the same price-class of approximatively half of the value of the PCSU1000, what i consider to much for my use.
I found also out that on the physical layer of USB it is very difficult to integrate optocouplers! The problem is the high speed and the bidirectional data. I read that the best way to implement USB isolation is to make it at a higher level. It is said that isolation on SPI an connection is easier and has less high data rates. But to implement that you must have access to that SPI.
The easiest and cheapest way to protect the PCSU1000 and the PC would be then to connect the PCSU1000 via an hub. Using an active hub with a good ground would even improve the protection.
And for any other risky measurement i will continue to use my good old hand held scope K7105
Yes, the isolated HUBs seem to be quite expensive.
Thank you for a good link concerning the USB isolation issues. The SPI is one solution. In the Velleman K8061 (Extended Usb Interface Board) is used UART and serial communication over the isolation border.
Indeed, your K7105 solves the isolation problem too!
