Load resistance

Ha, maybe I can add to the confusion?

I've not used 4-20ma stuff but have considered designing such a system.
Presumably there is a spec somewhere for this stuff?
Theoretically a constant current source needs an infinite voltage available so that it cope with any load.
In practical terms I'd imagine there must be a 'standard load' for these things and then some allowance for cable resistance...
your expected 24v DC supply seems like a sensible expectation.

In terms of you actual question about measurement...cable resistance can be measured with a multimeter. Load resistance measured as a voltage across it's terminals and current flowing at the time.

I shall watch this thread with interest.

Del

In milliamp loops in industrial instrumentation, it is important not to put so much stuff in the loop so that 20mA cannot be achieved.

Provided the loop is capable of driving the valve from 0 - 100%, the actual voltage at the positioner at any moment is as irrelevant as the cable resistance.

The maximum voltage from an analog output card is often limited to somewhere between 10 to 15 volts, usually to achieve an intrinsicly safe rating. The recieving devices are typically 100 or 250 ohms. You can add devices in series until you get to the maximum outpt voltage. Two devices with 250 ohm input resistance would each require 5 volts at 20ma., or 10 volts total.

In your post, you didn't say what the output current was when you measured the 6 volts.

From Wikipedia:

[edit] Analog

Analog current loops are used where a device must be monitored or controlled remotely over a pair of conductors. Only one current level can be present at any time.

4-20 mA ("four to twenty milliamp current loop"), also referred to as "process current", is an analog electrical transmission standard for industrial instrumentation and communication. The signal is a current loop where 4 mA represents zero percent signal and 20 mA represents the one hundred percent signal.^[1] A "mA" is a milliampere, or 1/1000 of an ampere.

The "live zero" at 4 mA allows the receiving instrumentation to distinguish between a zero signal and a broken wire or a dead instrument.^[1] The feature also allows low-power instruments to be directly powered from the loop, saving the cost of extra wires. The 4-20 mA standard was developed in the 1950s and is still widely used in industry today. Benefits of the 4-20 mA convention are that it is widely used by many manufacturers, relatively low-cost to implement, and it can reject many forms of electrical noise. The inherent noise rejection of 4-20 mA allows it to be used where the transducer is located far from the measuring instrument. Cable lengths of 50 meters or more are common.

Given its analog nature, current loops are easier to understand and debug than more complicated digital fieldbuses, requiring only a handheld digital voltmeter in most situations. Using fieldbuses and solving related problems usually requires much more education and understanding than required by simple current loop systems.

Additional digital communication to the device can be added to current loop using HART Protocol. Digital process buses such as FOUNDATION Fieldbus and Profibus may replace analog current loops.

[edit] Process-control use

For industrial process control instruments, analog 4-20 mA and 10-50 mA current loops are commonly used for analog signaling, with 4 mA representing the lowest end of the range and 20 mA the highest. The key advantages of the current loop are that the accuracy of the signal is not affected by voltage drop in the interconnecting wiring, and that the loop can supply operating power to the device. Even if there is significant electrical resistance in the line, the current loop transmitter will maintain the proper current, up to its maximum voltage capability. The live-zero represented by 4 mA allows the receiving instrument to detect some failures of the loop, and also allows transmitter devices to be powered by the same current loop (called two-wire transmitters). Such instruments are used to measure pressure, temperature, flow, pH or other process variables. A current loop can also be used to control a valve positioner or other output actuator. An analog current loop can be converted to a voltage input with a precision resistor. Since input terminals of instruments may have one side of the current loop input tied to the chassis ground (earth), analog isolators may be required when connecting several devices in series.

Taking the point of view of the source of current for the loop, devices may be classified as active (supplying power) or passive (relying on loop power). For example, a chart recorder may provide loop power to a transmitter instrument such as a pressure transmitter. The pressure transmitter modulates the current on the loop to send the signal to the strip chart recorder, but does not in itself supply power to the loop and so is passive. Another loop may contain two passive chart recorders, a passive pressure transmitter, and a 24 V battery. (The battery is the active device). Panel mount displays and chart recorders are commonly termed 'indicator devices' or 'process monitors'. Several passive indicator devices may be connected in series, but a loop must have only one transmitter device and only one power source (active device).

The relationship between current value and process variable measurement is set by calibration, which assigns different ranges of engineering units to the span between 4 and 20 mA. Occasionally the mapping between engineering units and current was inverted, so that 4 mA represented the maximum and 20 mA the minimum.

[edit] Long circuits

Analog current loops were occasionally carried between buildings by dry pairs in telephone cables leased from the local telephone company. 4-20 mA loops were more common in the days of analog telephony. These circuits require end-to-end direct current (DC) continuity. DC continuity is not available over a microwave radio, optical fiber, or a multiplexed telephone circuit connection.

Basic DC theory reminds us that current is the same at all points in a circuit. It was common to see 4-20 mA circuits that had loop lengths in miles or circuits working over telephone cable pairs that were longer than ten thousand feet end-to-end. There are still legacy systems in place using this technology. In Bell System circuits, voltages up to 125V DC were employed.

Another link:

http://www.scribd.com/doc/2291415/A-look-at-4-20-Milliamp-Signals

Good luck!

The valve positioner has a dropping resistor installed across its input terminals. It may be on the front or back of the terminals themselves, but is more likely on the printed circuit board inside. If you don't have a schematic of the card internals, you will need to follow the circuit from the terminals through the card-edge connector and the tracing on the card to find the correct resistor.

The sensing circuitry in parallel with the dropping resistor is very high impedance, so its effect on the current loop is negligible. The value of the dropping resistor is all you need to determine voltage drop at any input current.