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A 75kV 1000:1 Voltage Divider With 3G Input Impedance

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Proud Mary

Wed Jun 09 2010, 09:01PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

A 75kV 1000:1 Voltage Divider With 3G Input Impedance

Poor quality voltage dividers often draw so much current from the circuit under test that voltage readings are rendered meaningless.

The commonplace 1000:1 EHT probe intended for television service is almost invariably based around a 1G resistor, which means
that were it to be used - recklessly - at 50kV, it would be sucking 50uA out of the circuit to be measured.

One would have to build a larger EHT PSU simply to provide the current to allow reasonably accurate voltage measurement to take place, a
very silly state of affairs.

With this in mind, I decided to construct a voltage divider to meet my own requirements.

Firstly, I would increase the value of the dropper resistor from the usual - but woefully inadequate 1G - to 3G.

50kV is the maximum anode voltage I ever use with any of my XRD and XRF tubes - and more usually between 20kV and 40kV, according to the needs of the experiment, so I decided that a Voltage Divider that remained completely within all its ratings up to 75kV would provide an adequate margin of safety.

I also decided that I wanted not only the usual tap to match the 10M input impedance of DVMs, but a second tap to match the >100M input impedance of my Fluke 887AB Differential Voltmeter, and other very high input impedance instruments.

Here is a simplified outline of the circuit I have decided upon, to which refinements and safety features will be added, in the sweet bye and bye.

1276114478 543 FT0 Voltage Divider Schematic 400

Even at 75kV, it will draw only 25uA - not much of a burden on the most modest of EHT PSUs.

Selection of Resistors

I obtained ten 1G 20% 25kV 4W resistors, with a view to selecting the best three to get as near to 3G as possible.

The 25kV working voltage rating matched my spec for a series of three for a 75kV maximum, and the 4W rating was more than adequate for the 1.875W which would be dissipated in the 3G resistor chain at this maximum voltage.

The ten resistors from which the selection was to be made were first thoroughly cleaned with methylated spirits, and thereafter handled only with gloved hands, since even trace amounts of the salt in sweat would degrade the surface resistance of the vitreous coating, acting as a shunt in parallel with the desired resistance. I fastened labels on them with cable ties so as not to leave adhesive traces on the dielectric surface.

1276115590 543 FT0 Voltage Divider Resistors On Oven Top 400

I then measured the resistance of each using a Takeda Riken TR8601 High Megohm Meter on the 1000V range.

1276115713 543 FT0 Voltage Divider Resistor On Stand Meter Below 400

As you can see, in the first few cases, I wound a guard electrode round the resistor to null surface leakage. Using the guard improved the quality of the reading only very very slightly (for which I thanked my attention to the very thorough cleaning of the resistors) so I decided not to pursue it further,
since it would certainly change again when submerged in dielectric oil.

1276115997 543 FT0 Voltage Divider 1g Resistor On Stand 400

I recorded the following resistances for the ten components under test:

A: 1G21
B: 1G13
C: 1G01
D: 1G19
E: 0G91
F: 0G88
G: 1G15
H: 0G92
I: 0G20 - a total dud, though just the same as the others to look at.
J: 0G97

With the exception of Resistor I, a manufacturing failure, all the rest except Resistor A (1G21) were within the claimed 20% tolerance, and so no cause for complaint.

Here is Resistor H's reading of 0G92 to show the layout of the meter:

1276116478 543 FT0 Megohm Meter Reading 920m 400

Of the remaining nine resistors, I chose H (0G920), J (0G975) and B (1G130) to make the series value of 3G025, the nearest I could get to my target 3G.

I then screwed these together, put them on the stand, and after letting the meter settle for ten minutes, it sank from the tiniest bit above 3G, to 3G itself, where it stayed for the next half hour, until I switched it off. Applause in heaven!

Next: the resistor core assembly

To be continued

Proud Mary

Fri Jun 11 2010, 02:38AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

A 75kV 1000:1 Voltage Divider With 3G Input Impedance

The Resistor Core Assembly.

Nylon-66 discs of 45mm OD were prepared and bolted into the 3G resistor line to form the upper element of the voltage divider chain.

The resistor core assembly slides into a 50mm OD, 46mm ID, acrylic tube that will later be filled with vegetable dielectric oil.
Clearance of 0.5mm all around the nylon discs prevents the discs acting as pump plungers and obstructing convection, with the possible creation of pockets of unequal pressure during temperature change.

Next: the HV terminal assembly

IntraWinding

Fri Jun 11 2010, 05:55AM

Registered Member #2261 Joined: Mon Aug 03 2009, 01:19AM
Location: London, UK
Posts: 581

Very nice. I'm buying shares in 25mm acrylic tube manufacturers!!!

I don't like meths though! It's 'denatured' with a flexible range of nasties, so you don't really know what you've got, except that it's toxic and will leave a small residue of dye (and smells horrible!). I think Isopropyl Alcohol is a pretty much universal substitute without the drawbacks.

Proud Mary

Fri Jun 11 2010, 11:18AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

IntraWinding wrote ...

Very nice. I'm buying shares in 25mm acrylic tube manufacturers!!!

Just as well that my tube is 50mm, as I have said in the text.

IntraWinding wrote ...

I don't like meths though! It's 'denatured' with a flexible range of nasties, so you don't really know what you've got, except that it's toxic and will leave a small residue of dye (and smells horrible!).

I am sorry my projects keep failing to live up to your high expectations. As for methylated spirit, many people find that mixing it with cider makes it more palatable, though no less toxic.

