Capacitive high- voltage dividers

Choose the operating dielectric first because the calibrated division ratio and maximum pulse voltage depend on whether the divider is used in oil or air. The oil-calibrated models cover maximum pulse voltages from 300 to 500 kV, while their stated air limits range from 50 to 90 kV. The dedicated VD-305A-AIR is calibrated at 5,000:1 in air and is rated to 50 kV, with approximately 8 pF added capacitance. An oil-calibrated 5,000:1 unit can shift to about 11,500:1 in air, while the VD-500A can shift to approximately 23,000:1. Specify the intended dielectric environment when selecting the model rather than applying an oil calibration factor to an air installation.

The division ratio determines the approximate voltage delivered to the measuring instrument, so it must be considered alongside the instrument’s maximum input rating. At 5,000:1, an input of 300 kV produces a nominal 60 V output, while 50 kV produces approximately 10 V. At 10,000:1, 300 kV produces about 30 V and 500 kV produces about 50 V. These calculations use nominal ratios; the exact measured ratio marked on the individual divider should be used for final waveform scaling. Any external attenuation, cable effects and measuring-instrument limits must also be included when defining the complete acquisition chain.

Not without separately evaluating and calibrating the resulting measurement network. The quoted frequency ranges and droop rates are specified with a 1 MΩ load, so changing the input to 50 Ω can alter the division behaviour and invalidate the listed response figures. The internal 50 Ω resistor between the pick-up ring and output centre conductor does not by itself make the divider a conventional 50 Ω matched source. Additional cable, termination and instrument loading can also affect the observed waveform. For measurements that require 50 Ω termination, model the complete network and confirm its ratio, bandwidth and pulse response under the intended connection conditions.

Use the frequency range to assess the required spectral coverage and the rise-time figure to judge how closely the divider can follow a fast pulse edge. The VD-305A variants extend to 4 MHz and provide a usable rise time of 100 ns, while the VD-301 reaches 3 MHz with a 150 ns rise time. The higher-voltage VD-500A extends to 2 MHz and has a 200 ns usable rise time. Low-frequency limits and droop also matter when measuring wider pulses or signals with longer flat-top periods. Select a model with electrical headroom beyond the expected waveform content and verify the response of the divider, cable and measuring instrument as one system.

The nominal division ratio is stated as accurate to ±5% at the relevant calibration temperature, which is 35 °C for oil calibration and 22 °C for the air-calibrated VD-305A. Each unit carries its exact measured division ratio on the nameplate, and that value should be used instead of relying solely on the nominal 5,000:1 or 10,000:1 figure. The ratio is temperature compensated to ±1% over the range from 20 to 80 °C. This compensation describes divider-ratio variation rather than total measurement-system uncertainty. For tighter uncertainty requirements, calibrate the complete chain, including the divider, output cable, any external attenuation and the acquisition instrument.

No; the stated maximum pulse voltage ratings are based on pulse durations up to 5 µs. Longer pulses increase electrical stress duration, so the listed oil and air voltage limits should not be applied automatically outside that condition. A universal derating curve is not defined for the range, meaning longer pulse widths require an application-specific voltage review. Repetitive operation, duty cycle, dielectric condition and the surrounding high-voltage geometry may also need to be considered when establishing a suitable operating level. Treat the published rating as a short-pulse boundary and obtain an agreed derated voltage before using longer pulses.

The output connector shell must be connected to earth even though it is electrically isolated from the case when supplied. This isolation allows the installation designer to choose the location and arrangement of the earth connection, which can help control ground loops in the measuring system. The high-voltage conductor should have a smooth surface, adequate diameter and no sharp points or edges that could promote corona or arcing. The outer conductor of standard RG-58 coaxial cable can provide a practical smooth conductor for this connection. Keep the conductor as far as practicable from the acrylic chimney, divider case, output cable and other earthed structures.

The added capacitance ranges from approximately 8 pF for the air-calibrated VD-305A to 38 pF for the VD-500A. The standard and 10,000:1 VD-305A variants add about 18 pF, while the VD-301 adds approximately 28 pF. This capacitance can draw additional current during rapid voltage transitions and may influence a high-impedance or low-energy pulse source. The lowest-capacitance model is therefore not automatically the correct choice because voltage rating, calibration medium, division ratio and bandwidth must also match the application. Include the stated capacitance in the transient model of the test circuit and check that it does not materially alter the waveform being measured.