Hi,
excuse my silly question, but I am not good at electronics. I need to use a digipot to control an appliance that I don't have common ground with. I was thinking of using a mechanical pot, but that is too old fashioned plus depreciation happens.
Please direct me into the available options, is there a way to use a digipot in that case? I assume it has to be isolated from the appliance's circuit.
Thanks 
don't have common ground with
Can you get one?
Otherwise there are plenty of isolation chips and opto-couplers you can use to pass the control signals to the pot. Can the pot get power from the other end? If not you'll need an isolated DC/DC converter as well.
All common and cheap parts.
Rob
an isolated DC/DC converter as well.
All common and cheap parts.
I wouldn't call a isolated DC/DC converter cheap.
that I don't have common ground with.
that is the worrying thing, why.
If it is because you can't identify it you will need to in order to get the opto isolated thing to work.
What is the voltage that the original pot experiences. If it is too high it could rule out having a digital pot and you might have to look at the linear transfer opto isolator to generate a control signal. However, you haven't said what this pot is controlling, it could be totally unsuited to this sort of control.
I wouldn't call a isolated DC/DC converter cheap.
Well I guess everything is relative, but for my money $10 for a converter (unregulated) is pretty cheap.
Still you're right about identifying the requirements of the device being controlled.
Rob
Thanks. I am thinking of controlling a thermostat of a fan coil. The thermostat is pretty fancy, checks fluid and room temperature, opens/closes fluid valves and controls fan speed. It is still under warranty and I can't open the box, therefore my concern regarding the common ground. The thermostat's temperature sensors are on screw terminals and this is where I plan to interfere, by adding a digipot in series and one in parallel to the thermistor. Then thru ethernet enabled Arduino I will be able to monitor and control the room temperature by tricking the sensors to think it is colder or warmer than the thermostat setpoint.
Voltages on the temperature sensor side are low, haven't measured yet but assume <12V DC
All that is out of curiosity rather than practicality
It very much depends on what sort of temperature sensor you have. Yes that technique will work with something like a thermisistor but it won't with something like a current source sensor or even a digital one. One of those two wires will be enough to get at the common ground. Try it with a rotary pot first to see if it works.