MKS Micro-Baratron 870B, Instruction Manual, Page: 41 / 68

Chapter Two: Installation Electrical Information
31
Current Unit
The 870/872 transducer with the 4 to 20 mA output requires an external power source capable of
supplying the voltages listed in Table 5, page 22. Refer to Figure 8, page 32, for the electrical
scheme for a current (4 to 20 mA) unit. Ordinary unshielded twisted pair transmission wire can
be used for all connections.
The total external line resistance, including the sampling resistor, must not exceed the value R
LN
determined by:
R
LN
(ohms) =
(Excitation Supply Volts - 13) x 10 3
24
Figure 6: Line Resistance Equation
For example, R
LN
should not exceed 450 ohms for a 24 Volt supply.
0
200
400
600
800
1000
1200
Voltage
Resistance
13 18 23 28 33
Total Resistance in Line
Figure 7: Total Resistance in Line
The maximum voltage between ground (sensor case) and the (-) terminal must not exceed
36 Volts for mA units. The maximum voltage between the positive (+) terminal and the (-)
terminal must not exceed 32 Volts under any conditions.
Use the
ZERO
potentiometer to adjust the 4 mA setting (refer to How To Adjust the Zero ,
page 43). The
ZERO
pot is located on the side of the 870/872 unit (refer to Figure 1, page 25, and
Figure 3, page 27).