I agree Dan. The supply is indeed intrinsically safe on these modules. We used the LU Probe a lot in a place I used to contract to and I remember similar issues. We always provided a separate loop power supply.
danny.michael said:
According to the Siemens LU technical spec, the minimum operating voltage is 17.8 volts at 300 ohms loop resistance...
Danny, where did you get those specs from?
Lance Johnson said:
...when I connect the Siemens Probe LU it doesn't even power up...
Our friend seems to be using the Siemens Sitrans Probe LU. This is its
data sheet. According to this, the nominal voltage should be 24VDC @ 550Ω max., up to a max. 30VDC. So the Probe LU's working supply voltage range is 24VDC-30VDC. The 1794-IE8H provides the necessary HART minimum resistance of 250Ω (273Ω), which is also under the 550Ω. But the modules supply starts at 23.7VDC @ 0mA and drops to nearly 17VDC @ 22mA. The transmitter is 4-20mA, not 0-20mA, so when trying to power up, if the transmitter is at 4mA, the voltage will already be somewhat lower than 23.7VDC. If at 20mA, or above, forget it.
This is from the Flex I/O Selection Guide - Publication 1794-SG002D-EN-P
Analog Input Modules require supplied 24V DC (19.2…31.2V DC) and consume the currents listed in the module specifications (>17V @ 22 mA, >23V @ 0 mA). In addition, adequate power must be supplied to the 1794-IE8 and 1794-IF4I transmitters to deliver input terminal voltage or drive 20 mA into the input impedance listed. This power source may be the same as the module power and can be included in the power supply calculated.
I would look to put a separate supply on the loop for this passive transmitter, even if temporarily, just to test the theory. Flex offers the 1794-PS13 (1.3A) or 1794-PS3 (3A) PSUs, but any compatible PSU will do.
Regards,
George