Multiple DMM Current Readings with PXI-2564

[iiSLEDGE]

Hi All,

I have a PXI-2564 16-SPST module and a PXIe-4081 DMM module. Basically, my system consists of a DC power supply that will supply power to 5 small heaters from a single output. In addition to being able to turn the power on or off to an individual heater, I want to be able to scan through the 5 heater channels and get a single current measurement continuously. 

My basic attempt at a schematic has been to split the input voltage into 5 unique channel inputs, giving me control of each heater with ch0-com0 through ch4-com4. The current reading is where my confusion comes into play. I only have a single HI and LO lead for the 4081, so how can I measure 5 channels without physically moving the HI lead?

My attempt at this point is to create a 5x1 MUX on the 2564 that goes into the DMM, and then another 5x1 MUX handling the output of the DMM. The idea being at any given time, I would have 4 open and 1 closed relay on each MUX to chose which device I'm measuring. The issue is this then requires 5 more switches to keep the current routed to the heaters while 1 is re-routed through the DMM.

Am I over thinking this? Maybe have the wrong hardware for the job? Any help would be appreciated, as the NI SWITCH documentation contains little on the actual analog connections for Switch/DMM systems other than "Make the analog connections"

Thanks!

[Frank-L · Accepted solution]

iiSLEDGE,

Have you tried something like the following?:

Essentially hard wire 5 relays together to make a mux, and you do that three times. You then can use a precision resistor, and you take a voltage measurement across the resistor and and calculate current. This will allow you to power all 5 boards independently, while being able to measure current one at a time and not power off the heaters. This should take 3 relays per heater for a total of 15 relays.

For example, you can close relay 2 to power a heater, then close relay 3 and 5 to take the measurement across the resistor.

[iiSLEDGE]

Frank,

Thanks so much! I think I had the same idea as you at the same time. See below. Does this make sense?

[Frank-L]

iiSLEDGE,

That makes sense to me! To make sure all the current flows through the DMM, you would need to do a procedure where you do the following, but it should work!

1. close channel 9,
2. close channel 0 and channel 5
3. open channel 9
4. take a measurement
5. close channel 9
6. open channel 0 and channel 5

Also your drawing is so much better than mine!