Variance in 4-20 mA measurement

[jman03]

I am using an analog signal retransmission device to send a 4-20mA signal to a pair of VFD's. When I place a multimeter in series with the loop, it reads approx. 15mA which would be consistant with the process. The problem is that the VFD's are seeing a 21mA input. When I measure the voltage across the input to the VFD's, it reads 5.1 vdc which would be consistant with a 21mA input. I am confused as to how this is happening.
Any comments or suggestions?

 

[Crowbar1]

Sounds to me like your problem is you're using the wrong meter to calibrate your process. Is your multimeter capable of measuring current loops without loading them down? It would seem to me that your introducing an offset error by loading the circuit with the multimeter.

 

[ICSpecialist]

What other checks have you made? Have you checked the input to your retransmission device? How do you suppose 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. Chris Parrish
I&C Specialist
Dekalb County, GA Water & Sewer
cjparris@co.dekalb.ga.us

 

[grimmer]

Is this a loop signal isolator/converter? It sounds like you are converting a 4-20mA signal to a 1-5V signal.

If that's the case, has it been initially calibrated for
this application(if it's new) or has it been calibrated
recently to compensate for 'drift'(if it's old)?

As crowbar1 suggested, check your meter for proper
operation, though most DMMs these days are quite adequate
to measure these signals.

If the circuit was being overloaded, the signal should be
driven 'down', not 'up' as was suggested.

 

[ICSpecialist]

Oooh, I get it. Your current is correct, but you are assuming that it is high because of the 5.1 V. You probably have an odd resistor value(340 ohms?) at the end of the loop. It should be 250 ohms. Chris Parrish
I&C Specialist
Dekalb County, GA Water & Sewer
cjparris@co.dekalb.ga.us