BK Precision 177/V-95 User Manual

ADVANTAGES OF
THE VTYM
'I'he
greatest
of the many
advantages ofiered
by the
VTVM is its
high input
impedance. This feature
makes
possible
the measurement
of voltages
in
high
impedance
circuits
such as oscillator
grid
circuits and
AGC networks
with a much
higher degree
of
accuracy.
As a simple
illustrative
example
of this,
we have
showl
in Figure
1A
two 500K
resistors connected
in series across
a
100 volt
source
of voltage.
A simple knowledge
of
basic fundamentals
makes
it obvious
here
that the voltagc
drop
across
each resistor
will be 50 volts.
I\4easuring
the
voltage
across either resistor
with
a
20K
ohm-per-volt
non-electronic
type instru-
ment
on the 0-100 volt scale however, we
obtain a reading
of only
44 volts.
Why
the
discrepancy. As shown
in Figure 1A the
meter
(on
the
100
volt range)
can be
considered
a resistor of
2 meg.,
which is connected in
parallel
with the
500K
resis-
tor.
A simple
Ohm's Larv calculation
rvill readily
shorv the
total resistance
of the
parallel
circuit to be
400K,
resulting
in a
redistribution
of the
voltage drops in
the circuit. Now
the first
500K resistor has
approximatety
56 volts across
it, and
R(rorAL)
(ruerER
rN
PARALLEL
wlrH Rz)=ffiffffi;aoo
K
\oLTAGE AcRoss Rz AND
METER=Eoffi
xtQQ=44
voLTs
(APPRox')
FIGURE
IA
I
R=5OO
K
E=51
V
-t-
loo
v
_l_
-T-
loo
v
_.1_
/ruec.oN \
\ANY
SCALE/
R(roTlt-)=4goK
E= 49V
I
t
R(rorAu
(METER
rN PARALLEL
wrrH nz)=*#Hili$F.
.48MEG.=48o K
VoLTAGE AcRoss
R2 ANDMETan=#|ft5xloo=4e
voLTs
FIGURE
IB
the
other
500K
resistor in
parallel
with the meter
has approximately
44
volts
across
it. The
low resistance
meter
is loading or changing
the
impedance
of
the circuit
when
it is connected.
When a VTVI\{ is used
as shown
in Figure
18
we
have a
total
of 11 megohms
connected
in
parallel
with the
500K resistor,
and the total
resistance
of our
parallel
network
is now approximately
480K, resulting in the
voltage
across
the meter
and resistor
dropping
only to
49 volts or a mere
2/6 lowet
than the
actual operating
voltage.
More accurate
readings can only be
obtained
with a high-impedance
instument
such as the
VTVM.
R:5OO
K
E=56V
--MULrrMetEd-'
E_q?rf;.Ll"4ooK
ON IOOV SCALE=2 MEG.
I
VTVM

BK Precision 177/V-95 Specifications

General

Brand	BK Precision
Model	177/V-95
Category	Measuring Instruments
Language	English