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A Low-Lethality Dielectric Strength Tester
A Low-Lethality Dielectric Strength Tester
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Old 5th June 2018, 03:19 PM   #1
Elvee is offline Elvee  Belgium
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Default A Low-Lethality Dielectric Strength Tester

This tester comes as a natural companion for the thermal ohmmeter:
A Thermal Ohmmeter

To fully characterize insulating interface materials, one needs to know both thermal and electrical performances.
Of course, this tester can also be used for many other purposes, including totally unrelated ones, for example as a lab bias source for experimental electrostatic loudspeakers.

It is fully automatic, and reaches voltages of up to 10kV, but its most salient feature is its low-lethality: unlike many testers, it inflicts minimal or even zero damage to the sample tested, allowing many rounds of testing, reworking, etc on the same sample.

The low damage feature is achieved thanks to a minimal energy level: the peak current, power and duration of the discharge are very small, leading to a commensurately low energy delivered to the sample.

The first means of energy-limitation is a very large series resistance, totaling 220MΩ.
At the maximum output of 10kV, this result in a current never exceeding 45µA, and the worst-case instantaneous power dissipated within the sample is limited to ~115mW.
The duration of a discharge event is also limited: a detector inhibits the HV converter as soon as an event is detected, and the energy stored in supply capacitors is minimal: the supply is a directly rectified flyback, without intermediate multiplier stages and their capacitors.
The filter capacitor itself is minimal: 560pF.
At 10kV, it can store a maximum of 28mJ, of which 14 at most can be delivered to the sample, because of the series resistor.
As the detector's response time is 5 ~ 10ms, the HV converter will be turned off well before the capacitor has discharged significantly, thus the theoretical worst-case energy delivered to the sample is always going to be <15mJ, and in practice will be much lower, 1/10 or 1/100 or even less, because the voltage will generally not be maximal, and because of the realities of breakdown mechanisms.

How effective are all these measures?

They work stunningly (no pun intended!) well:
for example, with the tester in "hold" mode at 8kV, if you grab the ground wire with one hand, and near the "hot" terminal with the index of the other hand, at some point you see the detection has tripped, but you can only know by looking at the instrument: you feel absolutely nothing, not even a tingle, and there is no visible spark flying.
More to the point, if a fragile, carbon-rich sample like ordinary paper is tested many times in a row, the detected breakdown voltage is always in the same region, sometimes higher, sometimes lower, but there is no downwards tendency, meaning there is no significant carbonization or similar damage.
Attached Images
File Type: png BDT2.png (60.0 KB, 254 views)
File Type: jpg BDT1.jpg (290.8 KB, 251 views)
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Old 7th June 2018, 03:57 PM   #2
Elvee is offline Elvee  Belgium
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  • Power supply
The supply is slightly unusual because I didn't have the ideally suitable transformer in my stash, and I resorted to a 16V/20VA, the closest match.
With it, I couldn't use a normal, capacitor-only filter: the DC current required is a bit over 1A, and this would have resulted in a 2A rms current in the secondary, clearly excessive.
In addition, the pre-regulator DC voltage would have been ~22V, meaning lots of heat to dissipate.
Fortunately, I also had a 10mH choke, and with it, the rms current became just acceptable, and the full load voltage was 16.5V, ideal for the 15V LDO regulator.
I used a home-brew regulator rather than a monolithic one, because the supply voltage also serves as a reference for the DAC and needs to be stable and accurate.
Other than that, it has nothing special, but with the BD435, a dropout of 150mV @1A is sufficient to let it regulate, which is nice.
  • EHV generator
This generator is rudimentary and dreadfully inefficient: the HV winding is made on a simple cylindrical core, not a closed magnetic circuit, and the regulation is performed by varying the DC supply of the flyback stage, not by PWM.
The reasons for these choices are availability, simplicity, but also the quality and agility of regulation: a linear supply is superior to PWM in this respect, and it is more important than efficiency, because of the low level of power delivered.
The flyback switch, M1 receives a constant squarewave on its gate, and chops whatever voltage is presented to L2.
C10 is the classical FB capacitor, like in CRT supplies. It needs to be polypropylene.
The secondary voltage is rectified using a "pencil", selenium stack rectifier.
More modern options like Si diodes are of course usable, and would have a better efficiency, but I had a few available.
The resulting DC is filtered by a home-made 560pF/20kV.

A divider made of a string of high-voltage 33MΩ samples the voltage, bringing it to the wiper of R26, and the FB port of the regulator U4.
Its reference port receives the control voltage from the DAC.

