Instruction Manual
D103785X012
Wiring Practices
June 2017
10
As shown in figure 2‐2, the voltage available at the instrument depends upon:
D
the control system compliance voltage
D
if a filter, wireless THUM adapter, or intrinsic safety barrier is used, and
D
the wire type and length.
The control system compliance voltage is the maximum voltage at the control system output terminals at which the
control system can produce maximum loop current.
The voltage available at the instrument may be calculated from the following equation:
Voltage Available = [Control System Compliance Voltage (at maximum current)] - [filter voltage drop (if a HART filter is
used)] - [total cable resistance
maximum current] - [barrier resistance x maximum current].
The calculated voltage available should be greater than or equal to 10 volts DC.
Table 2‐1 lists the resistance of some typical cables.
The following example shows how to calculate the voltage available for a Honeywell
t
TDC2000 control system with a
HF340 HART filter, and 1000 feet of Belden
t
9501 cable:
Voltage available = [18.5 volts (at 21.05 mA)] - [2.3 volts] - [48 ohms
0.02105 amps]
Voltage available = [18.5] - [2.3] - [1.01]
Voltage available = 15.19 volts
Figure 2‐2. Determining Voltage Available at the Instrument
3 The voltage available at the instrument is not the voltage measured at the instrument terminals. Once the
instrument is connected, the instrument limits the measured voltage to approximately 8.0 to 9.5 volts.
1 Obtain filter voltage drop. The measured drop will be different than this value. The measured filter voltage drop
depends upon control system output voltage, the intrinsic safety barrier (if used), and the instrument. See note 3.
HART FILTER
(if used)
CONTROL
SYSTEM
+
-
COMPLIANCE VOLTAGE
VOLTAGE
AVAILABLE AT THE
INSTRUMENT
+
-
R
INTRINSIC SAFETY
BARRIER
(if used)
Control system compliance voltage
= Voltage available at the instrument 3
– Filter voltage drop (if used) 1
Example Calculation
18.5 volts (at 21.05 mA)
– 2.3 volts (for HF300 filter)
– Intrinsic safety barrier resistance (if used) x maximum loop current
– 2.55 volts (121 ohms x 0.02105 amps)
TOTAL LOOP
CABLE RESISTANCE
– Total loop cable resistance x maximum loop current
– 1.01 volts (48 ohms x 0.02105 amps for
1000 feet of Belden 9501 cable)
= 15.19 volts, available—if safety barrier (2.55 volts)
is not used
NOTES:
Calculate Voltage Available at the Instrument as follows:
THUM ADAPTER
(IF USED)
– Smart Wireless THUM adapter voltage drop (if used) 2
2 The voltage drop of the THUM adapter is linear from 2.25 volts at 3.5 mA to 1.2 volts at 25 mA.
