MKS Baratron 740C, Manual, Page: 40 / 84

Setup Chapter Two: Installation
30
Electrical Information for a Current Unit
The 7xxC Series transducers with 4 to 20 mA output require an external power source capable of
supplying the voltages listed in
Appendix A: Product Specifications, page 65. Figure 8 shows the
electrical information for a 4 to 20 mA unit. 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
R
e
si
st
a
n
ce
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
pot to adjust the 4 mA
setting. Refer to How To Check the Transducer Zero, page 41, to adjust the zero. The
SPAN
pot
adjusts the 20 mA setting. Refer to Span Adjustment, page 44, for more information.
Ordinary, unshielded twisted pair transmission wire can be used for all connections.