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A Low-Lethality Dielectric Strength Tester
A Low-Lethality Dielectric Strength Tester
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Old 19th June 2018, 10:34 AM   #11
Elvee is offline Elvee  Belgium
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Construction notes
  • High-voltage transformer
Here are the accessories I used to wind the transformer:

A Low-Lethality Dielectric Strength Tester-bdt10-jpg

The primary and secondary are wound separately. The primary is not particularly problematic: the 46t are hand-wound directly on the insulated ferrite core in three layers, and the coil is immobilized using cyanoacrylate glue.

I wound the secondary on a stationary drill, using the home-made flanges shown in the pic.
Their inner side is covered in polypropylene tape, to prevent adherence.
The core of the winding is a 10mm diameter silicone ring cut 1mm longer than the 5mm thickness of the finished coil: that way, when the screw is tightened, the core expands slightly, facilitating its removal when the coil is finished.

The coil is "scramble" wound: I made no attempt to achieve neat and orderly layers, I just guided the wire by hand, to try to achieve a reasonable regularity.
After the core was covered by the first layers, I immobilized them using cyanocrylate to avoid their disintegration at the removal stage.
During the course of the winding, I also poured glue drops from time to time, to reinforce the structure.
The turn number is approximate: my counter was broken and I just filled the flanges to the top (37mm dia.).

I stopped the winding using one more drop of glue, and I lightly impregnated it with modelling wax: I heated the coil by Joule effect and applied the wax against the outer layer.
I then let the wax penetrate for half an hour, and removed the heating power.
When everything had cooled down, I dismantled the winding props, and attached the teflon output cable using sewing thread and glue.
I then made an anticorona outer isolation using a ring of heatshrink tube punched in the middle.

A Low-Lethality Dielectric Strength Tester-bdt11-jpg

I secured the cable and obturated the hole using acrylic resin, fitted the sleeve of the inner termination, glued it and sprayed the whole winding in a polyurethane varnish.

When everything was dry, I assembled the primary and secondary: note that the winding directions have to be opposed: the outer side of the primary is the cold side, as is the inner side of the secondary.

This is necessary to avoid injecting noise in the breakdown detection circuitry.
Attached Images
File Type: jpg BDT10.jpg (197.1 KB, 101 views)
File Type: jpg BDT11.jpg (190.4 KB, 98 views)
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Old 21st June 2018, 03:49 PM   #12
Elvee is offline Elvee  Belgium
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  • The HV capacitor
I normally don't make my components myself, but I made this one because I had nothing suitable in my stock, and I didn't want to place an order just for that.

The base material is a polyethylene tape, 50mm wide and 0.14mm thick.

To build one electrode, I start with 200mm of tape, and I stick a strip of aluminum foil, 40mm wide, just in the middle, for the whole length.
I prepare the termination, a teflon cable stripped for 20mm having its individual threads arranged as a fan.
I then fold the tape assembly for the whole length, with the termination inside the fold, in contact with two sides of the aluminum.

I make the other electrode in exactly the same way, and carefully laminate them flat.

The two electrodes are then wound on an insulating tube or rod, approx. 10mm dia.:
A first electrode is taped to the cylinder, then the other, about 20mm further down.
Laterally, the two 20mm metal strips must be perfectly overlaid.
The winding is then completed, as tight as possible and immobilized with a bit of tape.
The whole thing is covered with a heat-shrink tube and retracted.

It is then dipped extremely slowly (1/2 hour) into a bath of liquid wax.
It is dipped verticaly, following the axis of the cylinder.
The purpose of this operation is to drive as much air as possible out of the capacitor.
The bath is then let to cool, and just before it solidifies, the capacitor is pulled out.

The result is a capacitor having two sheets of high quality polyethylene as a dielectric.
Attached Images
File Type: jpg BDT12.jpg (227.8 KB, 70 views)
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Old 21st June 2018, 04:03 PM   #13
Mike Gergen is offline Mike Gergen  United States
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Nice. I'm impressed.
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Old 25th June 2018, 07:35 AM   #14
trobbins is offline trobbins  Australia
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Chicken or the egg - HV tester first, or diy HV capacitor first !
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Old 27th June 2018, 08:54 PM   #15
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by trobbins View Post
Chicken or the egg - HV tester first, or diy HV capacitor first !
I had prior experience with this particular plastic film...

  • High-voltage precautions

The 10 kilovolt level reached in this tester is not huge, but it is higher than 99% of DIY projects, and deserves due respect.

