Installation
Model 2-Wire
39
5
You can use this current to operate a 4 to 20 mA indicator, recorder, controller or data logger – or a combination
of devices in the series. The following figure illustrates a simple system consisting of the infrared sensor, a digital
meter and a power supply. These components form a continuous current loop.
Figure 5-4: Principle Circuit Diagram: Infrared Sensor with Multiple Loads
The infrared sensor operates at any supply voltage between 12 and 24 V direct current. For indicators, recorders,
and other load elements, pay strict attention to the load resistance and, of course, the zero scale and full scale
currents. Part of the power supply voltage is dropped across the load and is not available for the infrared sensor.
In the following figure, a controller and an indicator are connected in a series in the loop. The 4-20 mA current
determined by the infrared sensor flows through these load elements, producing voltage drops proportional to the
resistance of each load element. The total load voltage is the sum of these voltage drops plus the drop across the
connecting wires.
Figure 5-5: Equivalent Circuit Diagram: Infrared Sensor with Multiple Loads
Assume the resistances are as follows:
𝑅
𝑊𝑖𝑟𝑒𝑠
= 3Ω
𝑅
𝐶𝑜𝑛𝑡𝑟𝑜𝑙𝑙𝑒𝑟
= 90Ω
𝑅
𝐼𝑛𝑑𝑖𝑐𝑎𝑡𝑜𝑟
= 7Ω
This adds up a total load resistance of:
𝑅
𝐿𝑜𝑎𝑑
= 𝑅
𝑊𝑖𝑟𝑒𝑠
+ 𝑅
𝐶𝑜𝑛𝑡𝑟𝑜𝑙𝑙𝑒𝑟
+ 𝑅
𝐼𝑛𝑑𝑖𝑐𝑎𝑡𝑜𝑟
= 3Ω + 90Ω + 7Ω = 100Ω
With a total load voltage at 20 mA maximum current:
𝑈
𝐿𝑜𝑎𝑑
= 𝑅
𝐿𝑜𝑎𝑑
× 𝐼
𝐿𝑜𝑜𝑝
= 100Ω × 0.02A = 2V
Loop
Current
Controller
Indicator
Infrared Sensor
Power Supply
R
Controller
Infrared Sensor
U
Sensor
min. 12 V
max. 24 V
R
Indicator
U
Supply
+
I
Loop
R
Wires
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