Operation Of The 4-20Ma S Ys Tem - Time Electronics 1044 Manuel Technique

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4. Operation of the 4-20mA S ys tem

Introduction
A common use of the 1044 would be to simulate a transducer or measure the current flow in
a transducer loop. The following brief explanation, which is intended as a guideline only,
may help when operating the 1044 in a 4-20mA system.
The 4-20mA System
The basic requirement for any transducer with remote display is:
A) To use as few wires as possible.
B) That errors are not introduced into the signal from the transducer by the effects of
interference due to lead resistance.
C) To supply power to the transducer to enable amplifiers to be built into the transducer
when the signal produced by the sensing element is too small to be transmitted without
amplification.
In the 4-20mA system the transducer is supplied with D.C. power from the 2 wire line. The
transducer takes the current from the line proportional to the stimulus (i.e. pressure,
temperature, etc.). By measuring the current flowing in the line, the output of the transducer
can be monitored.
Since the signal is transmitted as a current, lead resistance and voltage drop do not affect
the accuracy. The basic requirements are therefore fulfilled using just two wires - the
minimum possible.
To enable a certain amount of standardisation, and to compensate for voltage drop or
variation in the supply, transducers are designed to operate over a wide voltage range,
usually between 15 and 30 volts, and to take a 'zero' current of 4mA and a full scale current
of 20mA.
The simplest 4-20mA current loop system consists of 3 parts, although often 2 parts are
incorporated into the same unit. The 3 main parts are -
1) D.C. Power supply usually 24V.
2) 4 - 20mA current meter/recorder.
3) The transducer.
The D.C. power supply generates the drive power for the system and can be considered as
a battery.
The 4-20mA meter/recorder can be a passive moving coil mA meter with a zero offset. To
obtain a reading, current must be applied.
The transducer completes the loop and allows a current flow around the loop dependent on
the level of stimulus to the transducer. It can be considered as a potentiometer changing in
resistance to let between 4 and 20 mA to flow.
The 1044 can be used to check this system in either source or measure modes of operation
and has a 24V compliance voltage when set to current source mode that will power a loop.
V1.1 09/01/11 P a g e | 9 1044 Technical Manual