Martel Electronics MasterCAL - Manual (Page 60)

MasterCAL
Users Manual
50
Supplying Loop Power
The calibrator supplies loop power at 28 V or 24 V dc
through an internal series resistance of 250
Ω
. The
28 V setting supplies enough current for two or three
4-20 mA devices on the loop in addition to the two-
wire transmitter but uses more battery power. Use
the 24 V setting if there are two or fewer devices on
the loop in addition to the two-wire transmitter. (Each
device on a typical 4- to 20-mA loop has a resistance
of 250
Ω
, thus dropping 5 V at 20 mA. A typical
transmitter must have 11 V minimum in order to
operate correctly at its top end.)
When loop power is enabled, the mA (middle
column) jacks are dedicated to sourcing and
measuring the current loop. This means that the
SOURCE mA, measure RTD, and measure
Ω
functions are not available (see Table 8, later in this
manual.)
Connect the calibrator in series with the instrument
current loop as Figure 17 shows. Proceed as follows
to source loop power:
1. Press e for Setup mode.
2. Note that following Loop Power, Disabled is
highlighted. Press e .
3. Use the u or d arrow keys to select Enabled
24 V or Enabled 28 V.
4. Press e .
5. Press the Done softkey.