Voltage Available - Fisher FIELDVUE DVC6200 Instruction Manual

DVC6200 Digital Valve Controller
COMPLIANCE VOLTAGE
CONTROL
SYSTEM
+
−
2
Calculate Voltage Available at the Instrument as
follows:
Control system compliance voltage
– Filter voltage drop (if used)
– Intrinsic safety barrier resistance (if used) x maximum loop current
– Smart Wireless THUMt adapter voltage drop (if used)
– Total loop cable resistance x maximum loop
current
= Voltage available at the instrument
NOTES:
Obtain filter voltage drop. The measured drop will be different than this value. The measured filter voltage drop
1
depends upon control system output voltage, the intrinsic safety barrier (if used), and the instrument. See note 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.
The voltage available at the instrument is not the voltage measured at the instrument terminals. Once the instrument is
3
connected, the instrument limits the measured voltage to approximately 9.0 to 10.5 volts.

Voltage Available

The voltage available at the DVC6200 digital valve
controller must be at least 11 volts DC. The voltage
available at the instrument is not the actual voltage
measured at the instrument when the instrument is
connected. The voltage measured at the instrument is
limited by the instrument and is typically less than the
voltage available.
As shown in figure 2-10, 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.
20
INTRINSIC SAFETY
HART
BARRIER
FILTER
(if used)
1
3
Figure 2-10. Determining Voltage Available at the Instrument
THUM ADAPTER
TOTAL LOOP
(IF USED)
CABLE RESISTANCE
R
Example Calculation
18.5 volts (at 21.05 mA)
– 2.3 volts (for HF300 filter)
– 2.55 volts (121 ohms x 0.02105 amps)
2
– 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
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 11 volts DC.
Table 2-2 lists the resistance of some typical cables.
The following example shows how to calculate the
voltage available for a Honeywellt TDC2000 control
system with a HF340 HART filter, and 1000 feet of
t
Belden 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
VOLTAGE
AVAILABLE AT
+
THE
−
INSTRUMENT
September 2010