data-acquisition.us

[Jen]

An analog signal retransmission device is being used to send a 4-20mA signal to a pair of VFD's. When a multi-meter is in series with the loop, it reads approx. 15mA, which seems consistent with the process. The problem is that the VFD's are seeing a 21mA input. When the voltage across the input to the VFD's is taken, it is 5.1 vdc which would be consistent with a 21mA input. Confused here as to how this is happening?

[daqmaster]

You may be using the wrong meter to calibrate your process. Is this multimeter capable of measuring current loops without loading them down? You may be introducing an offset error by loading the circuit with the multimeter. Also, have you checked the input to your retransmission device? How do you think you are getting 21 mA? If the VFDs are both connected in series, make sure that their inputs are floating and not single-ended. Try removing one of them from the circuit, and check the current. Your retransmission device may not have enough drive (voltage) for two sinks plus your meter.