Initially, I had cleared away all the rows of copper pads surrounding the HV sections, but to my surprise, there were flashovers at voltages as low as 6~7kV.

Yet, the clearances and creepage distances should have been ample enough, but a lousy perfboard having its copper milled off behaves extremely poorly in this respect.
I then maniacally chased any copper I could still eliminate, but there were still breakdowns at 7~8kV.
In the end, I resorted to a kerf of the PCB in the most critical area, supplemented by an insulating shield and a polyurethane coating.
With a good quality, well cleaned epoxy PCB, clearances alone should be enough

All of that eventually cured the problem, and allowed safe operation at at least ~14kV: with a 10kV output, the voltage at the output of U4 is 7V, and even when the regulation is lost, the zener D2 limits the voltage to ~14kV.
Without the zener, the voltage could reach 20kV, which is clearly unsafe.

14kV might still look excessive, but one has to take into account temperature variations, and more importantly, the additional loading when a HV probe is connected to measure the voltage: the burden of the 100MΩ probe then becomes dominant, compared to the 132MΩ of the feedback divider, and the minuscule 220MΩ of the output resistor.

A jumper allows the connection of such a probe without tripping the detection circuit.
Conversely, another jumper inhibits the HV generation, to be able to debug the circuit comfortably, without the fear of HV being present.

Note that the power generated and the energy stored in the 560pF capacitor present no danger, but any contact with anything else than the normal, protected output could be somewhat unpleasant, to say the least.


I have built the circuit into an insulating case, and even the front panel is insulating because it makes things so much simpler, but it would be also possible to use metal-work: drastic insulation measures would be required though
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Old 7th July 2018, 09:45 PM   #16
Elvee is offline Elvee  Belgium
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  • Component selection

Most of the components used in this project are commodities. There are two exceptions though: the HV ones, and the resistors part of the BCD DAC.
The HV cap has been covered, the best solution is to find an off-the-shelf model, and the resistors also need to be specific HV types: if you use the Philips taxonomy as a guide, suitable types are VRxx.
For example, the four 33MΩ of the feedback divider are VR37, withstanding 3.5kV each.

For the 10x 22 MΩ, I used Yageo types, similar to the VR25, withstanding 1.5kV.

It would be possible to use ordinary CF or MF types rated at 250V, but the number required would be non-negligible....

DAC resistors
The values indicated on the schematic are theoretical ideals, supposing the CD4518's have a zero output resistance, which is not the case obviously.

In reality, they have around 250Ω output resistance, and this has to be factored in the selection.
Here is the way I did it, starting from ordinary 1% (good quality) types:

I picked 18 240KΩ in the drawer and I ranked them for accuracy, including the additional 250Ω.
I then saw that the four most accurate were not within 25ppm of the target value, and I tested some more samples to attain the target..
I then picked 120kΩ resistors until I found values exactly half of the 240kΩ for four of them (including the correction).

Finally, I populated the DAC according to the measured accuracy of the resistors: the most accurate for the heaviest weights.

The few other odd-valued resistors were made up from series combinations.

That way, I achieved a 1 in 104 resolution, but in reality that's a luxury: the 4-digit are required to avoid range switching, but an error of 2 or 3V on a 8kV breakdown voltage is in fact perfectly acceptable, and at lower voltages, the error will become unnoticeable.
I made the thing completely accurate simply because I could do it, but it it is certainly not necessary to do so
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Old 7th July 2018, 10:53 PM   #17
trobbins is offline trobbins  Australia
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Elvee, with that level of resolution, how have you approached confirming the accuracy level of the generated HV supply ?
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Old 8th July 2018, 05:07 PM   #18
Elvee is offline Elvee  Belgium
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I calibrated the output at 5kV and I then checked the real output against the displayed value for a number of voltages, higher and lower than 5kV.
The values were in good agreement, except that there is a small residual offset error, because the ~1mV offset of the regulator opamp has not been trimmed or compensated.
Of course, this couldn't catch isolated non-monotonicity errors caused by an incorrect resistor matching, but if the matching job has been carried out properly, such errors should not be present.
To make sure I made no silly mistake though, I also made a sanity check: I clocked the DAC at a rate high enough to allow a comfortable oscilloscope observation, and I examined the resulting ramp at a high magnification ratio, particularly concentrating on the large bit weights switchings, to make sure there was no obvious anomaly.

