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Testing HV multipliers with low frequency

Ed Breya
 

I finally got around to checking out another application for the HVCT. I wanted to see if the high frequency (20-100 kHz) type HV multipliers could be checked with 60 Hz HV. Of course they will still multiply, but running at 300-1000 times lower frequency than normal makes it a little tricky. The output is quite weak in current, so I used one the highest R HV probes I had on hand - an old Sencore with 1485 megs up front. I built a little switched terminator for it so it can run into the RET or DCL ports, to provide a scope display or DVM reading. Running into 1 meg instrument loads made it so I couldn't nicely scale the readout units, as I would have preferred, so the results from this probe are 10 kV/V, rather than 1 kV/V as on the built-in probe. It does however simplify the DVM measurements, since all expected readings are in the native 1V or 10V high-Z ranges, and no Rt changes are needed for the attenuation input resistance.

It works quite nicely. Currents are tiny, but the volts are plentiful. It's easy to figure the multiplication factor and breakdown limits. The guinea pig for this is a HP 0960-0117, from an HP182 scope. In normal use it runs about 40 kHz, 6300 Vpp input, and +19 kVDC output. It tested out just fine, and could easily reach over 40 kV out before breakdown started showing.

I have posted four new pictures in the HVCT album, showing some of results.

The "12 kV..." one shows the input "-DC sweep," which is actually just AC forcibly offset negative. The drive is set at about 12 kVpp, and the output is about 30 kVDC average - with about 15 kVpp ripple! This is of course due to running at only 60 Hz. The apparent multiplication is around 2.5, rather than the proper 3, also due to running so slow, and the probe load. Even though its R is quite high, it still takes enough current to prevent the output from reaching its (no load) full potential, which would be about 36 kVDC, with no ripple. Also note the little jagged spots in the output ramps. I'm not quite sure how to interpret them yet, but I believe they show the internal activity of the multiplication stage rectifiers through each cycle - pretty interesting.

The "-DC sweep..." one again shows the drive input, along with the input current, with the average DC output set right at 30 kV as read on the DVM. The little jags really show in the current.

The other two pictures just show the setup, and how it looks with AC drive, which is much wimpier than the offset DC drives, for technical reasons I'll explain eventually. The AC one would be a reasonable sinewave, but the multiplier load lugs it down greatly at the charge pumping parts of the cycle.

Anyway, I was pleasantly surprised at how well this simple setup works to assess the condition of this kind of multiplier.

Ed

Ed Breya
 

Oops - a favorable correction is needed. I found on further experimenting that there was quite a bit of capacitive coupling from the drive side wiring and the multiplier body into the middle and low end of the probe, which exaggerated the apparent ripple on the HV output. Rearranging stuff and pushing the probe and HV connection way out beyond the deck edge made it a truer reading. Instead of 15 kVpp shown in the "12 kV in..." picture, it now shows about 3 kVpp, or about ten percent ripple on 30 kVDC. It is also more rounded (like the sine drive) rather than triangle-shaped, and the little jags are still there. I'll put up a new image when I get a chance.

So, the effectiveness in getting high DC out of this multiplier at low frequencies is better than I thought, and the probe resistance is high enough to not upset things too badly.

Ed