Fluke 5725A instruction manual High Voltage Sense Assembly A6

5725A

Instruction Manual

to approximately 33Ω/°C by R615. Op amp U603A provides a reference voltage of about -0.77V. This reference voltage is applied to the circuit containing summing amplifier U603B, R611, and the linearized thermistor. The output of U603B is signal HV TEMP M, equal to 10 mV/°C.

A voltage proportional to the temperature rise from the heat sink to the junction of the transistor is added to HV TEMP M through R610. This is done via +400V IM from the Power Supply assembly. Resistor R610 is selected to properly scale this contribution to the 10 mV/°C output of U603B. Capacitor C610 provides averaging for the ripple on the current-sensing resistor, and CR604 prevents C610, an electrolytic capacitor from being reverse charged.

4-54. High Voltage Sense Assembly (A6)

In addition to ac voltage sensing for which it is named, the High Voltage Sense assembly provides four major functions for the 5725A. These additional functions are the following:

∙Analog monitoring of the 5725A status

∙Interface with the 5700A controller

∙Switching input signals from the 5700A

∙AC Line Power switching

Because it performs all the above functions, the High Voltage Sense assembly plays a primary role in controlling the instrument, attaining ac voltage specifications, performing 5725A calibration, and maintaining instrument safety.

The High Voltage Sense assembly has the following six sections: sense amplifier, analog monitor, serial interface/guard crossing, analog input switching, control, and mains switching. Each section is first briefly described, then described in detail. For reference, see Figure 4-5, the block diagram for the High Voltage Sense assembly.

∙Sense Amplifier Section

The sense amplifier section consists of a precision ac amplifier configured as an active attenuator, a buffer amplifier that isolates the 5725A high quality ground from the 5700A ground reference, and a precision true rms converter circuit for 5725A calibration. Together, these circuits accurately attenuate the output of the 5725A in ac voltage function to drive the 5700A Oscillator control circuitry. The sense amplifier section is essentially a gain-determining feedback element of a loop whose forward path consists of the 5700A Oscillator and the High Voltage Amplifier assembly (A3).

∙Analog Monitor Section

The analog monitor section is comprised of a bipolar output dac, a comparator, and a 16-channel multiplexer. The inputs to the multiplexer are scaled voltages representing critical parameters in the other 5725A assemblies, particularly those involving high voltage and/or current. Each input can be compared to programmed limits set by the dac under microprocessor control. The comparator gives simple go/no go decisions, which are then acted on in a more sophisticated way by the software. An additional function of the analog monitor section is to provide access to the B-RCL line used by the 5700A during 5725A calibration.

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