HP (Hewlett-Packard) 410C Manual De Usuario

TM 11-6625-1614-15

Model 410C

voltage or current being measured) to the neon-photo-

conductor chopper. Also applied to the opposite  side
of the chopper is the amplifier feedback voltage, which
is of the same polarity and approximately 5 micro-
volts lower in amplitude than the input voltage. The
modulator-chopper consists of two photoconductors,

A4V1 and A4V2, which are alternately illuminated by

two neon lamps, A4DS1 and A4DS2, respectively. The
neon lamps are part of a relaxation oscillator, whose
frequency is controlled by A3R5. The oscillator fre-
quency is nominally set to 100 cps for operation from

60 cps power line, or to 85 cps for 50 cps line. This

frequency is selected so that it is not harmonically

related to the power line frequency, precluding pos-
sible beat indications on the meter.

4-19.  As  the photoconductors are alternately illumi-
nated by the neona, their respective resistances are
low (conductive )  when illuminated and high (non-con-
ductive) when darkened. Therefore the input voltage
and feedback voltage are alternately applied to the
input amplifier. The amplitude of the resultant signal

to the amplifier is the voltage difference between the
input and feedback voltages.

4-20. The chopped DC signal is amplified by a three
stage RC amplifier, consisting of A3V1A, A3Q1 and

A3Q2. The amplified signal to the input of the de-

modulator-chopper is 180° out of phase with the out-
put of the modulator-chopper.

4-21.

The demodulator - chopper consists of two

photoconductors, A4V3 and A4V4, which are alternately
illuminated by neon tamps A4DS1 and A4DS2, respecti-
vely. Approximately 150 millivolts square-wave is
applied to the demodulator from the amplifier. Since
the same neon lamps illuminate both the modulator
and demodulator photoconductors, operation of the two
chopper is synchronous. Therefore, when A4V1 is
sampling the input voltage, A4V3 is clamping the
amplified and inverted difference voltage to ground.
Alternately, when A4V2 is sampling the feedback vol-
tage, A4V4 is charging capacitors A3C13 and A3C14
to the peak value of the square-wave. These capaci-
tors maintain this charge so long as the input voltage

remains constant by virtue of having no discharge

path and because they are being repetitively recharged
by the demodulator.

4-22. Therefore, a DC potential, proportional to the
difference between the input and feedback voltages, is
applied to the grid of the cathode follower and subse-
quently to meter circuit and DC AMPLIFIER OUTPUT
connector. A portion of the meter circuit voltage is

fed back to the modulator. The meter stabilizes when
the feedback voltage and input voltages are nearly
equal.

4-23.  THE FEEDBACK NETWORK.

4-24. The feedback network drives the meter and
determines the DC gain of the amplifier. The feed-
back is varied depending on the position of the  FUNC-
TION  and RANGE selectors. The different feedback

configurations are discussed below.

4-25.

FEEDBACK NETWORK FOR ±DCA. OHMS,

back configuration for ail positions of the FUNCTION

SELECTOR except ACV. The meter is electrically

inverted for ±DCV and ±DCA modes of operation. The

DC OUTPUT ADJ., A6R20 sets the output voltage. The
DC pot, A6R18 determines the amount of feedback to
the modulator. The resistor A2R30 is in the circuit

in the ± .015 DCV and ±1.5 µa modes of operation, to
decrease feedback and thus increase amplifier gain to
compensate for the decrease in input signal to the
modulator on these ranges.

4-26. FEEDBACK CIRCUIT FORAC VOLTAGE 

MEA-

SUREMENTS: Figure 4-6 shows the feedback confi-
guration for the ACV position of the FUNCTION SEL-

ECTOR switch, A2S2. The resistors that are placed
in the circuit by the RANGE switch program the am-

plifier gain to compensate for the non-linear response

of the AC probe. A6R16 and A6CR1 compensate the
non-linear response of the AC probe to the linear
calibration of the upper meter scale on the 5 volt
range.

4-27. POWER SUPPLY.

4-28. PRIMARY POWER. Either 115 or 230 volt ac
power is connected through fuse R1 (0.25 amp slo-blo)
and switch S3 to the primary of power transformer
T1. Switch S4 connects T1 primaries in parallel for

l15 volt operation of in series for 230 voit operation.

4-29.

UNREGULATED AND ZENER REGULATED

POWER SUPPLY. Full wave rectifier CR1 and CR2
produces unregulated +270 volts, which is used to

drive the photochopper neons. Unregulated +175 volts
and +140 volts are tapped off and are used to provide
B+ to the plates of A4V1B and A4V1A, respectively.

Zener regulators A7CR6 and CR7 provide regulated
+38 volts and -9 volts to bias A3Ql and A3Q2. Filtering
of the outputs is provided by the RC network consisting
of A7R1 through A7R3 and C5A through C5D.

4-30. SERIES REGULATED POWER SUPPLY. The
output of the full wave rectifier CR3 and CR4 is re-
gulatedbytransistor Ql, which is connected in series
with the output. Zener diode A7CR8 provides reference
voltage to the base of Q1. Regulated +6 volts is supplied
to the filaments of A3VlA/B and the AC Probe diode

A6V1.

+0.6 volts is provided through A7R10 to R3,

the OHMS  ADJ, control. Filtering of the outputs is
provided by C6A and C6B.

4-31. STANDBY FILAMENT SUPPLY.  The filament
tap (Tl, Pins 1 and 2) provides 6.0 volts actothe
filament of the AC probe diode, A8V1, so that the
filament remains warm when the Modei 410C is being
used in modes of operation other than ACV.  W h e n
FUNCTION selector A1S1 is switched to ACV, 6.0
volts AC is removed from the filament and 6 volts DC
is applied. Therefore,

the ACV mode is ready for

imrnediate use,

without waiting for the filament to

warm up.

4-2

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