Bronkhorst R Modules for Ex-proof applications - Manual (Page 15)

BRONKHORST HIGH-TECH B.V.
Control
circuit
Rk
XC
14 Vdc 13 Vdc
4 Cables
4.1 Introduction
An average two-wire cable has a capacity of 120 pF per meter and an inductivity of 0.67 µH/m.
For 100 m cable this results into:
C
cable
= 12 nF
L
cable
= 67 µH
4.2 The transmitter supply unit for flow sensors
If we take 100 m two-wire shielded 0,25 mm 2 than:
C
k
= 12 nF
L
k
= 67 µH
R
k
= 15.8 Ω (loopresistance)
L R
ratio
/ = 67/16 = 4,2 µH/Ω
For safety this is no problem.
At 20 mA the voltage loss over the cable is:
V = I x R = 0,02 x 15.8 = 0,3 V
4.3 The control board with XC coil
For the XC coil the capacity and inductivity is not important, but the voltage loss is the limiting factor.
The maximum voltage output of the controller is 14 Vdc.
For a two-wire shielded cable we don't want to allow more than 1 Volt loss.
So, on the coil V
max
will be 13 Volt.
for XC
I =
V
R
= =
13
65
200 mA
The cable resistance may be R = = =
0,2
1
I
V
5 Ω
A 0,25 mm 2 cable has a loop resistance of 1,6 Ω per 10 meter.
In other words: the maximum length is:
1,6
5
x 10 ≈ 30 m
Cable diameter max. length
0,25 mm 2 30 m
0,50 mm 2 60 m
0,75 mm 2 90 m
9.17.034 page 15