Spirax Sarco RIM20, Инструкция по установке и обслуживанию, Страница: 37 / 132

IM-P198-05 MI Issue 2 37
3.21 4-20 mA output connections
The standard RIM20 flowmeter has a single 4-20 mA loop. Two additional loops are available
on the optional communication board. The 4-20 mA loop current is controlled by the meter
electronics. The electronics must be wired in series with the sense resistor or current meter.
The current control electronics require 12 volts at the input terminals to operate correctly.
The maximum loop resistance (load) for the current loop output is dependent upon the supply
voltage and is given in Figure 26. The 4-20 mA loop is optically isolated from the flowmeter
electronics.
R
load
is the total resistance in the loop, including the wiring resistance (R
load
= R
wire
+ R
sense
).
To calculate R
max
, the maximum R
load
for the loop, subtract the minimum terminal voltage from
the supply voltage and divide by the maximum loop current, 20 mA. Thus:
The maximum resistance R
load
= R
max
= (V
supply
- 12V) / 0.020 A
R
load
(ohmns)
V
Supply
(volts)
Operating
range
Fig. 27 Load resistance versus Input voltage
V
Supply
(volts)
R
load
(o
hm
ns
)
+V
4-20 mA +
4-20 mA -
4-20 mA voltage = +V
For Hart communications signal
loop must have a minimum of
250 ohms load resistance R
L
mA meter
ac and dc powered flowmeters
Fig. 28 Isolated 4 - 20 mA output with external power supply
For HART ® communications the
signal loop must have a minimum
of 250 ohms load resistance RL .
4-20 mA +
4-20 mA -
mA meter
4-20 mA voltage = +V