Input Overload Protection; Rotary Knob Switch; Input Signal Conditioning Circuits; Volts Functions - Fluke 76 Service

76
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Input Overload Protection

The V  input is protected from overload by a network consisting of three metal-
oxide varistors (RV1, RV2 and RV3) and five current-limiting resistors (R1, R2, R8,
R12 and RT1). The presence of an extremely high energy signal causes R1 (500 , 2W),
a fusible resistor, to open. Thermistor RT1 rises to a high impedance during a sustained
voltage overload in the Ohms, or continuity mode. Transistors Q1 and Q2 form a voltage
clamp network. This clamp limits the overload current to U4 at 10 mA during Ohms and
continuity overloads. Power supply regulation and system operation are maintained
during any of these overloads.
The mA input is protected from overloads by F1 (1A/600V), while F2 (15A/600V)
protects the A input. Milliamp shunt resistor R5 is protected from overload currents
below the F1 fusing level by the U1 and CR1 diode clamp network.

Rotary Knob Switch

Input signals are routed from the overload protection circuits to a double-sided switch
wafer. This switch wafer provides the necessary connections to implement signal
conditioning and function-encoding for U4.

Input Signal Conditioning Circuits

Each input signal is routed through signal conditioning circuitry before reaching
multimeter chip U4. Incoming signals received through theV input are routed to
precision resistor network Z1. This divider network precisely scales the input for the
various voltage ranges and provides precision reference resistors that are used for the
Ohms and capacitance functions.
Input divider Z1 is used in two modes, series and parallel. In volts functions, a series
mode provides four divider ratios. In the Ohms function, a parallel mode provides five
reference resistors. During the following discussion, refer to schematic and signal flow
diagrams in Section 5.

Volts Functions

In volts functions, signal flow for input divider Z1 begins with a voltage that appears at
the V  input. (See Figure 2-2, 4V Range Simplified Schematic.) This input is
connected to the high end of the 9.996 M resistor (Z1-1) through R1, R8, R12, and
RT1. If the AC volts function is selected, dc blocking capacitor C1 is also connected in
series. In AC the signal is routed to U2 through the AC buffer in U4. If the DC volts
function is selected, C1 is shorted by S1 (contacts 6 and 7).
Internal switches connect the 9.996 M and 1.1111 M resistors (Z1-2 and -3). The low
end of the 1.1111 M resistor (Z1-7) is connected to the COM input through S1 contacts
11 and 12. This produces the divide-by-10 ratio needed for the 400 mV ac, 4V ac, and 4V
dc ranges. The 400 mVac and 4Vac ranges require frequency compensation, which is
supplied by C20 (not shown in Figure 2-2.)
For the 40V range, internal switches connect the Z1-4 (101.01 k ) resistor to provide a
divide-by-100 ratio. In the 400V range, Z1-5 (10.01 k ) produces a divide-by-1,000
ratio. And in the 600V range, the Z1-6 (1.0001 k ) resistor provides a divide-by-10,000
ratio.
2-4
2-4.
2-5.
2-6.
2-7.