Fluke 5720A service manual 2-157. 1100V DC Range, HVDC Power Supply Filter Circuit

5700A/5720A Series II Calibrator

Service Manual

2-157. 1100V DC Range

Refer to Figure 2-26 for the following discussion. The High Voltage assembly (A15) amplifies the output of the DAC assembly (A11), set to the 11V range, by a gain of -100 to generate the 1100V dc range.

For operation in the 1100V dc range, DAC OUT HI and DAC SENSE HI from the DAC assembly, set to the 11V range, are tied together and connected to the input of the dc HV amplifier circuit by relay K1. The dc HV amplifier circuit, the HV dc output series pass and current limit circuit, and the HV dc power supply circuit on the High Voltage Control assembly (A14) constitute an overall amplifier with an inverting gain of 100. This gain is determined by the 70 kΩ input and 7 MΩ feedback resistors on the HR7 assembly.

High voltage dc output, generated on the High Voltage Control assembly (A14), is obtained by filtering an approximate trapezoidal wave. The overall loop gain of this amplifier divides this ripple so the high voltage dc output is clean and stable.

Signal routing to the front panel binding posts is done in the same manner as the ac 1100V range.

2-158. HVDC Power Supply Filter Circuit

The high voltage dc power supplies are generated by the High Voltage Control assembly (A14) in conjunction with the Power Amplifier assembly (A16). The High Voltage Control assembly generates an amplitude-controlled square wave, HVCL, from the magnitude control circuitry. The magnitude control circuit contains all the circuitry on sheet 2 of the High Voltage Control assembly schematic.

Signal HVCL is amplified by the Power Amplifier assembly with its output, PA OUT DC, connected to one side of the primary winding of transformer T1 by relay K1. The other side of the primary is connected to PA COM by R67. The secondary windings of T1 are connected to bridge rectifier CR1-CR4 by relays K9 and K6. The dc voltage from this rectifier is called +HVDC and -HVDC. Resistors R3-R5 form a 600 kΩ bleeder resistor for C1. Line HVDC is selected between +HVDC and -HVDC by relays K4 and K11. Line HVDC is connected to HV OUT by relays K5, K12 and K3.

During operation with a negative DAC voltage and a positive output from the High Voltage/High Current assembly, +HVDC is connected to HV OUT by relays K4, K11, K5, K12 and K3. +SP C is created from -HVDC by relays K4 and K8. During operation with a positive DAC voltage and a negative output from the High Voltage/High Current assembly, -HVDC is connected to HV OUT by relays K4, K11, K5, K12 and K3. -SP C is created from +HVDC by relays K4 and K8. Zener diodes VR4 and VR5 keep -SP C and +SP C, respectively, from exceeding 16V. The dc voltage level of +SP C and -SP C is controlled by the HV dc output series pass and current limit circuit on the High Voltage/High Current assembly. This in turn controls the magnitude of HVCL which sets the level of HVDC.

2-159. HV DC Output Series Pass and Current Limit Circuit

The HV dc output series pass circuit controls the level of +SP C when the high voltage output is positive, and -SP C when the high voltage output is negative. Typically +SP C and -SP C are approximately ±6.8V dc with ripple equal and opposite polarity from the HVDC ripple.

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