Instruction Manual

Instruction Manual
D103557X012
Wiring Practices
January 2015
10
Figure 2‐1. Determining Voltage Available at the Instrument
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.
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 2.
LINE CONDITIONER
OR HART FILTER
(if used)
CONTROL
SYSTEM
+
-
COMPLIANCE VOLTAGE
VOLTAGE
AVAILABLE AT THE
INSTRUMENT
+
-
R
INTRINSIC SAFETY
BARRIER
(if used)
Logic solver or control system compliance voltage
= Voltage available at the instrument
– Line conditioner or filter voltage drop (if used)
Example Calculation
18.5 volts (at 21.05 mA for Honeywell TDC2000)
– 2 volts
– 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)
= 15.49 volts, available—if safety barrier (2.55 volts)
is not used
1
2
NOTES:
1
2
Calculate Voltage Available at the Instrument as follows:
Compliance Voltage
If the compliance voltage of the logic solver or control system is not known, perform the following compliance voltage
test.
1. Disconnect the field wiring from the system and connect equipment as shown in figure 2‐2 to the system terminals.
Figure 2‐2. Voltage Test Schematic
CIRCUIT
UNDER
TEST
VOLTMETER
MILLIAMMETER
1
kW POTENTIOMETER
A6192‐1