Low Battery Detection; Power Reset; Input Conditioning; Difference Amplifier (Voltage Input) - Fluke 39 Service Manual

39/41B
Service Manual

2-15. Low Battery Detection

The low battery detection circuit monitors the battery voltage and sends a signal to the
microcontroller when the battery voltage falls below 4.22 volts. R49 and R53 set the
reference for the circuit to 1.47 volts. R52 and R30 divide the battery voltage down to
stay within the common mode range of the op amp. R58 provides hysteresis, to prevent
oscillations. LOW_BAT goes low when the battery drops below its minimum value.

2-16. Power Reset

Various parts of the digital circuitry require a power reset signal to initialize their
operation upon power-up. U26 and Q19 monitor the VCC power supply. When the VCC
voltage goes above ≈2.7 Volts, Q19 turns on. This causes Q17 to turn off and allows C46
to start charging. When the voltage across C46 reaches ≈1.8V dc, Q2 turns on, pulling
the RESET signal low. Because both senses of the reset signal are needed, Q3 inverts the
RESET signal and provides the signal RESET*.

2-17. Input Conditioning

Both the voltage and current inputs provide gain adjustments and filtering of the
incoming signal before it is sent on to the a/d converters.

2-18. Difference Amplifier (Voltage Input)

U10 and Z6 form a difference amplifier. The difference amplifier improves the
instrument's operation on three-phase delta power systems. It also provides isolation and
a protective impedance in both the high and common inputs. The gain of the amplifier is
1/500.

2-19. Protection Circuit (Current Channel)

RT1, R38, R18, R23. R17, Q6, and Q7 form a protection circuit to limit the input voltage
and current applied to U18 during accidental overvoltages. The emitter of Q7 clamps any
input voltage to ≈ ±10 Volts. R17 further limits the current into the input of U18 to
500 µA.

2-20. Gain Amplifier

Both the voltage and current inputs adjust the gain based on the range selected. There are
three hardware ranges for each input; 256, 512, and 1024 volts peak for the voltage
input, and 20, 200, and 2000 Amps peak for the current input. The ranges for the current
input assume that the input is 1 mV/Amp. The gain for both inputs are controlled by the
microcontroller through a latch (U1). Resistor networks Z4 & Z7 are ratio matched to
0.1%, with the resistor between pins 1 and 8 as the reference.

2-21. Latch (U1)

The latch (U1) controls the switches that select the gain for both the amps and volts input
stages. The switches controlled by U1 change the feedback path of the amplifiers U9 and
U18, thus changing the gain. The D inputs to the latch are directly connected to the DSP
bus. The signal to latch the data is generated by U3 (CS_GAIN). The latch is memory
mapped in the "Y" data space at address $FFD0.
Tables 2-1 and 2-2 show the relationship between the control signals and the selected
gain.
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