Loop Isolation Techniques

Loop-powered-Isolators, Isolating transmitters and Isolation barriers

By Moshe Weissberg – MESCON Technologies, Inc.

 

There have been many misconceptions relating to the differences between the various types of current-loop isolating techniques, methods and instruments. The purpose of this article is to clarify this confusion, to differentiate between the various instruments and emphasize the instances where they could be used.

In truth, except for one unique instrument (Mescon’s 2-2-2) none of the so-called, loop-isolators provides isolation to an existing non-isolated loop. What they serve is to extract, duplicate and/or convert a 4-20mA current loop signal, while providing galvanic isolation between the original signal and a newly generated one.

Mescon provides one of the world’s greatest varieties of loop isolating methods, including instruments, which are uniquely available from Mescon Technologies.

 

1. Loop-Powered-Isolator.

Mescon Model Isotec-2

When connected in a 4-20mA process loop, this unit will duplicate the input signal, providing an isolated and identical output. The Isotec-2 is used when a 4-20mA signal is to be transmitted to a device with common ground potential or is biased at unknown values and thus requires isolation from the loop. It DOES NOT (!) convert a non-isolated loop into an isolated one!

Advantages: the input/output are fully specified over a 0-50mA range. The technique is simple and precise and does not require periodic calibration. As such, zero and span pots are not even provided. Output signal accuracy is insensitive to load variations and provides superior temperature stability. The input requires a minimal voltage drop in addition to the output load. The units are low cost, do not require a power supply and are very simple to hook-up and operate.

Disadvantages: the input voltage drop depends on the output load, thus higher voltage may be required from the loop power supply. Provides limited drive capability, usually up to 600 Ohms.

 

2. Loop-powered isolation-converter

Mescon Model Isotec-3

While functionally similar to Model Isotec-2, the Isotec-3 is capable of converting the 4-20mA signal into standard zero-based, low impedance voltage outputs such as: 0-1V, 0-2V, 0-5V or 0-10V, which are proportional to the 4-20mA loop signal.

The voltage output is isolated from the current loop and can provide a suitable signal for many devices requiring zero based voltage inputs, such as computers, recorders and data acquisition systems.

Advantages: Very high accuracy yet low cost. NO POWER SUPPLY IS REQUIRED. The output is load independent hence calibration is easily performed in the field or in the lab. The Isotec-3 is Very easy to install and operate.

Disadvantages: requires moderate voltage drop from the input loop power supply thus may limit loop drive capability.

 

3. Loop-isolation barrier

Mescon Model 2-2-2

Basically, this unit connects between a current loop power supply and a non-isolated two wire transmitter. Using unique transformer isolating technique it provides a true isolation-barrier to the loop which, in effect, converts a non-isolated 2-wire transmitter into an isolated one(!). As such, the 2-2-2 supplies the power to the non-isolated transmitter.

Advantage: solves isolation problems, which no other method can. This is the only true loop isolator. Very simple to hook up and operate. Enables economic field installation while maintaining isolation. Simple and low-cost. Unique. Provides additional design flexibility.

Disadvantages: requires higher operating voltage than a standard isolating 2-wire transmitter.

 

4. Loop isolating transmitters and signal conditioners

Mescon Models 20/3, 40/3, 60/3

This family of isolating two wire transmitters and signal conditioners serve to duplicate and/or convert a current loop signal into a current or a voltage output while providing input/output isolation (Model 20/3), or triple (input/output/power) isolation (Models 40/3 and 60/3). They provide the most functionally flexible arrangement for signal duplication, conversion and isolation.

Advantages: extremely low current loop voltage drop enables multiple loads on input current signal. High accuracy output signal is independent of input and output loads. Easy to change range and calibrate.

Disadvantages: because of the more complex nature of the circuitry these units usually cost more than other loop isolating devices. Also, they need a power supply to operate.