I started some troubleshooting tonight. An ADC that works fine when powered by a 9V battery (or laptop USB cable) gives bad readings when powered by a simple 9VDC wall wart.
The ADC is reading thermocouples which are grounded to a brass boiler in a coffee machine. The boiler is very well connected to the earth ground of my electrical system. (Yes, I must use grounded thermocouples for this application).
I suspect the problem has to do with a ground loop, so I measured the voltage between the (-) output from the wall wart, and the earth ground of my electrical system.
To my surprise, my DMM is showing a 50VAC potential between the (-) output and earth ground. So I checked a different wall wart, and read a very similar number.
I'm having a hard time accepting that the output of the wall wart is really parked 50V above ground. So what the heck do my readings mean?
If there really is a 50V difference, then do I need to add an earth ground to the ADC circuit (which is now floating)?
Jim
The ADC is reading thermocouples which are grounded to a brass boiler in a coffee machine. The boiler is very well connected to the earth ground of my electrical system. (Yes, I must use grounded thermocouples for this application).
I suspect the problem has to do with a ground loop, so I measured the voltage between the (-) output from the wall wart, and the earth ground of my electrical system.
To my surprise, my DMM is showing a 50VAC potential between the (-) output and earth ground. So I checked a different wall wart, and read a very similar number.
I'm having a hard time accepting that the output of the wall wart is really parked 50V above ground. So what the heck do my readings mean?
If there really is a 50V difference, then do I need to add an earth ground to the ADC circuit (which is now floating)?
Jim