Normal Operation; Circuit Descriptions; Power Supplies; Power-On Circuit - Fluke 39 Service Manual

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Service Manual
8. About 140 ms after the power button is first pressed, a dark screen is displayed. If
2-5.

Normal Operation

The normal sequence is as follows:
1. The Tester simultaneously samples volts and amps inputs and stores the samples in
2. U2 calculates the results using algorithms stored in the program section of SRAM.
3. The calculated results are displayed on the LCD. The software determines which
This entire sequence takes about 300 ms, that is, it repeats about three times per second.
During normal operation, the instrument always operates in this sequence with one
exception. When the instrument is in the "HOLD" mode, sampling and calculations are
halted until the hold mode is exited, or the display is changed to another screen.

2-6. Circuit Descriptions

2-7.

Power Supplies

There are six power sources required for the Tester's analog, digital and LCD circuits. A
+5V dc and -6V dc source is required for the analog section as well as a 2.1154V dc
reference voltage for the a/d converters. The digital section requires 3.3V dc for the DSP
and associated kernel components. The LCD module requires -22V dc for power and a
variable source of -15 to -19V dc to control the LCD contrast. A power-on circuit is
incorporated to control the application of power to the various sections of the unit.
2-8.

Power-On Circuit

The Tester has a soft key power-on circuit. Closing the power switch turns on Q11. This
turns on Q10, which starts the switching regulator (U30). To maintain power to the rest
of the circuitry, the microcontroller (U2) sets the signal PWR_ON to a high level (2.4V
dc) and starts toggling W_DOG*. When W_DOG* goes low, it turns on Q20, charging
C71 to VCC. Q9 remains on as long as the voltage on C71 remains above the threshold
voltage (2.15V typical). With Q9 and Q13 both on, Q10 remains on when the power
switch is released.
If something causes U2 not to toggle W_DOG*, Q9 turns off after one to three seconds,
causing Q10 to turn off and remove power from the instrument. This circuit arrangement
allows the instrument to always recover from any microcontroller crash. Q8 signals the
microcontroller that the power switch has been pressed after power has been applied.
The microcontroller considers this a signal to turn off the instrument.
2-9.

VCC

U30 is a buck switching regulator that changes the battery voltage (4V to 6V dc) to the
3.3V dc needed for the digital circuitry. The battery is always connected to the power pin
of U30. U30 is in the standby mode until pin 1 goes to the battery voltage level. R31 and
C50 set the soft start time. Feedback from the voltage divider (R48 and R41) sets the
2-6
the power button is held down during power-up, a checkerboard test pattern then
appears on the LCD. When the button is released, the instrument resumes normal
operation.
SRAM.
The results are stored in SRAM as well.
mode the instrument is in, and displays the corresponding screen and information.