Linear Post Regulators (I); Input Overload Protection; Volt/Ohms Input Protection - Fluke 867B Service Manual

a 3 volt minimum value for VCC. C19 delays this trip for at least 100 ms after power is
applied to the GMM. CR5 assures that the cycle is completely repeated by discharging
C19 at power down.

Linear Post Regulators (I)

The output voltages from the rectifiers on the DC-DC converter are set about 0.6-1.0 volt
higher than the required output levels. The linear regulators used here are op amps with
transistor emitter follower outputs. The transistors increase the power available to drive
the load. Since the op amps are driven from the raw DC voltages (U4) or a combination
of the raw voltages (-24) and a higher regulated voltage (+5.2, for U2), the base of the
transistors can be driven high enough to allow the transistors to be nearly in saturation.
This method allows the circuit to regulate with minimal voltage drop across the
regulators, thus saving power.
All of the regulators derive their voltages from U31 (the 2.5-volt reference.) The circuit
comprised of U2, Q23, R132, and R128 amplifies by 1.325, giving an output of a
nominal 3.31 volts. The circuit of U4, Q18, R89, and R39 amplifies by 2.07, giving an
output of a nominal 5.175 volts. The circuit of U4, Q19, R19, and R13 amplify the
output of Q18 by -1 (inverting) to provide a nominal -5.175 volts.
The 475Ω resistors in the base circuit of each transistor provide current limit.
The SOT-23 transistors used in this circuitry cannot handle short circuits
at their outputs. Momentary shorts due to probing will damage these
devices.
Zero ohm resistors R52, R87, R62, R48, and R60 provide isolation of various circuit
blocks. R60 can be used to disable most of the digital circuitry if trouble is suspected in
the digital hardware. The other resistors allow separation of the analog loads going to the
A/D Converter and analog circuits.

Input Overload Protection

All ranges and functions of the 860 series are designed to meet or exceed IEC 1010-1,
1000V, CAT III protection.

Volt/Ohms Input Protection

The Volt/Ohms input (Ë) incorporates three separate protection circuits. In each
circuit, a series impedance provides current limitation, and a shunt voltage clamp
prevents overvoltage.
•
The High Voltage DC Path is used for DC measurements in the 30V, 300V, and
1000V dc ranges. A 10-MΩ resistor in the Z5 resistor network provides current
limiting for this path. Internal clamp diodes on U30 hold the low end of this resistor
within a diode drop of the ±5V supplies. Zener diodes VR4 and VR5 provide the
return path to common from the 5V supplies during an overload.
Note
Theory of Operation

Input Overload Protection

2-20.
2-21.
2-22.
2
2-13