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7-24.
RELAY MODULE A1A4. The relay module (figure 7-5) is mounted on one end plate of the power
supply. Primary power is applied to'the module through a line transformer or the DC input. When an AC input is
used, the RELAY ENABLE line is high, the relay is open, and the power supply operates from the AC input. The
MAIN AC and MAIN AC lines receive a 13.5-volt AC rms input from the line transformer. After full wave
rectification, the DC power is routed throughout the power supply on the DC bus. Filtering of the DC power is
done on the switcher module.
7-25. When the DC input is used, the RELAY ENABLE line is low, the relay is closed, and the power supply
operates from the de input. The battery charge voltage is boosted to 32 volts using the AC bus voltage to bias
an AC boost winding center tap.
7-4
SECTION 7
LOW VOLTAGE POWER SUPPLY (A1)
7-1. GENERAL. The low voltage power supply (figure 7-1) converts an AC line voltage input or a DC voltage
input to the required DC operating voltages. The power supply is composed of four modules, each module
containing a printed wiring board. These modules are the relay, control, switcher, and output modules.
Protection circuits protect the power supply against short circuits, high internal temperatures, and high and
low DC bus voltages.
7-2.
INPUT POWER CONTROL. When AC power is applied to the power supply, the output of the AC
rectifier and filter circuit provides the DC bus voltage. An AC sense circuit provides a control voltage when AC
power is present. This control voltage isolates the DC voltage input from the DC bus and drives the front panel
AC indicator.
7-3. The off, standby, or on operating mode of the power supply is selected by the control circuiJ. When the
~~~r,1is'ilf1! t11Br~?Jitli:y'SlanaaHl,anlliC~-O'Pifsrr9~'1f~l't>1''Wft!tl
an
g
th~ !l~tl~~6i'fli~9~l1lf'6~.
YJhen
t~e
'ii~~lyzer
is
inrf~~he
chopper,generatoi'fiis•of¥1and tiie'YfTitG'u!l6'cY:::Siiitctil7d
~nd !:lattliiiZctifirg~r
are:Oril.
When the
anaiy~er
is on,
th~fr~cWf!itfy'stand~;d
a"nd chopper
g~;e·r~tor
are on and th; ba.ttery charger is.off.
Thus, the battery is charged when the analyzer is in the off and standby modes of operation. The frequency
standard operates in the standby and on modes of operation.
7-4. The battery charger requires a voltage higher than the nominal DC bus voltage. This increased voltage,
32V, is provided by the AC boost circuit.
7-5. To operate the power supply using a DC voltage input, the AC power input must be removed, disabling
the AC sense voltage. When the AC power is removedand. the analyzer
tu.~ned?fl·
no power is present on thP
DC bus .. When the,analyzer is
s'Nit~heg
to,
thestaf1C!15sli-lid'ae,'t~.~ QC:lreia~'closes( connectingth~
DCvoltage,;!
\nJ?utt~a,~~1g:oi6~~.<~JJ,9.,J.P.e.!uppi~'V5if~9e~<>lltl'~T~Ci~MYi~ta.n'd~i'Jfwh£i';iJ1;·i@ilyierlssWf!clf~~'6~:!h;
bhopper;generato'i' is erial:ileo' and normal operation occurs.
7-6.
DC OUTPUT CONTROL. Regulation of the DC output voltages is accomplished by using the +5 volt
output as feedback. This feedback voltage is compared to a stable reference voltage (7.9V). The resultant
control voltage determines the on time ofthe pulse width modulator, thus regulating the input voltage to the
chopper circuits. The output transformer winding ratios determine the output voltages with respect to the
+5-volt feedback.
7-7. The chopper generator provides a 7-volt reference voltage, a 20-kHz squarewave chopper drive signal,
and a 20kHz triangular waveform output for pulse width modulator control. The pulse width control compares
the triangular waveform with a control voltage. When the control voltage is equal to the mean DC voltage of the
triangular waveform, the pulse width modulator has a 50 percent duty cycle. For control voltages that are
above or below the mean DC voltage of the triangular waveform. the duty cycle is proportionately increased or
decreased.
7-8. The filtered DC output from the pulse widths modulator is chopped 50 percent through the primary
windings of output transformer T2 at a 20kHz rate. The DC output is alternately switched between each half of
the primary winding of T2. Current through the primary winding center top is passed through a current
transformer whose output is used for overcurrent protection.
7-1
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