4. VERIFICATION OF NORMAL OPERATION
It is the purpose of this section to aid the user in determining, in the event of a malfunction
to which the Model IL78A is suspected of contributing, whether the instrument is
functioning normally or whether it is the source of the observed trouble. In the
event the unit requires repair, the user may also contact the factory Service
Department or the local Daytronic Representative for assistance. Daytronic service
information is located on the last page of this document.
If the instrument is suspected of faulty operation, follow the steps below.
(a)
I
f the unit is totally inoperational (front-panel power indicator does not light),
check the primary power input terminals for proper connection. Input power can
be from 11 to 28 Vdc and will draw less than 2 watts. If properly connected,
the front panel Green LED will be illuminated. Before reapplying power, visually
inspect the power supply and the input power connections for any discrepancy
which could have caused the overload.
(b)
If the transducer has some preloading, the BALANCE controls may not allow
successful zeroing of the instrument output. This condition can be remedied by
connecting a resistor (50 k- 200 k range, metal-film type) from the +Signal
terminal of the transducer to the + or –Excitation Sense terminals. The Excitation
terminal to which the connection is made is determined by the direction of the
loading or off-zero reading.
(c)
Figure 7. Star Bridge Construction
20
The inability to balance correctly, where the instrument output readstotally off scale
and the BALANCE controls have no authority, can very likely be the result of a
damaged or defective transducer or cable. This possibility can be confirmed (or
eliminated) by substituting a transducer and cable known to be in good condition
or by simulating a balanced transducer, using either a commercially available
transducer simulator or the simple star bridge arrangement shown in Figure 7. The
star bridge simulates a conventional four-arm bridge in an exact condition of
balance. To construct a star bridge, connect four 10% carbon resistors as shown
in Figure 5. Use 180-ohm resistors to simulate a 350-ohm bridge and use 56-ohm
resistors to simulate a 120-ohm bridge. Neither the resistor values nor
temperature characteristics are critical since the balance condition of a star
bridge is not determined by the resistance values. Solder two resistors together,
then solder the remaining two resistors together. Next, connect the two
junctions together using a separate wire as shown. There is a good reason
for this method of construction, and it should be followed. Connect the substitute
or simulated transducer to the instrument I/O connector using a short 4-wire cable
configuration as shown in Figure 4. Attempt to balance the substitute-simulated
transducer. If conditions now appear to be normal, the transducer or cable is at
fault. If the previous difficulties persist, the IL78A is faulty.
Verification of Operation IL78A.3
AC Strain Gage Conditioner
Module
IL78A
