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RCA Volt-Ohm-Milliammeter
WV-38A V O M
Circuit Description
Metering Circuit
The WV-38A meter circuit for the A C Volts, D C Volts, and D C cur-
rent functions consists of the calibrating potentiometer R22, connected in
series with the 50/xA meter movement.
In the ohms function, the calibrating potentiometer is replaced by
R25, the front panel Z E R O O H M S control.
Some models of the WV-38A have two special silicon diodes (CR3 and
CR4) connected across the meter terminals to prevent damage to the meter
movement due to accidental overload.
DC Volts
The eight ranges of the D C volts circuit are obtained by adding suc-
cessively higher, values of resistors in series with the meter circuit.
The .25 V range is connected directly to the meter circuit. The voltage
divider network used for the remaining ranges is composed of R27, R l , R2,
R3, R4, R5, and R29.
AC Volts
The diodes C R 1 and C R 2 , along with R20 and R21, form a full-wave
rectifier circuit, converting the A C signal input to pulsating D C voltage.
The six A C voltage ranges, as with the D C voltage section, are ob-
tained by using a voltage-divider network. The resistors in this network are
RIO, R l l , R12, R13, R14, and R28. R24 is a 6K potentiometer for cali-
brating the low A C ranges. R23, also a 6K potentiometer, is for calibrating
the high A C ranges.
DC Current
The D C current ranges are obtained by switching the appropriate
values of shunt resistance across the meter circuit to maintain 50/iA full-
scale deflection. The ranges and shunting resistors are listed below:
Range
Series Resistors Shunting Meter
50/iA
No shunt used
1 ma
R6, R7, R8, R9, and R26
10 ma
R6, R7, R8, and R26
100 ma
R7, R8, and R26
500 ma
R8 and R26
10 Amp
R26
Resistance
In resistance measurements, the ohms control, R-25, is connected in
series with the meter, and is used for calibrating or "zeroing" each range.
In the R x 1 and R x 100 ranges, the 1.5V " D " cell battery is applied
to the circuit. In the R x 10,000 range, the four 1.5V penlight cells are also
used, resulting in 7.5 volts. The battery voltage is applied to the meter
circuit and shunt resistors of the particular range so that when the test leads
are shorted, 50/*A of current pass through the meter, causing full-scale
deflection.
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