Hi, I'm new with arduino and electronics in general.
In my project I need to measure an analog signal from a control board (signal between 0 and 5 V).
This board has a ground reference on the phase of power supply (220V).
So with my arduino uno I need to measure all analog signal with reference of that gnd.
Is it safe to link that gnd to the gnd pin on my arduino board?
(arduino will not be linked to my pc, obviously)
thanks in advance
Antonio
Although it's more or less safe to connect to that ground under normal circumstances, I would not recommend it because these circumstances can change. I'd use an optocoupler to galvanically separate your Arduino from the high voltage circuit. An analog optocoupler like this one http://www.conrad.ch/ce/de/product/140035/OPTOKOPPLER-HCNR201-000E should do the job.
In my project I need to measure an analog signal from a control board (signal between 0 and 5 V).
This board has a ground reference on the phase of power supply (220V).
I assume that control signal is "safe" and fully-isolated from 220V? (The ground does not have to be isolated, but the "hot" 220V DOES need to be isolated!!!!)
Yes, you need the ground reference, and it's OK to connect the Arduino's ground to the control board's ground and/or to your house-wiring ground.
For example, when you measure voltage with a voltmeter there are two leads and both wires need to be connected to get a measurement. The black lead on your voltmeter is the "ground" or reference and in order to measure the analog voltage, that black lead needs to go to the ground on the control board. If the black lead is not connected, or if it's connected to a different/isolated ground, you won't get a good reading. (The ground on the control board is likely connected to the 220V ground already.)
The internal low-voltage ground is often connected to the 120/220V ground, but it's not always required. (For example, the signal grounds on a stereo amplifier are often connected to "earth ground" through the power cord.)
If you do decide to reference the Arduino ground to one of the mains phase conductors instead of mains earth, here are a few things to watch out for:
What power supply will you use for the Arduino? Many wall warts have the negative side of the output connected to the mains ground pin - these are obviously unsuitable for your application.
You must be prepared for the Arduino ground reference to become live at any time, for example if the live and neutral are the wrong way round, or the neutral connection breaks. So the Arduino and everything connected to it must be double insulated from anything the user can touch. Working on such a system is dangerous unless you use an isolating transformer.
What else will be connected to the Arduino? In particular, is there anything that needs a different ground reference?
The internal low-voltage ground is often connected to the 120/220V ground, but it's not always required. (For example, the signal grounds on a stereo amplifier are often connected to "earth ground" through the power cord.)
And also as an example Stereo Hi-Fi equipment in the 70s used only two prong AC cords with no attachment to the household 'ground system'. This had an advantage of allowing the Hi-Fi common ground used by all the interconnect cables between the devices to be isolated from power ground and more immune to AC ground loop caused hum.
So it can be confusing especially for newcomers to electronics. Connecting any two devices to each other requires a 'common reference' connection between them, but this may or not not have any requirement to be household AC ground referenced. AC power grounds primary purpose is safety related to carry fault currents to insure that circuit breakers or fuses open up over allowing 'hot' metal surfaces ready to find a ground path through a unlucky person who happens to be in contact with something else referenced to AC power ground.
Ground is such an overloaded term, often misused or misunderstood to also mean circuit common which may or may not be the case, so almost always requires further information to be fully understood.
With our commercial Power equipment we recently had a situation where there was 30VAC between neutral and earth. Of course my 3 phase systems barked at that and threw a few sparks - we turned it off.
The culprit was likely a large 3 phase motor - as the earth leakage from active manifested across each phase but each phase had a slightly different earth leakage.
My advice would be to take a common 'ground' from the low power side - that is, from the circuit supplying the analog AC signal.
Similarly when connecting a microcontroller to a sensor there must be a common reference ie a negative of the dc power supplying the circuit. You can common the circuit negative to all board power ie 5Vdc, 12Vdc etc - include the sensor negative eg the opamp -ve and it will work.
Of course you will need to make sure that the analog signal is in the 0-5V range. Suggest using a voltage divider and place the signal somewhere in the middle of the range, then you need to ensure that any attenuation in the signal falls within the voltage spec. to the analog input - easy to do - use a CRO to check signal levels as rms before connecting to the Arduino. Suggest using a 100:1 probe to inhibit circuit impedance problems. 10:1 probes and lower will add impedance and corrupt the signal flow, so when you disconnect the CRO your calibrations will be off.
Good luck.