DayTronic 3000 series, Instruction Manual

Page: 21 / 30

Model 3178
where X = Equivalent Input, % of full scale
R
b
= bridge resistance, ohms
K = transducer sensitivity, mv/v full scale
R
c
= calibration resistance, ohms (59 K installed)
Sample Calculation: Assume that K = 3.000 mv/v for a 5000-pound load cell
(fullscale) with a bridge resistance of 350 ohms.
4. BLOCK DIAGRAM DESCRIPTION
49.4% of full scale = 2472 pounds
The purpose of this section is to explain how the Model 3178 works by using a
simplified block diagram. This section is not intended to provide a detailed explana-
tion of electronic circuits for personnel untrained in electronic technology. How-
ever, it provides an adequate overview of operation for those familiar with basic
electronic circuit operation. Throughout the following, refer to Figure 6.
Power Supplies.
Primary power (115 volts ac, 50-400 Hz) is applied to the
instrument by means of a rear-panel ac connection point and the supplied
3-conductor power cord. A rear-panel slide switch is used to turn ON primary
power. Overload protection is provided by a 0.25 ampere fuse mounted near the ac
connection point. When the slide switch is ON, primary power is applied to the
power transformer which provides the necessary power-line isolation and the low
ac voltages required to develop the regulated dc voltages used in the 3178. The
secondary of the power transformer has a grounded center tap, and a Diode Bridge
functions as two full-wave rectifiers to produce ±9 volts regulated dc. Two
three-terminal integrated-circuit
Regulators
are used to develop these regulated
voltages. The reference terminal of each
Regulator
is biased with one or two diodes
to make certain that a minimum regulated voltage of 9 volts is achieved. The proper
diode biasing is accomplished at factory check out.
A dc reference voltage of +2.5 volts dc is further developed from reg9
volts by the use of a third three-terminal
Regulator.
This precision dc reference is
used to control the amplitude of the ac excitation signal and is further discussed in a
following paragraph.
14
X = 25000 R
b
K R
c
X =
59000 x 3
25000 x 350
=