Enviro EA800 - Manual (Page 16)

12 D-011-0152
Sensors
Power Supply / Sensor Voltage Selection
In order to determine the power supply voltage necessary to ensure correct full-scale operation, it is necessary to
identify all voltage drops within the current loop.
Figures 6 and 7 show an EA800 drop of 4.0 VDC. This value accounts for the voltage drop generated by
connecting the 4-20mA sensor to the EA800 using 1000 feet of 22 AWG wire.
Now it is only necessary to identify the operating voltage range of the 4-20mA sensor, more importantly the
minimum operating voltage. Once identified, the minimum operating voltage of the sensor can be added to the
4.0V voltage drop of the EA800 (+ wire) to determine the power supply voltage necessary to ensure correct
full-scale operation.
Example:
■ Assume that a 4-20mA sensor whose operating input voltage range is 10 – 30V has been selected for use.
Therefore the minimum operating voltage of the sensor is 10V (V
sensor
). When the 4.0V drop of the EA800
(and wire) is added, it can be determined that at least 14VDC is needed to power the loop.
■ If the EA800 is connected to a 12VDC power supply, use of the sensor in this example requires an external
power supply of at least 14V as shown in Figure 7.
It is important to not exceed the maximum operating voltage of the 4-20mA sensor, as specified within the sensor’s
product specification.
The following formula provides the basis for the selection matrix shown in Table 7. Please use Table 7 to select
either the proper sensor rating to be used with a known power supply, or select a power supply for a known sensor
rating.
R
L
< (V
cc
– x) .023
where:
■ R
L
= Loop resistance of 200
■ V
cc
= Power supplied to EA800 or Aux Power Out
■ X = Sensor voltage (max)
Table 7 4-20mA Voltage Select Matrix
Power Supplied to EA800
or AUX. Power Out
V
sensor
(max.) [Maximum
sensor voltage rating]
Power Supplied to EA800 or
AUX. Power Out
V
sensor
(max.) [Maximum
sensor voltage rating]
11 VDC 7 VDC 19 VDC 15 VDC
12 VDC 8 VDC 20 VDC 16 VDC
13 VDC 9 VDC 21 VDC 17 VDC
14 VDC 10 VDC 22 VDC 18 VDC
15 VDC 11 VDC 23 VDC 19 VDC
16 VDC 12 VDC 24 VDC 20 VDC
17 VDC 13 VDC 25 VDC 21 VDC
18 VDC 14 VDC 26 VDC 22 VDC