True-Rms Converter; Touch-Hold Circuit; Voltage Protection; Current Protection - Fluke 8050A Instruction Manual

THEORY OF OPERATSON
TRUE-RMS CONVERTER
3-28.
As
Part
C
of Figure 3-5
shows, when
the
2
mS,
2000,
or 2
kO
range
is
selected,
the voltage
drop
across
the
unknown
resistance
is
measured
in relation
to the
voltage
drop
across the
known
reference
resistor,
and
since the
same
current
is
flowing
through both
resistors,
the value of the
unknown
resistance
can
be
computed
using the formula:
Vrx
__
Rx
-VflREF
~
RrEF
(Minus
VfiREFis necessary
for de-integration during
the
Read
period.)
3-29.
As
Part
C
of Figure
3-5
shows,
when
any range but
2
mS,
2000, or
2
kO
is
selected, the voltage drop
across
the
unknown
resistance
is
measured, and C7
charges
up
to
the
ohms
voltage source,
VH.
During
the
Read
period,
the
a/d
buffer subtracts the voltage on C7 from VL,
thereby obtaining "Vuref.
3-30.
For conductance measurements,
the
microcomputer
inverts the
kO
measurements (S=l
/
O) by
reversing the order of the Integrate
and Read
periods of
the
a/d
converter.
3-31.
Tru
e-RMS
Converter
3-32.
The
true-rms converter
is
made
up
of
two
ac
buffers
and a
hybrid true-rms
converter.
3-33.
AC
BUFFERS
3-34.
The
ac
buffers consist of operational amplifiers
U23, U21, and
their associated
components. Through
the
buffers, the
input
signal
is
scaled to a level within the
range of
the
hybrid true-rms
converter.
Each
buffer has a
gain of
I
or
1
0
which
is
controlled
by
the
microcomputer.
Refer
to
Table
3-1
for the buffer gains selected for each
range. In the
dB
function with the 200
mV
range
selected,
the buffers auto-range through XI, XIO, and
XIOO
gains
depending
on
the input
signal
level.
Thus
in this
mode,
the
8050A
appears
to
have a
single
range from -60
dBm
to
+8
dBm
(600n
Reference impedance). Upranging
occurs
at
the linear ac range equivalent of 20,000 counts;
downranging
occurs
at
the equivalent of 1,800 counts.
3-35.
The
output
of the
first
buffer
is
divided
in
half
and
then amplified by
a factor of
two
in the
hybrid true-rms
converter. This
reduces the required
dynamic
range of
the
true-rms converter
amplifier
by
a factor
of two, thereby
accommodating waveforms
with
crest factors
up
to 3 at
full scale.
3-36.
HYBRID TRUE-RMS
CONVERTER
3-37.
An
rms
amplitude
is
the value of alternating
voltage
that
results in
the
same power
dissipation
in
a
given
resistance
as
a dc voltage of the same
numerical
value.
The mathematical formula
for
computing
the
rms
value of a dc voltage
is:
V
where Vi
is
the
instantaneous voltage
at
any
given point
in
time
and
VF
is
the average of VF.
The rms
converter
in
your
8050A
monitors the
instantaneous voltage and
computes
the
rms
value of the input
signal. Figure 3-6
shows
the
mathematical
derivation of the
implicit
rms
conversion
circuit in
your
8050A and
a block
diagram
of
that
circuit.
3-38.
Touch-Hold
Circuit
3-39.
The
touch-hold
circuit operates
in
conjunction
with the
80T-H Touch-Hold
Probe.
The
touch-hold
circuit
works
in
all
measurement
functions except
mA
and
dB.
If
any
valid function
is
selected,
and
the control
switch
on
the
80T-H Probe
is
pressed, the
touch-hold
circuit places
a
logic zero
(-5V)
on
the
T
& H
input (pin
16)
of
the
microcomputer. At
this signal, the
microcomputer
freezes the display with the data present
when
the control switch
was
pressed.
Touch-Hold
will
not
operate
if
Fuse FI and/or F2
is
blown.
3-40.
Voltage Protection
3-41
.
In the volts
mode
of operation, protection against
inputs
and
transients
above
the input ratings of
the
8050A
is
provided
by metal oxide
varistors
R V
1
,
R
V2,
and
R
V3,
and by Rl,
R2,
and
Ql.
RVl, RV2,
and
RV3
clamp
the
voltage across
the
measurement
circuitry
at
approximately
±1200V
while
R1
and
R2
limit
the input
current.
3-42. In the
kO
mode
of
operation, protection
is
provided by thermistor
RTl
and
the
clamp/zener
action
of
Q2.
As RTl
heats up,
its
resistance increases sharply.
3-43. Current Protection
3-44. In the current
mode
of
operation,
diode bridge
U28
and
diode
CRl
clamp
the voltage across the current
shunts until the fuses FI
and
F2
blow.
Backup
fuse
F2
is
used
to clear
open
voltages between
250V
and 600V.
Table
3-1.
AC
Buffer
Gains
RANGE
FIRST
BUFFER
SECOND
BUFFER
OVERALL
GAIN
200 mV,
dB
only
Auto-range only
XT
XIO,
XIOO
200 mV,
Linear
XI XIO
XIO
2V
X1
X1 X1
20V
X1
XIO XIO
200V
X1
X1
XI
750V
XI
XI
X1
3-8