Fluke 863, Руководство по обслуживанию

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867B,863
Service Manual
2-14
•
The Millivolt Sense Path is used for DC measurements in the 3V and 300 mV dc
ranges, and for all Ohms, Diode Test, and Component Test measurements. This path
uses two 100-k
Ω
resistors (R99 and R20), two metal-oxide varistors (RV2 and RV3),
and internal clamp diodes (U30).
1.
When S1 is open, the varistors clamp the voltage below 2500V to prevent an arc
condition at the rotary selector switch (S1). R99 limits the current in the
varistors.
2. When S1 is closed, the clamp diodes in U30 become the voltage clamps. R99
and R20 are then used to limit current. In mV DC, Diode Test, Ohms, or
Component Test, a steady-state high voltage overload encounters a 200-k
Ω
input
impedance.
•
The Source Path is used any time the GMM is sourcing current to a device under test
and for AC-coupled voltage measurements. R103 and RT1 (in series) provide the
current limitation for this path. When S1 is open (volts measurements), two varistors
are used as overvoltage clamps (RV1 and RV2). In Component Test, S1 is closed,
and Q8 and Q9 become the voltage clamps. In Ohms or Diode Test, S1 closes, and
Q7 and Q15 become the voltage clamps. For all of these circuits, the nominal current
limiting impedance is 4.6 k
Ω
. During an overload, however, RT1 transitions to high
impedance before the clamp components fail. This circuit has no fusing components,
allowing it to fully recover from both high voltage transients and steady-state
overloads.
External Trigger and Logic Activity Input Protection 2-23.
The external trigger input receptacle consists of the following two signal paths, each
with a separate input protection circuit.
•
The DC path is used only for logic activity. A 1.5-M
Ω
resistor (R91) is used for
current limitation. The low end of this resistor is clamped by chip diodes on U30.
•
The AC path is used for both logic activity and external trigger signals. The current
limitation for this path is provided by R75 (1 M
Ω
) in parallel with C36 (100 pF).
External clamp diodes (CR26) protect U30 from high voltage transients. These
clamp diodes hold the low end of R75 within a diode drop from the ± 5V supplies.
Zener diodes VR4 and VR5 provide the return path to common from the 5V supplies
during an overload.
Amps / mA / µA Input Protection 2-24.
Fuses and diode clamps provide overload protection for the Amp and mA/
µ
A input
circuits. Both of these circuits are designed to prevent GMM damage from a 1000V
unlimited energy source applied at the test leads.
•
The Amps input is protected by F1, an 11A, 1000V AC/DC, 17000A breaking
capacity fuse.
•
The mA/
µ
A input is protected by F2, a 440 mA, 1000V AC/DC, 10000A breaking
capacity fuse. Prior to F2 clearing, the clamp diodes CR1 and CR13 protect
measurement circuitry from overcurrents.
Input Signal Conditioning 2-25.
AC Volts 2-26.
AC Volts measurements are connected at the
Ë
and
â
inputs. Input protection
is provided by R103 and RT1 for all ranges. Relay K1 connects C31 to the 1.111 M
Ω