Time Electronics 1077 Milliamp Transducer Simulator - Manual (Page 8)

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1077 Technical Manual P a g e | 8
4.
Operation
4.1. Principle of Operation of the 2 Wire 4/20mA System
To fully understand the operation of the 1077 a working knowledge of the 4/20mA system is
required. A short explanation follows:
The basic requirements 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 or voltage drop 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 by a 2-wire line. The
transducer takes a current from the line proportional to the stimulus (pressure, temperature,
etc.). By measuring the current flowing in the line the output of the transducer can be
monitored.
As the signal is transmitted as a current, lead resistance and voltage drip do not affect the
accuracy. Hence the basic requirements are fulfilled using just two wires - the minimum
possible.
To enable a certain amount of standardisation and to compensate for voltage drop and
variation in the supply, transducers are designed to operate over a wide voltage range
(usually between 15-30 volts) and to take a 'Zero' current of 4mA and a 'full scale' current of
20mA.
The simplest 4/20mA current loop system is of 3 parts, although often 2 parts are
incorporated in the same box. The 3 main parts are:
1. D.C. Power supply usually 24V.
2. 4-20mA current meter/recorder
3. The transducer.
The DC 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 injected.
The transducer completes the loop and allows a current to flow round the loop dependent
on the level of stimulus to the transducer, it can almost be considered as a potentiometer
changing in resistance to let between 4 and 20mA flow.
The 1077 can be used to calibrate/test any part of this system, by using the 1077 in its 3
operating modes.