NIVELCO MicroTREK - Page 27

BKI16ATEX0018X ♦ htk4014a0600p_03 ♦ 27 / 76
Power supply
Nominal voltage 24 V DC
Maximum voltage (U
input
): 35 V DC
Minimum voltage (U
input
): dependent on load impedance, see graph below
Load impedance R
A
Loop resistance, R
loop
R
HART
+ R
cable
+ R
ammeter
Ohm
Minimum load impedance R
A
0
Ohm
Maximum load impedance R
A
750
Ohm
Non-hazardous-duty version
=
U
S
R
A
HART
4 ... 20mA
U
E
MikroTREK 2-wire
mA
L
R
HART
resistance for HART® communication 250 Ohm, recommended
loop current, mA
voltage, V
0246810121416182022
8
9
10
11
12
13
14
15
16
17
18
19
20
voltage drop
over 250
resistance at
4 mA
Ω
minimum input voltage V
(measured at MicroTREK terminals)
inp ut
voltage drop
over 250
resistance at
20 mA
Ω
minimum y ,V , , for a loop
resistance, R of (measured at
the terminals of the power supply)
suppl voltage
supply
loo p
250 Ω
Line A = minimum voltage at the MicroTREK 2-wire terminals
Line B = minimum supply voltage (for voltage drop caused by a 250 Ohm loop resistance)
Example for calculating the power supply: The voltage drop is tested at 22 mA: U
power minimum 22
= 22 mA x load impedance + U
input minimum 22
U
power minimum 22
= 22 mA x 250 Ohm + 10 V = 5.5 V + 10 V = 15.5 V
In order to cover the whole current range, the voltage drop must also be tested at 4 mA:
By analogy, the following applies: U
power minimum 4
= 4 mA x load impedance + U
input minimum 4
U
power minimum 4
= 4 mA x 250 Ohm + 18 V = 1 V + 18 V = 19 V
At a load impedance of 250 Ohm a power supply voltage of 19 V is sufficient to energize the current device range of 4 to 20 mA.