The cold side of the EHV supply is not directly tied to the ground, but to the breakdown detector U11, thus when a current is drawn from the test output, R28 pulls a negative current from the (-) input and as a result, the output goes positive (BTW, I see that I messed up this part of drawing, there will be a version 1.2)

In the mean time, here is already the second part of the circuit, comprising the small control logic, the BCD-DAC, and the display
Attached Images
File Type: png BDTL.png (87.8 KB, 232 views)
File Type: jpg BDT3.jpg (544.1 KB, 230 views)
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Old 7th June 2018, 08:55 PM   #3
Elvee is offline Elvee  Belgium
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Here is the corrected schematic, since the "Edit the first post" feature does not seem to work properly.
If a moderator sees this, he can replace the schematic of the first post by this one (and delete this post, as it will become redundant)
Attached Images
File Type: png BDT2.png (68.7 KB, 221 views)
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Old 8th June 2018, 01:55 AM   #4
trobbins is offline trobbins  Australia
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Good on ya Elvee for yet another interesting test instrument.

I don't recall 'gimped' as a winding technique ??

Why do you refer to C10 as a 'feedback' capacitor - I would expect it to have more a dV/dt and peak V snubber type action.

I'm sure you will describe how you made your step-up transformer and 20kV cap - can't wait for that.

I guess somebody else would have been preparing this thread if you hadn't made the current limiting acceptable for human testing!
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Old 8th June 2018, 04:08 AM   #5
JMFahey is offline JMFahey  Argentina
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Excellent Elvee.
Thanks *a lot* for your very useful contribution.
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Old 8th June 2018, 09:50 AM   #6
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by trobbins View Post
I don't recall 'gimped' as a winding technique ??
Gimped is the translation of a very technical french term, "guipé", which means that the main, central wire is wrapped inside other wires, in this case synthetic silk.
It is useful for reducing the capacitance, because the winding is more aerated, and also to build odd-shaped windings, because of the friction.

Here, both features are useful.

There is perhaps a more commonly used term than "gimped" in english?

Quote:
Why do you refer to C10 as a 'feedback' capacitor - I would expect it to have more a dV/dt and peak V snubber type action.
FB means flyback: it makes the stage operate in class E or quasi-class E.
When the MOS opens, a positive half-sine arch is generated, and the voltage returns to the ground, where it stays clamped by the body diode.

A Low-Lethality Dielectric Strength Tester-bdtx-png

Here, with the low repetition rate, I am not sure it works in pure class E, but even if the cap voltage returns to Vsupply before the next pulse, it is relatively unimportant since the voltage is limited to few volts.

Quote:
I'm sure you will describe how you made your step-up transformer and 20kV cap - can't wait for that.
I certainly will

Quote:
I guess somebody else would have been preparing this thread if you hadn't made the current limiting acceptable for human testing!
Even without the limitation, the power output is sufficient to deliver a good jolt, not kill somebody

Quote:
Originally Posted by JMFahey View Post
Excellent Elvee.
Thanks *a lot* for your very useful contribution.
Thanks
Attached Images
File Type: png BDTx.png (75.4 KB, 192 views)
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Old 8th June 2018, 04:04 PM   #7
Elvee is offline Elvee  Belgium
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Most of the Power/HV section is obvious enough for not requiring detailed explanations, but some details probably look puzzling:

-A 2kV spark-gap is connected between the earth and the ground (0V).

What is the point of such a bizarre arrangement?

First of all, the instrument is designed as a class 2 device, with proper insulation and an insulating case, therefore not requiring earthing, and although the power cord is equipped with an earth wire, it is not permanently connected to a node inside the instrument.

When the tester is connected to a DUT, it is possible for the DUT in question to be fully floating, but it can also have a path to the earth. Such a path could be explicit, but it can also be more informal, like a leakage of several gigaohm, caused by the surface the DUT is placed on for example.

If the cold terminal of the tester is connected to the earth-related side of the DUT, no particular problem arises, but if the connection is reversed, the 0V of the tester will in fact assume a potential of -Vtest wrt. the earth.

Since the average potential of the mains is the earth, the transformer will see this potential between its secondary and primary windings, which could initiate a discharge and damage the insulation.
The spark-gap limits the voltage to 2kV which in practice is safe enough for a split-bobbin transformer.

Wouldn't it be simpler to permanently connect the earth and the 0V?

It is in fact the normal, preferred configuration, and the earth and 0V terminals are side by side on the front panel, and are normally strapped together, but I wanted to keep the option of a floating instrument open: it could be useful when testing some parts of a large stationary equipment where earth is present, but not explicitly connected to any side of the section tested.
For example, imagine you want to test a terminal block inside a machine: none of the terminals is connected to the earth, but the block itself is attached to an earthed chassis.
If you test the terminal-to-terminal breakdown voltage with the tester earthed, you are in fact going to test the terminal to earth insulation, which might be smaller than the actual terminal to terminal insulation.


