 ### Experiment 09 (24/10/02) - Measurement errors with pulsed dc sources

© Engineer Xavier Borg - Blaze Labs Research

The aim of this experiment is to confirm the error between the real voltage and the reading of a DC voltmeter with signals of different waveforms, and most important how to obtain the real measurement. This is important, because when we read the hv voltage that we supply to our hv devices, the hv probe is not the only device which introduces errors. One has to consider whether the DC signal being measured is pure dc or any type of pulsed dc.

Equipment used

Signal generator set at 1 V peak dc @ 20 kHz
Oscilloscope set at 0.1 V/div
Standard (not True RMS type) Multimeter

Setup

The signal generator is set at 1 V peak output, and at a frequency of 20 khz. The output from the signal generator is fed to an oscilloscope in parallel with a multimeter, in parallel with a 10 kOhm resistive load. Various waveforms are then selected to check the results on both oscilloscope and multimeter.

Pulsed sine wave testing

This is the kind of waveform one would normally get from a monitor's hv output at 20 khz. As you can see, at 1 V peak signal (as shown on the oscilloscope), the multimeter shows 0.226 V. If this was a reading from a hv probe, one would read 22 kV, when the actual peaks reach 100 kV! Of course this reading varies according to the multimeter frequency response, and duty cycle of the signal. Pulsed square wave testing

This time, a square wave of 20 kHz, 1 V peak is fed to both meter and oscilloscope in parallel. As you can see, at 1 V peak signal (as shown on the oscilloscope), the multimeter shows 0.436 V. If this was a reading from a hv probe, one would read 43.6 kV, when the actual peaks reach 100 kV! Pulsed triangular wave testing

This time a triangular dc waveform at 20 kHz, 1 V peak is fed to the instruments. At 1 V peak signal (as shown on the oscilloscope), the multimeter shows 0.193 V. If this was a reading from a hv probe, one would read 19 kV, when the actual peaks reach 100 kV! Conclusions

1. The voltage of a pulsed dc source cannot be simply measured by connecting an hv probe to a multimeter.
2. Errors will depend on type of multimeter used. Its input capacitance, and its internal integration circuit may give further frequency dependent errors, which get worse with higher frequencies.
3. The best way to measure pulsed dc, would be to first obtain the 'derating' factor for the particular waveform, by using a similar setup shown above, and then apply the factor to the voltages read from the hv probe. For example, if the voltage of the primary winding of my hv flyback was as shown in the pulsed sine experiment above, then I would apply a derating factor of 1/0.226 to the multimeter reading to get the correct value.