Brochure/Catalogue

3-port Isolation
• 3-port isolation is the most universal form of signal
isolator
• An optical coupler or transformer isolates the input
from the output circuit. Together with the clearance
and creepage distances it serves to dene the isolation
level. For example, the input signal is converted by
means of pulse-width modulation into a frequency
signal and demodulated again on the output side to
form an analogue value. An amplier then generates
a standardised analogue signal. A galvanic isolated
DC/DC converter feeds the input and output circuit
with a potential free supply voltage. It also determines
the isolation level through its data, air and creepage
distances. In the case of these three isolation paths (input/
output, input/auxiliary voltage, output/auxiliary voltage)
one refers to 3-port isolation.
Temperature Signal Measuring Method
• Measurement using resistors (RTD)
When measuring with temperature-dependent resistors a
current of approx. 1.5 mA is passed through the resistor
from a constant current source in the signal converter.
An operational amplier is used to measure the potential
drop at the resistor (2-wire circuit).
In order to take account of lead length, the voltage drop
is measured at the return conductor and calculated with
double the value (3-wire circuit). This simulates the wire
resistances from the feed and return lines.
Accurate measurements are achieved by separately
measuring the voltage drop at the feed and return lines
(4-wire circuit). The values for the supply lines are
calculated against the measured value.
Temperature Signal Measuring Method
• Measurements using thermocouples
When conducting measurements using thermocouples
the voltage that is generated when two differently alloyed
metals come into contact with each other is measured.
A differential amplier is then used to recondition
the signal. The easiest (and the most cost-efffective)
method of subsequent processing is conducted by
means of an amplier circuit, which converts these
signals into standard signals. High-end components
process the measuring signal using a microprocessor,
which simultaneously reconditions the signal (ltering,
linearisation)
Cold Junction Compensation For Thermocouples
• Recording temperatures by using thermocouples
encounters the problem of a thermal voltage forming at
the clamping terminals on the signal converter on account
of the different materials in the conductors and bus bar.
This voltage then counteracts the thermal element‘s
voltage.
In order to compensate for the error to the measured
value which arises here, the temperature is measured at
the clamping terminal. The microprocessor in the signal
converter reads the value measured there and calculates
it against the measured value. This procedure is known as
cold junction compensation.
Voltage at the measuring point (V
meas
)
+ Voltage at the terminal (V
terminal
)
= Voltage at the thermocouple (Vt
hermo
)
=> Temperature at the thermocouple (T
thermo
)
Linearisation
•
Temperature-dependent components do not normally
have linear characteristic curves. To ensure that further
processing can take place with the necessary accuracy,
these characteristic curves have to be linearised to some
extent. The graph showing measurements of thermocouples,
in particular, reveals signicant deviations at some points
from the “ideal graph”. As a consequence, the signal which
has been measured is worked up by microprocessor.
Technical data
W
Technical appendix/Glossary
W.72729490000