2
3. Theory of Operation
A 4-20 mA loop is a series loop in which a transmitter will vary the
current flow depending on the input to the transmitter. In the HX71
Series the amount of current allowed to flow in the loop will vary
depending on the relative humidity or temperature being measured
by the sensor(s). Some advantages of a current output over a voltage
output is that the signal measured is less susceptible to electrical
noise interference and the loop can support more than one measuring
instrument as long as the maximum loop resistance is not exceeded.
A typical application utilizing a current loop will normally consist of
a power supply, the transmitter and a meter, recorder or controller
to measure the current flow. The loop resistance in the sum of the
measuring instruments and wire used. The maximum allowable loop
resistance for the HX71 to function properly is found by using the
following formula:
Rmax = (power supply voltage – 8 volts) ÷ 0.02 amps
EXAMPLE: (When using a 10 Vdc power supply).
Rmax = (10 – 8) ÷ 0.02 amps = 800 ohms max loop
resistance
Figure 1 - Basic Transmitter Set-up With Current Loop Output
3
Theory of Operation
HX71 CABLE
HX71 CABLE
RED WIRE
RED WIRE
SHIELD WIRE
SHIELD WIRE
BLACK WIRE
BLACK WIRE
PANEL METER
PANEL METER
POWER SUPPLY
POWER SUPPLY
+
10V
24V
–
50%
50%
+
–
+
–
+
–
WHITE WIRE
