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The 4-20 milliamp (mA) circuit is the most common way to transfer an instrumentation signal from one device to another. 4 mA usually translates to a zero value and 20 milliamps indicates some full-scale value. For example, if we want to keep track of the level of water in a 16-meter storage tank, 4 mA would mean the tank was empty, 20 mA would be full, and each milliamp in between would translate to one meter of water. A transmitter at the tank would sense the level by pressure or ultrasonics and translate that into the 4-20 mA instrumentation signal. It may then send the analog signal to a gauge, chart recorder, PLC or other data-acquisition hardware.

The circuit is commonly powered by a 24 volt DC supply. When this voltage is used, up to 4 devices may be connected to a single loop to transmit and read the common signal (each requiring roughly 5 VDC). Normally, the transmitter will receive its operating power from the loop and may require no other electrical connection. Other devices usually receive power from another source. There may be an indicating gauge, a chart recorder and a telemetry device all reading the same signal by looping them in series. Often, the receiving devices will have a precision 250 ohm resistor across their input terminals either externally or internal to the unit. When the 4-20 mA loop is connected across this resistor, the 4-20 milliamps is translated to 1-5 volts DC and this voltage signal is read by the instrument. Transmitting the 1-5 volt signal not ideal because some of this signal will be lost over distances (due to line resistance) and hence the signal would be inaccurate. But, a characteristic of an isolated electrical circuit is that its current will be constant at every point along the circuit; therefore, current makes a better carrier for precision signals.



The power supply may be connected at any point in the loop. Its positive terminal will connect to a positive terminal of another device and its negative terminal connects to a negative terminal of still another device in the loop. All other connections in the loop are positive to negative. Shielded cables are used to guard the low-level signals against outside interference. Shields of each cable run are grounded at one end only, preferably at a common point. This is to prevent current from traveling down the shield and causing interference. Ungrounded ends should be insulated to prevent accidental grounding or shorting.

You will often find devices, such as sensors and transducers, coupled to a serial I/O module, like this one from Measurement Computing:


above: Single Channel Voltage or 4-20 mA Input Module

Here are its feature:

* Voltage or current inputs
* 16-bit conversions
* 1 digital input
* 2 digital outputs

Specifications:

Input type: mV, V , or mA
Channels: 1
Input range: ±150mV, ±500mV, ±1V, ±5V, ± 10V and ±20mA
Current range: ±20mA
Sampling rate: 10 samples/sec
Bandwidth: 4 Hz
Accuracy: ±0.05% or better
Zero drift: ±0.033 ppm/°C
Span drift: 25ppm/°C
CMR @ 50/60 Hz: 150 dB
NMR @50/60 Hz: 100 dB
Over voltage protection: ±10V

General Specifications:

Display: LED: 4½ digit(I-7011D)
Power consumption: 1.5W

Digital Input: Channels: 1, Logic 0: 0 to 1V, Logic 1: 3.5V to 30V

Digital Output: Channels: 2 Open collector to 30V, 30mA load max.

Notice that this module is will accept both digital and analog inputs. It will output in digital only. Also notice that this module accepts both voltage or current inputs.