Proud Mary

Fri Jun 11 2010, 01:48PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

A 75kV 1000:1 Voltage Divider With 3G Input Impedance

High Voltage Terminal Assembly

I found a tough clear plastic furniture coaster that matched splendidly into the inner diameter of the 50mm tube.

After drilling a pilot hole with three increasing bit sizes using a hand drill, I worked the hole to the right size with a hand reamer.
I used hand tools to reduce the chance of splitting, cracking, and melting.

1276263533 543 FT90813 Voltage Divider Top Cap And Reamer 400

The hole was sized to be a snug fit for the ceramic step inside this HV bulkhead feedthrough:

1276263637 543 FT90813 Voltage Divider Exploded Terminal View 400 Pic 2

At this stage, I have just screwed it loosely together, but RTV will be applied to the threaded rod and insulator mating surfaces in the final version.

1276263805 543 FT90813 Voltage Divider Terminal Mounted On Cap 400 Pic 3

And here is the HV terminal as it will appear in place:

1276263975 543 FT90813 Voltage Divider Hv Terminal In Place 400 Pic 4

To be continued...

IntraWinding

Fri Jun 11 2010, 01:51PM

Registered Member #2261 Joined: Mon Aug 03 2009, 01:19AM
Location: London, UK
Posts: 581

Proud Mary wrote ...

I am sorry my projects keep failing to live up to your high expectations. As for methylated spirit, many people find that mixing it with cider makes it more palatable, though no less toxic.

Oh - sorry I come across that way. I'm very interested in your projects and appreciate you posting them. I wanted to contribute in the areas where I feel I have experience.

Hon1nbo

Fri Jun 11 2010, 02:41PM

Registered Member #902 Joined: Sun Jul 15 2007, 08:17PM
Location: Pacific Northwest USA
Posts: 1042

Very well thought out! - now I wonder, should I be checking the voltage divider on my HV meters?

-Jimmy

Proud Mary

Fri Jun 11 2010, 05:26PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

DaJJHman wrote ...

Very well thought out! - now I wonder, should I be checking the voltage divider on my HV meters?

Thanks, Jimmy!

I guess what I've done so far is the easy part - the two-tier measuring resistor and safety spark gap assembly, the hermetic dual impedance output ports, and base seal with oil port and over-pressure device, will surely tax my ingenuity as I come to them.

Herr Zapp

Fri Jun 11 2010, 07:59PM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

PM -

Looking very nice so far.

One question: do you have any mechanical "compliance" designed into your resistor stack to isolate it from thermally-induced changes in the length of the outer tube? Acrylic has a high CTE, and as your tube warms and cools it will grow and shrink substantially in length, especially compared to your resistors, which probably have low CTE ceramic cores. Think of your resistors as strain gage elements ......

Be very careful about exposing your acrylic tube to denatured alcohol. As IntraWinding noted, the "denaturing" agent can be acetone, gasoline, or any one of a number of aeromatic solvents that can severely attack (crazing, cracking, etc.) acrylic or polycarbonate. Over the years I've seen a number of incidents where molded or machined plastic components in commercial products were attacked when inadvertantly exposed to denatured alcohol, where isopropyl alcohol had been specified.

Regards,
Herr Zapp

Proud Mary

Fri Jun 11 2010, 08:37PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Herr Zapp wrote ...

One question: do you have any mechanical "compliance" designed into your resistor stack to isolate it from thermally-induced changes in the length of the outer tube? Acrylic has a high CTE, and as your tube warms and cools it will grow and shrink substantially in length, especially compared to your resistors, which probably have low CTE ceramic cores. Think of your resistors as strain gage elements ......

As you may have noticed, the resistor assembly core is not fixed to the tube walls, and there is to be a Hooke's Law device - a compression spring doncha know

- between the HV terminal assembly and the top of the stack. An over-pressure device fitted into the base seal will accommodate any egregious pressure rises, while a compressable pressure compensation device in the oil will moderate lesser pressure changes. I haven't made my mind up yet, but this is likely to take the form of a small roll of closed-cell neoprene hidden out of view at the base - Boyle's Law, Charles's Law, and Young's Modulus all rolled up in a sausage!

Herr Zapp wrote ...

Be very careful about exposing your acrylic tube to denatured alcohol. As IntraWinding noted, the "denaturing" agent can be acetone, gasoline, or any one of a number of aeromatic solvents that can severely attack (crazing, cracking, etc.) acrylic or polycarbonate. Over the years I've seen a number of incidents where molded or machined plastic components in commercial products were attacked when inadvertantly exposed to denatured alcohol, where isopropyl alcohol had been specified.

You may rest assured that I have no plans to use methylated spirit to clean the acrylic tube.

You may be interested to know that the formulation of methylated spirits is strictly controlled by UK law, to put the ethanol beyond all prospect of economic recovery by bootleggers. The government recipe goes like this:

"Completely denatured alcohol must be made in accordance with the following formulation: with every 90 parts by volume of alcohol mix 9.5 parts by volume of methanol or a substitute for methanol and 0.5 parts by volume of crude pyridine, and to the resulting mixture add mineral naphtha (petroleum oil) in the proportion of 3.75 litres to every 1000 litres of the mixture and synthetic organic dyestuff (methyl violet) in the proportion of 1.5 grams to every 1000 litres of the mixture."

Source: Statutory Instrument 2005 No. 1524, known as
'The Denatured Alcohol Regulations 2005.'

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