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

THEORY OF OPERATION
TRUE-RMS CONVERTER
3-28. As Part C of Figure 3-5 shows, when the 2 mS,
20on, or 2 kn 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
-V !lREF
=
RREF
(Minus V OREF is necessary for de-integration during the
Read period.)
3-29. As Part C of Figure 3-5 shows, when any range but
2 mS, 20on, or 2 kn 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 -V nREF.
3-30. For conductance measurements, the
microcomputer inverts the kn measurements (S= l / n) by
reversing the order of the Integrate and Read periods of
the a/ d converter.
3-31. True-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, U2l, 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 l or l 0 which is controlled by the microcomputer.
Refer to Table 3-l for the buffer gains selected for each
range. In the dB function with the 200 m V range selected,
the buffers auto-range through XI, XIO, and XlOO 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 l ,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 de voltage of the same numerical
3-8
_ ...
...,....,~
'"' 0
T·--1-·1-
value. The mathematical formula for computing the rms
value of a de voltage is:
Vrms=VW
where Vi is the instantaneous voltage at any given point in
time and Vi 2 is the average of Vi
2 •
Therms 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 Fl 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 RV l, R V2, and R V3,
and by RI, R2, and QI. RVI, RV2, and RV3 clamp the
voltage across the measurement circuitry at
approximately ±1200V while RI and R2 limit the input
current.
3-42. In the kn mode of operation, protection is
provided by thermistor RT l and the clamp/ zener action
of Q2. As RTI heats up, its resistance increases sharply.
3-43. Current Protection
3-44. In the current mode of operation, diode bridge
U28 and diode CRI clamp the voltage across the current
shunts until the fuses Fl and F2 blow. Backup fuse F2 is
used to clear open voltages between 250V and 600V.
Table 3-1. AC Buffer Gains
RANGE
FIRST SECOND OVERALL
BUFFER
BUFFER GAIN
200 mV,
Auto-range only
X1, X10, X100
dB only
200 mV, X1
X10 X10
Linear
2V X1
X1 X1
20V
X1 X10
X10
200V X1
X1 X1
750V X1
X1 X1