The tempco of the VR37 resistor is quoted as +/-200ppm/°C, and this would be a limiting factor, but the actual tempco is clearly much lower, comparable to good MF resistors.
Note that not all high voltage resistors are that good: for example, the Yageo ones are less stable.

Of course, both the measurement and the regulation are made upstream of the 220 meg output resistor, and in actual use if there is the slightest leakage current before the breakdown occurs, it will cause a deviation from the displayed value, but that's the principle of this instrument
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Old 9th July 2018, 12:26 AM   #19
trobbins is offline trobbins  Australia
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You have a 5kV reference?

Could you please go through the DUT breakdown detection process again wrt to U11, and what level of current through the DUT is allowed for parasitic capacitance charging (which would depend on generator voltage ramp up rate) and what rate of DUT breakdown voltage (or current ramp up rate) is needed to trigger a breakdown detection, given C12 filtering across R19.

I have been doing and setting up for some valve and ss diodes PIV testing. I'd anticipate your tester may be fine for such an application, whether for valve or ss diodes, as the diode loading capacitance would typically be very small, and the DUT conduction current suitably low, up until vacuum related or avalanche related breakdown processes started - and the DUT current would be limited to well below damage level (eg. for ss diodes). I have done a few quick checks of ss diodes with an insulation resistance meter at 1kVDC - even modern 1N4004's didn't show conduction more than 1uA - and was setting up a cheap battery powered dc/dc to provide additional circa 500VDC to go in series with the insulation meter's 1kV.

Last edited by trobbins; 9th July 2018 at 12:28 AM.
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Old 9th July 2018, 05:38 PM   #20
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by trobbins View Post
You have a 5kV reference?
No, I don't: I use a high resolution multimeter (Datron 1071) and a high voltage probe that I can calibrate at safe voltage (100V), whilst retaining a sufficient resolution.
I set the voltage to ~5kV in pause mode, and I measure the actual output (upstream of the 220 meg, of course, and with the detection inhibition jumper in place).

Quote:
Could you please go through the DUT breakdown detection process again wrt to U11, and what level of current through the DUT is allowed for parasitic capacitance charging (which would depend on generator voltage ramp up rate) and what rate of DUT breakdown voltage (or current ramp up rate) is needed to trigger a breakdown detection, given C12 filtering across R19.
U11 is wired essentially as a transimpedance amplifier measuring the DC return current of the HV supply.
However, it is somewhat degenerated because I felt the need to include R28 to isolate further the opamp from HV events, event though D8, D9, C12 already provide a certain level of protection: when you have some experience with HV, you tend to become paranoiac about these details.
The degeneration doesn't fundamentally change the way the TIA operates, it just makes the calculations of gain a bit more complicated.
Because the TLC274 has an output capable of reaching 0V, the voltage on C13 under quiescent conditions is effectively zero (even a LM324 almost works... but not quite).
The time-constant imparted by C12 is ~1.3ms, smaller than the 2.7ms of C14, reasonably lower than the maximum possible clock rate (6ms).
Going much lower would not really make sense, because of the 220meg resistor and the 560pF supply filter cap.

At the lowest practical detection threshold of 100nA (which could easily be lowered if necessary) and the maximum ramp rate of 10kV/min, the maximum capacitance in parallel with the DUT is (100e-9*60)/10e4=600pF.
At the 1kV/min rate, this becomes 6nF, and in manual mode, it can increase to ~18nF


Quote:
I have been doing and setting up for some valve and ss diodes PIV testing. I'd anticipate your tester may be fine for such an application, whether for valve or ss diodes, as the diode loading capacitance would typically be very small, and the DUT conduction current suitably low, up until vacuum related or avalanche related breakdown processes started - and the DUT current would be limited to well below damage level (eg. for ss diodes). I have done a few quick checks of ss diodes with an insulation resistance meter at 1kVDC - even modern 1N4004's didn't show conduction more than 1uA - and was setting up a cheap battery powered dc/dc to provide additional circa 500VDC to go in series with the insulation meter's 1kV.
The tester will work well when the DUT breaks down in a brutal way, ie it suddenly becomes a negative resistance: this is often the case for HV devices, but for components exhibiting a progressive increase of the leakage current or a zener-like characteristic like controlled-avalanche rectifiers, the 220 meg will affect accuracy.
If you intend to use the tester for such applications, it would probably be possible to correct the displayed voltage according to the threshold current setting, but this would add some complications.
The other option would be to measure the actual output voltage, downstream of the 220 meg, but I don't think it would be simpler
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Last edited by Elvee; 9th July 2018 at 05:41 PM.
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