I would definitely not use the PropScope for measuring mains voltage with any kind of probe -- at least not without transformer isolation. Otherwise, you risk not only a shock hazard but possible damage to the PropScope and to your PC.
Phil, I know that you are trying to be helpful but this type of vague "don't do it" response is less than scientific.
Not knowing your relationship to this forum or to parallax, are you speaking in an official capacity about use of the propscope product?
I am new to USB scopes vs benchtop units. Can you help me understand why there is not a safe choice for a probe to do this?
For example, I am familar with the need to isolate a bench top mains-powered scope when measuring mains. For a USB scope, could you not simply hook it up to a laptop and have that whole setup float?
The problem with a non-isolated probe for mains probing is that logic ground on most PCs is also connected to mains ground. To measure the mains voltage directly, one pin has to be connected to the hot side, and one to neutral. If you happen to connect the hot side to logic ground, that will cause a direct short through the USB cable and PC, resulting in almost certain damage. But even if you connect neutral to logic ground, you will get a ground fault condition and return current flowing through the mains' protective ground wire, which it's not designed to carry.
For that reason, I would definitely use a small transformer for isolation, which will avoid the issues mentioned above. It doesn't even need to be 1:1 if you calibrate it from a known line voltage obtained from a battery-powered VOM.
BTW, I'm not a Parallax employee, so my forum postings should in no way be construed as any kind of "official" word from Parallax. For that, if it's what you want, you should contact Parallax's tech support.
For a USB scope, could you not simply hook it up to a laptop and have that whole setup float?
Although that would eliminate the problems mentioned above, there is still a shock hazard from any exposed metal parts on the PropScope and laptop. I wouldn't do it. Unlike battery-powered test instruments whose metal parts are well-insulated or deeply recessed, the PropScope and most laptops are not designed for that kind of service.
back to the original question about suggestions on a specific appropriate probe to use for this purpose, assuming that an isolated probe is used, what are the remaining concerns for safety and/or risk of damage to the scope and a laptop?
An AC-output wall transformer could be connected directly to the PropScope inputs for measuring a voltage proportional to the AC line voltage. Just make sure to get one whose outputs are well within the PropScope's -10V - +10V limits, or else add a resistive voltage divider to limit the voltage that the PropScope sees.
greggler
Posts: 5
Comments
-Phil
-Phil
Not knowing your relationship to this forum or to parallax, are you speaking in an official capacity about use of the propscope product?
I am new to USB scopes vs benchtop units. Can you help me understand why there is not a safe choice for a probe to do this?
For example, I am familar with the need to isolate a bench top mains-powered scope when measuring mains. For a USB scope, could you not simply hook it up to a laptop and have that whole setup float?
For that reason, I would definitely use a small transformer for isolation, which will avoid the issues mentioned above. It doesn't even need to be 1:1 if you calibrate it from a known line voltage obtained from a battery-powered VOM.
BTW, I'm not a Parallax employee, so my forum postings should in no way be construed as any kind of "official" word from Parallax. For that, if it's what you want, you should contact Parallax's tech support.
Although that would eliminate the problems mentioned above, there is still a shock hazard from any exposed metal parts on the PropScope and laptop. I wouldn't do it. Unlike battery-powered test instruments whose metal parts are well-insulated or deeply recessed, the PropScope and most laptops are not designed for that kind of service.
-Phil
-Phil