Power Supply; Power Supplies - Fluke 5700A Series Service Manual

Table 2-2. Supplies Generated by the Digital Power Supply
Signal Name Test Point
+75V OUT TP2
+35V OUT TP5
+12 VOLTS TP8
-12 VOLTS TP10
+5V TP12
COMMON TP13
2-25. +5V Power Supply
The unregulated +5V supply uses CR25-CR28 in a full-wave rectifier configuration with
filter capacitors C12, C13, and C14. Capacitors C20 and C21 filter out high-frequency
noise. Inductor L8 is a common-mode choke and C11 is a bypass capacitor. The regulator
is fused by 3.15A slow-blow fuse F5.
The regulated +5V is generated by three-terminal low-dropout +5V regulator U3 with
heat sink. The regulator's output voltage is increased about 0.2V by CR35, a germanium
diode connected between the regulator's ground pin and COMMON. Capacitor C14 is for
bypass. Capacitor C23 filters out high-frequency noise. Diode CR29 protects the
regulator against input shorts, and diode CR30 protects the regulator from reverse
voltage. Inductor L7 and C16 further filter the output to P41.
2-26. ±12V Power Supplies
A full-wave rectifier made of diodes CR17-CR20 and filter capacitors C6 and C7
generate the unregulated +12V and -12V supplies. AC inputs are fused by F3 and F4,
both 2A slow blow.
The regulated +12V supply is generated by a three-terminal +12V regulator U1 with heat
sink. Capacitors C5 and C9 are for bypass. Diode CR21 protects the regulator from input
shorts, and diode CR23 protects the regulator from reverse voltage. Capacitor C22 filters
out high frequency noise.
Three-terminal -12V regulator U2 with heat sink generates the regulated -12V supply.
Capacitors C8, C10, and C19 are for bypass. Diode CR22 protects the regulator from
input shorts, and diode CR24 protects the regulator from reverse voltage.
Inductors L3-L6 filter the regulated outputs. R7 further isolates the ±12V FAN lines from
the ±12V power lines. The +12V FAN and -12V FAN lines power the two 24V dc fans
inside the calibrator.
2-27. +35V Power Supply
The +35V power supply powers the grid drivers and anode drivers on the front panel
output display circuitry.
A full-wave rectifier made of diodes CR9-CR12 and filter capacitor C3 generate the
unregulated +35V supply. Its input is fused by F2, a 0.125A slow-blow. Capacitor C18 is
for bypass. Capacitor C25 filters out high frequency noise.
Zener diodes VR14, VR15 and transistor Q5 generate the +35V regulated output. Zener
diodes VR14 and VR15 (both 18V) make up the control element which sets the output
voltage. Transistor Q5 is used as an emitter follower. CR13 is the constant current source
supplying current to the zener diodes and the base of Q5. Components R5 and Q6 make
up the current-limiting circuit. During an over-current condition, the voltage drop across
R5 turns Q6 on, thus drawing current away from the base of Q5 and limiting current flow
Nominal Tolerance
Output
73V +/-8%
35V +/-7%
12V +/-5%
12V +/-5%
5.2V +/-5%
Theory of Operation
Digital Section Detailed Circuit Description
Current
Limit
121 mA
52 mA
1.5A
1.5A
2.4A
2
Rated
Output
100 mA
40 mA
700 mA
450 mA
2.0A
2-19