LabVIEW
Connect String Pots and Linear Pots to NI 9216
Sorry for bothering, but you mean by common of the DAQ card the AI- corresponding to each AI+ ?
I am just trying to figure out the physical connection for all the pots, is there a way I could group the wires attached to the PS terminals? also for the wires going from the -ve terminal of the PS to the AI- of each pot how could I make this connection (physically)
Attached the Power supply and the pinout for NI 9205.
Many Thanks
You could probably imagine something like the following:
Since they have shared GND values in this schematic, you'll also be able to make other connection types as described above, but the Differential setting should be fine for this too.
I only drew 3, but hopefully extending to 10 is fairly straightforward 🙂
Make sure you choose an appropriate power supply voltage and have sufficient current (I = V/R_total * 10), where V is the voltage supply value and R_total is the full resistance of the potentiometer (not the varying fractional amount).
V_measured is that same I * R_fractional.
Edit: note I placed a "GND" terminal in the schematic by convention (and because it's usually required in schematics for Electrical Rules Checking or SPICE simulations) but you don't physically have to wire a separate ground - the power supply negative terminal is likely grounded by a shared bar between what appears to be the black and yellow banana sockets.
If there's no connection between the negative voltage line and the power supply ground, you might want to make that connection (or else your entire experiment will be floating, which has various pros and cons).
If your power supply has a metal case it might be earthed (not the same as ground really). Otherwise it is usually at most soft-earthed (if it has a 3 prong outlet connector). Soft earth means that some reference point, usually ground, is coupled to the earth terminal through a capacitor to allow AC noise signals to flow to earth.
If both your sensor and your analog measurement device don’t have a solid reference point (that capacitor doesn’t count as such), your circuit is floating and you measure all kind of superimposed voltages and can get physical damage to the electronic circuitry from sporadic discharges of static charge building in the circuit. It won’t usually damage it right away but slowly detoriate the semiconductors on the input circuitry. More obvious, those discharges will creat voltage jumps in your measured signal as the circuit tries to adjust to the voltages created by the discharge currents in it.
If you ground/earth both sides you get another problem in the form of a ground loop. Small potential differences on both sides can cause significant current flows between the two ground potentials which will cause ringing and in extreme cases can destroy your equipment.
The best is to make always sure the entire circuit is earthed at exactly one point and one point only. Which point that is will depend on the components you use. The NI 9205 is not earthed in itself. Your pots do not seem to have a connection between one of the poles and the housing, which when from metal might form an earthing point if mounted on a location electrically connected with earth.
Leaves your power supply which nowadays often also isn’t hard earthed.
@askhassan wrote:
I am really thankful for this schematic ! Its really clear now. I just need to clarify something else
As written in the pots data sheet, the resistance is 10 kOhm, and the NI 9205 takes a voltage range of +/- 10 V , so this is the maximum voltage for excitation. My question is that the I (Current) from Power supply would be (10 V / (10kOhm x 10)) ?
Not quite. Using the formula you gave, adding more resistors will reduce the current. But each resistor is dissipating power, so the total current should be going up with more resistors.
Put another way, the resistors are in parallel, so the total resistance of the 10 potentiometers is 1/((1/10k)*10) = 1k -> I = 10V / 1kOhm = 10mA.
@askhassan wrote:
I just did not quite understand the last point for grounding the Power supply, isn't it grounded already by the power chord to the electricity port ?
I'm fairly sure I can see a bar connecting the yellow and black outputs on your power supply image. I would expect that probably one of those has an earth symbol under/over it, and is connected to the earth pin of your power socket in the wall.
As rolfk explained in much more detail, this might not be the case - see your power supply datasheet or manual.
Regarding the cDAQ earthing - this grounds the chassis and the internal connections, but the 9205 module has Channel-to-Earth isolation (see datasheet) that means the channels are not connected to the chassis ground. The image on p5 of the datasheet shows the internal connections - only the COM port appears to be grounded. You could connect this to your AI- or AISENSE pin, but then you'll want to take care for ground loops as mentioned in rolfk's post.
Sorry to ask this, but do you have any electrical engineering background? While the higher art of proper grounding is a topic one could write an entire book about and still not having covered everything, some of your questions are pretty basic EE stuff that you learn in the first year of any electrical engineering class. Learning on the job is an interesting principle but can be also an expensive exercise for whomever you are meant to do this!
