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

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Proud Mary

Wed Jun 16 2010, 04:30PM

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

Metalwork on the Base Unit

I decided to use an old steel instrument case for the lower base unit, which meant cutting a 50mm hole through the top to admit the acrylic tube.

1276705300 543 FT90813 Enclosure With Chassis Cutter 400

The chassis cutter quickly became so hard to turn that I had to put it on the floor and use my own weight to budge the Allen key.
But eventually - hey presto

1276705443 543 FT90813 Enclosure With 50mm Hole Small

So now to sanding, priming, undercoating, and the design of the front panel.

Proud Mary

Thu Jun 24 2010, 04:04PM

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

First Fitting Together of Large Parts

Proud Mary

Sat Jun 26 2010, 01:03PM

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

The Combined Oil-Filler Earth Terminal

I discovered the usefulness of bolt-in car tyre valves a while back when experimenting with a butane parallel-plate proportional counter I'd built inside an Eddystone diecast metal box. I decided to kill two birds with one stone for my voltage divider, and use one of the bolt-in valves as a combined oil filler port, and cold end terminal lug.

To match the compression seals on the tyre valve, I centred a 12mm hole in a 50mm acrylic furniture leg coaster using hand tools, so as not to split or melt the plastic.

1277555525 543 FT90813 Earth End Coaster With Valve First Small A

This is how the valve parts all comes together.

After applying a dab of silicone grease to the compression seals, I screwed the valve into place using a deep 14mm socket and a screw driver. (There is a screw driver slot cut into the inboard end of the valve assembly.)

1277556010 543 FT90813 Earth End Coaster With Valve In Place 400 C

1277556091 543 FT90813 Earth End Coaster With Valve In Place 400 D

The valve core can now be removed with a valve core tool

1277556176 543 FT90813 Earth End Valve With Tool Inserted 400 E

so that the oil filler tube can be inserted:

1277556319 543 FT90813 Earth End With Filler Tube Through Valve 400 Oblique G

Filling will be a leisurely business with such a narrow tube, but as I expect the oil to stay in good condition for several years, a few hours idled away in a deck chair with a cold drink while the oil goes drip -- drop -- drip--drop-- will not make me leap wailing into the grave.

Once the resistor tube is full, (except for a little argon-filled headroom) the valve core is screwed back in, and the valve cap screwed down.

The oil filler assembly also serves as a rugged Earth terminal, the cold lower end of the voltage divider.

IntraWinding

Sat Jun 26 2010, 02:02PM

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

Looking very impressive!
Is the argon to prevent oil oxidation but still allow some room for expansion and contraction of everything?

Proud Mary

Sat Jun 26 2010, 02:39PM

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

IntraWinding wrote ...

Looking very impressive!
Is the argon to prevent oil oxidation but still allow some room for expansion and contraction of everything?

Yes. Just so. The argon-filled headroom is to buffer those small thermal pressure variations which would otherwise cause a leak, the oil being incompressible for all practical purposes. By first flushing the resistor tube with inert gas, I hope to reduce the possibility of oxidative polymerization, and aerobic microbial decay, though I might still add the moth larva from a bottle of mezcal con gusano just to see if anyone notices.

(advertisement photo)

I bought this disposable cylinder of 'pure' argon (~95%) together with the regulator and hose for about 30 squids on ebay some while ago for use with butane in a sealed proportional counter experiment. A little argon goes a long way when filling small spaces!

1277562005 543 FT90813 Argon Gas With Regulator 400

(Original ebay ad photo)

Patrick

Sat Jun 26 2010, 11:18PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

i am making my own ceramic pieces i will post a pic under the HV forum shortly.

must get sleep...

I'm back baby!

i like your project, but is this for DC measurement? will stray capacitence be a factor?

also where did you get this ceramic insulator:

1277619414 2431 FT1630 1276263805 543 Ft90813 Voltage Divider Terminal Mounted On Cap 400 Pic 3

Proud Mary

Sun Jun 27 2010, 08:06AM

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

Patrick wrote ...

i am making my own ceramic pieces i will post a pic under the HV forum shortly.

must get sleep...

I'm back baby!

i like your project, but is this for DC measurement? will stray capacitence be a factor?

also where did you get this ceramic insulator:

Yes, it is for DC meausurement, and I will be calibrating it and making any necessary tweaks before it is put into service.

As for the insulator, I bought a box full of assorted ceramic and glass feedthroughs a few years ago from an old radio ham, or the wife of an old ham who'd passed away, I can't remember now...

Patrick

Wed Jun 30 2010, 03:49AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

im building a capacitive voltmeter attachment fot my computer usb based o-scope. i would like to have your input now that im nearing the completion of construction. it will be a 1:10,000 divider, driving a instrument 3 op-amplifier bnc-output. mine will be for AC, down to about 2Hz @ -3dB roll off.

i will post a full topic thread in a High Voltage Forum thread, this weekend, when i have less homework.
EDIT: ok my probe thread is up, for your perusal.

here is the sam goldwasser papers, my example is most close to the last schematic.

Proud Mary

Tue Jul 06 2010, 02:57PM

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

Measuring Resistor and LCD Voltmeter Unit

The measuring resistance assembly and an LCD voltmeter module are to be fitted inside this diecast metal box of
120mm x 95mm x 25mm. I called these "Eddystone diecast metal boxes" out of habit, but this
one is made by STC.

Here I am using a jigsaw to cut out the rectangular window for the LCD display, following the joining-up-the-drill-holes school of hole cutting.

1278427304 543 FT90813 Measurementboxjigsaw

Here are the main structural elements, including the primary partition made from PC board and adhesive Cu foil. Four parallel resistors will be soldered between the ceramic stand-off and the brass Earth screw.

The large feedthrough capacitor is 3.9nF 350V, and the three small ones are 39nF 150V, which is what I had to hand, rather than what I would choose in an ideal world, where everyone gets what she wants.

As you can see, I am taking a very firm line with any rogue spikes and EMI that might have been thinking of deliberately taking a wrong turn up my arm, or marching into the feeble disgestive system of the meter module.

803

Tue Jul 06 2010, 08:26PM

Registered Member #2807 Joined: Fri Apr 16 2010, 08:10PM
Location:
Posts: 191

It's like art! Good job Mary!!amazed

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