INM 5500 Rev 11
55
Please note that the safety drive on the MTL5581 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.
6 .7 .5
MTL5581 - mV/Thermocouple Isolator
Single channel, mV/THC input for low power signals
The MTL5581 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.
Hazardous area
Safe area
PWR
MTL5581
Terminal
Function
1
THC/mV input –ve
2
THC/mV input +ve
11
Output –ve
12
ve
13
Supply –ve
14
ve
OFF position
ON position
Sw 1
Sw 2
Safety drive switches
Sw1
OFF
ON
Sw2
OFF
ON
Drive direction
‘+’ Upscale ‘–’ Downscale
Safety drive
OFF
ON
–
Vs–
Vs+
20 to 35V dc
+
Output*
+
–
* Use compensating
cable with
thermocouple inputs
6
5
4
3
2
1
7
8
9
10
11
12
13
14
mV
mV
mV
Figure 6 .31:
Top label
for MTL5581