-The ground connection has a series resistor of 6.8K and a ferrite bead, and both the ground and earth connections pass through a common ferrite ring.

Compared to the 220MΩ limiting resistor, the added impedances looks ridiculously small, so what's their point?

The 220MΩ resistance does statically limit the current to <45µA, but if the DUT is capacitive, that is not necessarily the case: obviously, if you push a capacitor to its breakdown voltage, the main discharge current will be caused by the capacitor itself, and could reach many amperes, but this current will be completely internal, and won't return through the tester.
There is however one case when the current will return through the tester: when the DUT has a significant capacitance wrt. the ambient space: if a large isolated conductive object is charged through the 220MΩ, the discharge current will return to the earth through the earth/ground connection, the transformer's capacitance, and the spark-gap, if the earth connection is not used.
The very steep pulses of this discharge current did funny things to the digital section, which is why I had to add damping/limiting components.

-Finally, and I see that I will have to prepare a version 1.3 of the schematic, what is the purpose of R29/C15?

In fact, R29 connects to the output of the opamp.
When the DAC out returns to zero, after having reached the maximum, U4 injects a transient into R26 through the loop compensating components, and the kick-back of this transient generates a false detection event in U11 U3.
This is a minor nuisance, because it cannot be confused with a real event, but it forces you to clear an inexistent breakdown detection.
The "good" solution would have been to buffer the wiper of R26, but I had no spare opamp left, and I had no room to add one, so I added this compensation network, which is not perfect but sufficient.
The output of U4 is not ideal, because it regulates the HV and contains a little ripple compensation, but that was the only low-impedance node available for that.
The consequence is a detection floor limited to 100 ~ 200nA, because of the added noise, but that's at the resolution limit of the potentiometer anyway, thus little harm is done
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Old 8th June 2018, 05:13 PM   #8
Elvee is offline Elvee  Belgium
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Here is the final (?) version of the schematic:
Attached Images
File Type: png BDT2.png (61.3 KB, 32 views)
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Old 10th June 2018, 08:58 PM   #9
Elvee is offline Elvee  Belgium
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  • Digital section
The core of the digital part is the 4-digit counter.
The outputs are used for the DAC, a variation on the R2R theme, and for the display.
The 7-segments are handled directly by 4543's: I don't bother to use limiting resistors, it is much easier to rely on the self-limiting characteristics of the outputs.
Here with a 15V supply, the current and hence the dissipation would have been uncomfortably high which is why I used a 8.2V zener D10 in series with the display common.
The method might look brutal, but it works very well provided all the 4543 have the same origin.

The main clock is generated by oscillator U3; two basic rates are possible: 1kV/minute is for "serious" tests, requiring repeatablity and accuracy, 10kV/min is for exploratory purposes, because 1kV/min is slow, especially if the breakdown voltage is high.
It is also possible to manually fine tune the voltage, at ~1/3rd the nominal rate.
The advancement can also be halted completely; the implementation is not very orthodox: the input of U3 is simply left floating, but it does not seem to pose a problem, and even if there was an increment every several minutes, it wouldn't be problematic (but nothing of the kind has been observed).

The top voltage can also be limited thanks to a crude address decoding.

Two flip-flops, U4, U5 register the run/stop and breakdown conditions.
When a breakdown condition is detected by the analog section, it freezes the count, blanks the DAC output thanks to an analog switch, and makes the display blink rapidly thanks to U6.

Here is a more detailed view of the EHV section and resistors strings:

A Low-Lethality Dielectric Strength Tester-bdt4-jpg


A Low-Lethality Dielectric Strength Tester-bdt5-jpg


Here is the 2kV spark-gap:

A Low-Lethality Dielectric Strength Tester-bdt6-jpg

This is the converter controller:

A Low-Lethality Dielectric Strength Tester-bdt7-jpg
Attached Images
File Type: jpg BDT4.jpg (533.9 KB, 152 views)
File Type: jpg BDT5.jpg (542.0 KB, 152 views)
File Type: jpg BDT6.jpg (522.3 KB, 152 views)
File Type: jpg BDT7.jpg (526.7 KB, 152 views)
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Old 17th June 2018, 05:01 PM   #10
Elvee is offline Elvee  Belgium
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Here is a convenient accessory for the tester: it is a sample-holder.

It allows testing under controlled and ~repeatable conditions: same pressure, same geometry of the test electrode, etc

A Low-Lethality Dielectric Strength Tester-bdt8-jpg

A Low-Lethality Dielectric Strength Tester-bdt9-jpg

And here is a short video showing the tester in action:

YouTube
Attached Images
File Type: jpg BDT8.jpg (221.0 KB, 117 views)
File Type: jpg BDT9.jpg (185.9 KB, 118 views)
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