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Capacitive Voltage divider, for O-scopes (10,000:1)

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Patrick

Mon Sept 27 2010, 09:04AM

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

ok, i will microwave it.

also, i will microwave just unglazed clay, then clay with glaze, then clay with polyester epoxy coat.

but can someone give me an estimate on voltage per centimeter for typical microwave ovens? if such a thing is possible.

Proud Mary

Mon Sept 27 2010, 12:47PM

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

Patrick wrote ...

ok, i will microwave it.

also, i will microwave just unglazed clay, then clay with glaze, then clay with polyester epoxy coat.

but can someone give me an estimate on voltage per centimeter for typical microwave ovens? if such a thing is possible.

As no more than a mere stab in the dark, and definitely sticking my neck out, if we take a common MO magnetron:

LG Magnetron Tube model: 2M213
Typical Oven Output Power: 700W
Frequency: 2460MHz
Peak Anode Voltage : 3.95kV

and assume that waves having the same amplitude as the peak anode voltage will appear at the nodes across a quarter wave section inside the oven, then you would have 3.95kV of microwave RF across 30.5mm, but distributed, of course, along a curve.

There are many obvious variables here, including possible impedance transformation, networks of standing waves inside the oven, parasitic resonances, frequency dependent attenuations, oven geometry, and who knows what else, so I would only look to the figure I've suggested as perhaps indicating not much more than an order of magnitude, which I hope will be better than nothing.

IntraWinding

Mon Sept 27 2010, 02:47PM

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

I'd be a bit worried a microwave was too tough a test. Whilst cold glass insulates, it conducts when very hot. You might get thermal runaway burning spots (or whatever) into a glaze that would work fine with just EHT applied.

Is there no chance of testing it with pure volts?

I know ideally it should be tested at well above the maximum use voltage, but if that's not possible then at least a test at maximum will spot a major problem without setting fire to any oscilloscopes

Patrick

Tue Sept 28 2010, 03:33AM

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

I was thinking IntraWinding might be right, I microwaved it at 1100 Watts for 1 minute. it was warmer than my skin temp, but not too hot to pick up. I wonder if i microwave porcelain (40-50% Alumina) if i will get the same temp rise, i think this may not be accurate, Molecular Orbital theory is complicated and heat rise may not be based on conduction. i will put some kV on it to see if i can detect uA leakage.

Patrick

Wed Sept 29 2010, 03:34AM

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

maybe two sharp points, the insulator and a microamp meter all in series.

Patrick

Mon Oct 04 2010, 12:09AM

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

Im getting ready to do a test in the next few days. Id like to see if anyone knows what a transistor driven ( squarewave+555 timer ) ignition coil looks like on a scope with a professional/conventional HV divider probe? I'd like to see something simialar out of my own scope/probe for credibility comparison. such a scope pic would be nice.

EDIT: the voltage division does take place, about 2500:1 with 3.5pf and 2.9nf into a 10Mohm DMM. (at 7000Vac) the unglazed insulator act like a resistor and the E-field is unevenly divided, much as i had feared.
the insulator must be porcilain the chinese ceramic has to much talc,so itshivers the glaze off, and becomes conductive. also, the ceramic has no aluminum oxide, while porcilain is about 50% AlO3.
i will next make a shield that puts the ball 3 or 4 diameters below the sheild edge.

so becuase of the inadequate shileding, and the insulator not insulating, the division ratio is less then ideal.

i am thinking this new shield arrangement will be better.

in this pic there is 1.050 inches of oil between the copper guard ring and the hot ball-rod.

Patrick

Tue Oct 05 2010, 01:31AM

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

pic

Here is the copper guard ring.

1286242270 2431 FT91689 Clip Image004122

This is the only real pic on iggy's scoped I could find.

IntraWinding

Wed Oct 06 2010, 01:40AM

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

I thought the whole HT capacitor needed to be guarded in a conductive tube?

Patrick

Wed Oct 06 2010, 02:50AM

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

The HV cap is fully shielded. Are the renderings not persuasive?

EDIT: i was cosidering potting the whole cap in polyester resin from castin' cast, instead of olive oil. like a 3 inch daimeter and 3 inch tall cylinder of resin. the volume would be 21.2 cubic inches of cast. however i dont now what that would do to the capacitence response of high freq stuff. but thats small stuff i want the whole thing working as a prototype, then ill refine it.

IntraWinding

Wed Oct 06 2010, 06:58AM

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

So is the whole of the yellow tube in your 3D drawing metal with the copper guard ring an addition to prevent the effect of sharp edges at the opening?

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