Detailed Circuit Description; Input Overload Protection; Rotary Switch; Input Signal Conditioning Circuits - Test Equipment Depot 88 Service Manual

2-3. Detailed Circuit Description

Each of the functional blocks in Figure 2-1 is discussed in greater detail in the following
paragraphs. Refer to the schematic diagrams in Section 5 for details not shown in the
figures in this section.
2-4.

Input Overload Protection

Overload protection for the T input is provided by a network of two metal-oxide
varistors (RV1 and RV2), three current-limiting resistors (R1, R2, and RT1), and spark
gap E1. The 1-kΩ, 2W fusible resistor R1 opens when an extremely high energy signal is
present. Thermistor RT1 rises to a high impedance during a sustained voltage overload
in the mV dc, ohms, or diode test functions. A voltage clamp network is formed by
transistors Q1, Q2, and Q6, diodes CR7 and CR8, and resistor R58. During ohms and
diode test overloads, this clamp circuit limits the overload current to U4 to 10 mA.
Power supply regulation and system operation is maintained during any of these
overloads.
Overload protection for the mA input is provided by F1, rated at 1A/600V. The A input
is protected by F2, rated at 15A/600V. In addition, the microamp shunt resistors (R4 and
R43) are protected from overload currents below the
F1 fusing level by the U1 and CR1 diode network.
The Input Alert feature provides a beeper warning signal when an input jack is
connected to a current input (mA or A) and the rotary switch is not set to a current
function (mA or A). The Meter detects the presence of an input connection by using split
jacks at the mA and A inputs. One side of the jack is connected to an overload protection
resistor (R7 for mA, R10 for A). R7 and R10 are connected to U4 sense lines AP4 and
AP5 (pins 89 and 88).
Resistors R8 and R48 provide VDD pull up for AP4, and R11 is the pull up resistor for
AP5. When a connection is made at mA or A, the sense side of the jack is pulled to
COM. This condition is detected and compared with the selected function by U4. If a
conflict exists, the beeper warning is activated.
2-5.

Rotary Switch

Input signals are routed from the overload protection circuits to a double-sided switch
wafer, which provides the necessary switching to implement the various signal
conditioning.
2-6.

Input Signal Conditioning Circuits

Each input signal is routed through signal conditioning circuitry before reaching U4.
Input signals received through the T input are routed through Z1, a precision
resistor network. This input divider network provides precise input scaling for the
various voltage ranges and precision reference resistors for the ohms and capacitance
functions. The capacitors in parallel with the various resistors in Z1 are used for high
frequency compensation.
The input divider is used in two modes: in voltage functions, a series mode is used to
provide four divider ratios; in the ohms function, a parallel mode provides five reference
resistors. During the following discussion, refer to the schematic and signal flow
diagrams in Section 5.
Theory of Operation

Detailed Circuit Description

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