4-20 mA Current Loops Made Easy

In the context of analog sensor data transmission, the 4-20 mA current loop has emerged as a prevalent method for conveying the sensor data that has been acquired. The design of sensors or transducers typically encompasses the measurement of a range of values pertaining to the measured parameter, which is designated as the measurand.

It is imperative to convert the measurand value to a current within the measuring device, thereby ensuring that the current in the loop is proportional to the measurand value. The loop current ranges from 4 mA to 20 mA, and we call this range the span of the transmitter. When configuring the transmitter, we set one end - point of the measurement value to correspond to 4 mA, and the other measured end - point value to correspond to 20 mA.

The 4-20 mA current loop is the standard for signal transmission and electronic control in most analog control systems, and it's a marvel of modern engineering! In a current loop, a DC loop power supply supplies current. The current flows through the transmitter via the field wiring that connects to a loop load resistor in the receiver or controller, and then returns to the loop supply. All the elements form a series circuit! All current-loop-based measuring systems use at least these four elements.

2-Wire Current Loop Transmitters

Engineers designed 2 - wire loop - powered transmitters as amazing electronic devices. You can connect them in a current loop without using a separate or independent power source. Designers ingeniously made them draw their power from the current flowing in the loop. This versatility enables their use in a wide range of applications, including sensors, transducers, and other field instruments. These devices are particularly useful in systems where supplying separate power to all components can be challenging. Engineers can place these devices in an enclosure where access might be tricky or even in a hazardous location, making loop - powered devices the perfect solution.

Figure1: 2-Wire Current Loop Transmitters

Take a look at Figure 1, which shows a 2-wire loop-powered device connected to a current loop. This remarkable device functions as a current-sinking component within the loop circuit, making it a game-changer for efficiency and safety. The power to drive the device is supplied entirely by the unused current below 4 mA in the loop. 2-wire loop-powered transmitters are popular, but 3-wire is usually more costly.

3-Wire Current Transmitters

3-wire transmitters are a game-changer when it comes to loop-powered transmitters. Their loop current is developed from a DC power supply, which supplies more current than just the loop current. The whole transmitter runs on this power supply, which can be a lot more than what typical 2-wire loop-powered devices need.

But here's the best part: a 3-wire system is a sourcing element, meaning it supplies the current loop even though it uses its own power.  3-wire transmitters often come with a lower price tag than 2-wire options, making them an attractive choice for many users. Take a look at Figure 2 below for a clear picture of a typical 3-wire loop. It's essential to remember that a 3-wire transmitter should never be connected to a 2-wire loop system.

Figure2: 3-Wire Current Transmitters

Engineers don't directly connect the high side of the power supply to the loop. Instead, they connect the return side of the power supply via a grounded point. This means that engineers need to carefully consider grounding issues for a 3-wire transmitter to prevent potential ground loops. If engineers use a 3-wire transmitter in an application and need to separate the signal from the power supply, they can do one of several things.

One option is using a separate DC power supply for each 3 - wire loop output device to avoid interaction with other loops. Another is using a loop isolator module with methods like transformers or optical couplers for galvanic isolation. These devices can accept a 4 - 20 mA signal, act as a repeater or retransmitter, and deliver an isolated 4 - 20 mA signal. Another option is for engineers to use a 4-wire transmitter with built-in isolation.

4-Wire Current Transmitters

4-wire transmitters offer the current sourcing advantages of 3-wire devices and also provide galvanic isolation for the current loop output. However, 4-wire devices cost much more than 3-wire devices. Because of this, engineers generally use them where isolation is needed, or they are part of a combination device with an approved safety barrier for current loop operation in a specific hazardous location. Figure 3 below shows the block diagram of a 4 - wire transmitter. One important thing to note is that the 4-wire device itself uses a separate DC power supply for operation, similar to a 3-wire transmitter. It supplies loop current this way.

Figure3: 4-Wire Current Transmitters

Pros & Cons of 4-20 mA Loops

Working in an industry that requires process control is an exciting challenge. It's thrilling to determine if the pros outweigh the cons. The right decision can save both time and money.

Pros:

• The 4-20 mA current loop is the dominant data transmission standard in many industries.
• Connecting and configuring it is recognized as the simplest analog data transmission method.
• It uses less wiring and connections than other methods, greatly reducing startup/setup costs.
• It is superior over long distances of field wiring, as current won't diminish, unlike voltage does.
• Most electrical noise and related EMI (electromagnetic interference) have relatively little impact on it.
• It permits a local or a remote readout or monitoring device to be inserted in series with the loop.
• It is self-diagnostic of faults in the measuring system because 4 mA is equal to 0% system output,   so any loop current substantially lower than 4 mA becomes an immediate indicator of loop fault.

Cons:

• 4-20 mA current loops can only transmit one specific sensor or process signal per loop.
• Isolation requirements become exponentially more complicated with a larger number of loops.
• Multiple loops are required for applications where there are many sensor or process output that must be transmitted. A lot of field wiring will be needed, which can lead to serious issues  with ground loops if the independent current loops are not properly isolated from each other.

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