Loop Isolation Techniques
- Loop-powered-Isolators, Isolating transmitters and
- 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
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
- 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.
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.
- 2. Loop-powered isolation-converter
- Mescon Model Isotec-3
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
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
Disadvantages: requires higher operating voltage than a standard isolating 2-wire
- 3. Loop-isolation barrier
- Mescon Model 2-2-2
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.
- 4. Loop isolating transmitters and signal conditioners
- Mescon Models 20/3, 40/3, 60/3