MTL MTL5021 - MTL5081 Millivolt Isolator

A switch located on top of the module enables or disables a safety drive
in the event of thermocouple burnout or cable breakage; a second
switch permits the selection of upscale or downscale drive as required.
6.28.1 Safety drive
Please note that the safety drive on the MTL5081 responds to line
breakage (an open circuit) or in the event of thermocouple burnout. It
does not provide detection of a short circuit. It can, however, be set
upscale or downscale. Selection is made by switches located on top of
the module (see figure 6.64)
* Note that the above statement Vout = Vin is subject to transfer accuracy
and drift specified in the MTL Intrinsic Safety catalogue.
6.28.2 Wiring connections
See figure 6.65 for wiring connections.
27
INM5000-6 Jul 2010
6.27.2 Testing
Default parameters for the MTL5074, unless custom configured by MTL,
are:
Input type 3-wire RTD
Linearisation enabled
Units °C
Cj compensation disabled
Damping value 0 sec
Smoothing value 0 sec
Output zero 0°C
Output span 250°C
Tag and description fields clear
Open circuit alarm high
Transmitter failure alarm low
Cj failure alarm low
Line frequency 50Hz
Note: the configuration of MTL5074 cannot be changed without the aid
of a PCS/PCL45 configurator (see INM073A Connection and
Configuration Manual for full details). These devices also enable more
comprehensive testing of the converters.
Make the safe- and hazardous-area connections shown in figure 6.62.
Note that the range of the dc ammeter 'A' must be suitable for 4 to
20mA currents and the variable resistor 'R' should have a range of 0-
200Ω and, ideally, should be a decade resistance box. Switch on the
power supply and check that the green power LED comes on (if it flashes,
the unit is incorrectly configured or the test wiring setup is faulty). Check
that the loop output current at 'A' varies as the variable resistance at 'R'
is altered and check the table below for particular settings (194Ω
@250°C and 100Ω @0°C for a PT100 sensor).
6.28 MTL5081 millivolt isolator
The MTL5081 takes a low level dc signal from a voltage source in a
hazardous area, isolates it and passes it to a receiving instrument
located in the safe area. Although used with thermocouples, this unit
does not have cold junction compensation facility. It is intended for use
with thermocouples utilising external cold junction compensation. When
using with thermocouples, in order to minim ise the temperature gradient
between the safe- and hazardous-area terminals, it is recommended that
the units are mounted on vertical DIN rail (see figure 6.63).
Input Loop output Power
resistance current LED
(R) (A) (green)
194Ω 20mA On
100Ω 4mA On
Open circuit 22.25mA Flashing
Short circuit 22.25mA Flashing
7
8
9
10
11
12
13
14
6
5
4
3
2
1
mV
I
20 to 35V dc
Note: The MTL5073 (which is no longer manufactured)
had an internal link between terminals 11 and 13.
Vs
Vs+

+
Loop output
4 to 20mA
A
+

VR1
200W lin
Figure 6.62: Test circuit for MTL5074
Figure 6.63: MTL5000 Series units mounted on vertical DIN rail
ON
MTL5081
PWR
LF
PR
CH1
ON
OFF
+
SAFETY
DRIVE
SAFETY DRIVE LINE
ON/OFF +/- BREAKAGE Vout
OFF N/A NO Vin*
OFF N/A YES undetermined
ON + NO Vin*
ON + YES >+100mV
ON – NO Vin*
ON – YES <–100mV
Figure 6.64: Top label of MTL5081 showing positions of
safety drive switches and an explanation of their functions
Terminal Function
1 THC/mV input –ve
2 THC/mV input +ve
11 Output –ve
12 Output +ve
13 Supply –ve
14 Supply +ve
7
8
9
6
5
4
3
2
1
13
14
10
11
12
mV
mV
Vs+
Output
Vs
20 to 35V dc
mV

+

+
Figure 6.65: MTL5081wiring diagram and connections
Hazardous area
Safe area
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