Fluke 5720A 2-25. +5V Power Supply, 2-26. ±12V Power Supplies, 2-27. +35V Power Supply

Theory of Operation 2

Digital Section Detailed Circuit Description

Table 2-2. Supplies Generated by the Digital Power Supply

Signal Name

+75V OUT

+35V OUT

+12 VOLTS -12 VOLTS +5V

COMMON

Test Point

TP2

TP5

TP8

TP10

TP12

TP13

Nominal Output

73V

35V

12V

12V

5.2V

Tolerance

+/-8% +/-7% +/-5% +/-5% +/-5%

Current

Limit

121mA

52mA

1.5A

1.5A

2.4A

Rated

Output

100mA

40mA

700mA

450mA

2.0A

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

Afull-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

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