Intro4u2u

The Four-to-Twenty Milliamp Circuit is the most common way to transfer an instrumentation signal from one device to another. 4 milliamps would usually translate to a zero value and 20 milliamps would be some full-scale value. For example, if we want to keep track of the level of water in a 16′ storage tank, 4 mA would mean the tank was empty, 20 mA would be full, and each milliamp in between would translate to one foot 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 and send it to a gauge or a chart recorder in a control room.

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. Typically, 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. We might have an indicating gauge, a chart recorder and a telemetry device all reading the same signal by looping them in series as shown in the diagram below. 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. The reason we don’t simply transmit the 1-5 volt signal is because some of this signal would be lost over distances due to line resistance and therefore the signal would be inaccurate. However, a characteristic of an isolated electrical circuit is that it’s amperage will be constant at every point along the circuit so therefore amperage 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.