Fluke 5720A service manual High Voltage Assemblies A14 and A15, 2-156. 1100V AC Range

5700A/5720A Series II Calibrator

Service Manual

2-155. High Voltage Assemblies (A14 and A15)

The High Voltage/High Current assembly (A15) and the High Voltage Control assembly (A14) are used in with conjunction with other assemblies in the calibrator to generate the ±1100V dc, 1100V ac, and the 2.2A ranges. The two assemblies work together in generating these ranges. This theory of operation explains how these ranges are generated and discusses the individual circuits on each assembly. Refer to the High Voltage/High Current assembly simplified schematics (Figures 2-24 through 2-28) or the schematic diagram to understand this theory of operation better.

2-156. 1100V AC Range

Refer to Figure 2-25 for the following discussion. The ac signal generated by the Oscillator Output assembly (A13) is amplified by the Power Amplifier assembly (A16). The output of the Power Amplifier, PA OUT DC, is routed to the High Voltage Control assembly (A14), where it is further amplified by transformer T1 to generate the 1100V ac range. This high voltage signal from T1 is also the feedback signal to the Power Amplifier assembly. Relays K14-K16 connect the transformer windings in series during operation below 120 Hz.

Line PA OUT DC is connected to one side of the primary winding of step-up transformer T1 by relay K1 on the High Voltage Control assembly (A14). The other side of the primary winding is tied to PA COM through R67. One side of the secondary winding is tied to PA COM by relays K9 and K6. The other side of the secondary winding, the high voltage ac signal, is connected to HV OUT by relays K5, K12 and K3. Line HV OUT is connected to the OUTPUT HI binding post by relays K9 and K1 on the motherboard.

This high voltage ac signal is also connected to HVAC by relay K5. Line HVAC is the feedback signal to the Power Amplifier assembly. During high voltage operation, the input resistance of the Power Amplifier assembly is 5 kΩ. Line HVAC is connected to the feedback resistance, which is a series of resistors totaling 500 kΩ. This creates a gain of 100 to the Oscillator Output.

The SENSE HI binding post is connected to HV SENSE by relays K3, K2, and K10 on the motherboard. HV SENSE, which is tied to HV OUT at the load, is connected to INT HV SNS by relay K13 on the High Voltage Control assembly (A14). INT HV SNS is routed to the High Voltage/High Current assembly (A15), where it is connected to the ac sense buffer circuit by relay K6. This circuit attenuates the high voltage signal by 100 and connects it to OSC SENSE HI through relay K4B. The level of attenuation is determined by the 7 MΩ input and 70 kΩ feedback resistors on the HR7 assembly. OSC SENSE HI is used by the Oscillator Output assembly, which adjusts its output signal to maintain an exact feedback signal level. HV OUT and HV SENSE are routed to the High Voltage assembly (A15) where components CR9, CR10, and R33-R35 keep the voltage difference between them at 0.7V should they become disconnected at the load. Relay K11 connects HV OUT to HV SENSE during calibration in the ac function.

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