6.7.5 MTL4581 - mV/Thermocouple Isolator
Single channel, mV/THC input for low power signals
The MTL4581 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. The module
is intended for use with thermocouples utilising external cold-junction compensation.
A switch enables or disables the safety drive in the event of thermocouple burnout or a
cable breakage; a second switch permits the selection of upscale or downscale operation
as the application requires.
Please note that the safety drive on the MTL4581 responds to a line breakage (i.e. an open
circuit) or a thermocouple burnout. It does not provide detection of a short circuit. It can
however, when chosen, be set to drive the output either upscale or downscale. These
options are selected using the switches located on the side of the module.
Safety drive switches
Line
breakage
V
out
value
Sw2
Safety drive
Sw1
Drive direction
OFF N/A NO V
in
*
OFF N/A YES undetermined
ON + NO V
in
*
ON + YES > +50mV
ON – NO V
in
*
ON – YES < –50mV
* Within V
in
/V
out
transfer accuracy and drift error as specified in the product datasheet.
Hazardous area Safe area
Output*
Vs–
Vs+
20 to 35V dc
6
5
4
3
2
1
mV
mV
9
8
7
12
11
10
13
14
mV
* Use compensating
cable with
thermocouple inputs
Terminal Function
1 THC/mV input –ve
2 THC/mV input +ve
8 Output –ve
9 Output +ve
11 Output –ve
13 Supply –ve
14 Supply +ve
Figure 6.38:
Top label for
MTL4581
OFF position
ON position
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