DayTronic 3000 series, Instruction Manual

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Daytronic Corporation
The -9 volts regulated is used to light the front panel indicator (LED) which
indicates the application of ac power to the instrument.
The + unregulated voltage from the diode bridge is routed to the Digital
Indicator and HI/LO Limits circuit boards when these items are supplied (Models
3278 and 3378). It is used to d5 volts regulated for the TTL logic employed
in these circuits. Refer to the Digital Indicator and HI/LO Limits
Instruction
Manuals.
The secondary of the power transformer also supplies 5 volts ac to the Digital
Indicator circuit board when the Model 3278/3378 is supplied. This ac voltage is
used to develop unregulated +6 volts. Refer to the Digital Indicator
Instruction
Manual.
AC Excitation.
The 3.28 kHz ac excitation is produced with a
Wein Bridge
Oscillator.
The oscillator output is applied to a full-wave rectifier to obtain a dc
voltage proportional to the ac amplitude of the oscillator output. The rectifier
output is applied to the inverting input of an
Integrating Amplifier.
The non-
inverting input of the amplifier is connected to the precision
Reference
voltage
(+2.5 volts dc). If the amplitude of the oscillator varies, the output of
the Integrat-
ing Amplifier
changes the resistance of an
Automatic Gain Control
element (FET)
to return the oscillator amplitude to its nominal value of 2 volts ac. The
Integrating
Amplifier
thus serves as an error amplifier, and the integrating element (capacitor)
deletes 60-Hz ripple.
A pair of operational amplifiers is used as a phase splitter to eliminate the
ground reference of the oscillator signal so that the transducer bridge is floating.
This reduces signal-carrier common-mode voltages to a negligible value. Both plus
and minus
Sense
wires are returned from the transducer bridge to the instrument to
sense and regulate the excitation voltage at the transducer (Figure 6 shows the 3178
connected to the transducer via a 7-wire cable. Refer to Figure 4). Excitation
Phase
and
Amplitude
balance controls are connected across the
Sense
lines. A pair of
Power Drivers
is used to provide the 80 milliamperes of drive current required by
the transducer bridge.
When more than one 3178 (or a combination of 3178 or 3130 LVDT Condition-
ers) is being used in a measurement setup, beat frequencies may be produced from
the 3-kHz oscillators contained in each instrument. To prevent beat frequencies
from occurring, one unit can be designated the
master,
and the remaining units can
be driven from the oscillator contained in the
master
unit. The remaining units are
15