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DIY High voltage probe

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Pinky's Brain

Fri Dec 10 2010, 01:31AM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
Location:
Posts: 837

Well, what is the self inductance of say a 20 cm ID 30 cm OD aluminium foil ring conducting across it's 10-30 micron thickness?

My guess ... the number has an exponent so far into the negative numbers it has a prefix I don't know the name too (last one I know from memory is femto, yes ... sadly pathetic, but still).

Patrick

Fri Dec 10 2010, 01:48AM

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

Pinky's Brain wrote ...

Come to think of it, discrete elements are actually kind of nasty for a capacitive divider ...
..... might not need series resistance at all.

lookie here pinky

, i tried to get rid of the resistence. it did not work well...

Pinky's Brain

Fri Dec 10 2010, 03:37AM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
Location:
Posts: 837

Okay I should have said "You might not need a distributed series resistance at all.".

klugesmith

Fri Dec 10 2010, 04:29AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1714

Pinky's Brain wrote ...
Well, what is the self inductance of say a 20 cm ID 30 cm OD aluminium foil ring conducting across it's 10-30 micron thickness?

Sorry to go off track, but I've never seen a 4hv thread wander so close to some calculations I made 5 years ago,
about self-inductance and resistance of aluminum beverage cans around their circumference.
Both are approximately proportional to the inverse of the conducting width (the cylinder length).
For a 4 inch length (roughly a whole 355ml can) it's about 30 nH and 1 mOhm,
so a circulating current would decay with characteristic time constant of 30 us.

For very short sections (length << diameter) the inductance and time constant are smaller
than that inverse formula would indicate. A can section 1/4 inch long (conducting width 1/4 inch)
would be 16 mOhm and only 128 nH, so its L/R is only 8 us.

Those old R values were based on sheet resistance of 0.5 mOhm/square, about the same as 1 oz copper foil.
Not sure if it's the value I measured on many can specimens. The measured value was about twice what you would figure from the handbook resistivity of Al, for the measured thickness (and for the measured weight after chemically stripping the paint). Measured R was consistent with the handbook resistivity of the aluminum alloy that can bodies are made of. But this is far off track for the thread. Someday we'll have a thread about can-crusher modeling.

Patrick

Fri Dec 10 2010, 05:19AM

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

did one of pinkys posts disappear, the one with the schematic link!?

Pinky's Brain

Fri Dec 10 2010, 06:02AM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
Location:
Posts: 837

It did disappear, but that was because I realised that the structure I was describing would use 100 pieces of aluminum foil of around 2 m^2, consequently with a structure of around 1.5 meter diameter (assuming 20 pF total series capacitance, 2 meter height and 100 plates).

The schematic is here.

radhoo

Fri Dec 10 2010, 04:55PM

Registered Member #1938 Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 699

Guys, indeed there are some issues with the probes, but let's try to keep things simple.
I have an update on my setup that I'd like to present. My probe is a resistive divider only.

case1:
+HV Supply <----[1 GOhm resistor]--[uAmpmeter]-----GND (and)
case2:
+HV Supply <----[1 GOhm resistor]--[uAmpmeter]----- -HV Supply (because my supply is bipolar:

).

Details: I am using a 100uAmeter. My HV supply is fed by my variable low voltage power supply, so I can use the pot to adjust the output voltage. I put in 12V and get a ~6cm spark (~ 60KV)
I also put a 1MOhm resistor in parallel with the uAmeter.

case1: the uAmeter shows ~ 30uA
case2: the uAmeter shows ~ 60uA

Questions:
1) Is it correct to add a 1MO resistor in parallel to the uAmeter, if my goal is to have uAmps = kVolts on the scale?
2) After running test2/case2 and powering off, the uA was continuously showing a small value ~20 on its scale. Shaking it and waiting some time put the needle back on 0. Why?
3) Is it ok to do measurements like in case 2? The negative component goes straight into the uAmeter, or I must only use it like in case1 , with ground and through a HV resistor? If this is the case how to measure my bipolar supply.
4) How to I measure the impedance of this uAmpmeter?
5) Is there anything else I should know here? I'm planing to finalize this probe and need to make sure that what I do is correct. Here are some pictures:

Thanks

Pinky's Brain

Fri Dec 10 2010, 05:09PM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
Location:
Posts: 837

radhoo wrote ...
1) Is it correct to add a 1MO resistor in parallel to the uAmeter, if my goal is to have uAmps = kVolts on the scale?

It doesn't do a thing ... you can just leave it out.

wrote ...
3) Is it ok to do measurements like in case 2?

In theory ... as long as it doesn't put you yourself too near that -HV.

The meter probably won't much like having a corona and the accompanied static build up though (which might explain the faulty measurement). Ideally you would have two high voltage resistors at either side, so the meter is at ground.

wrote ...
4) How to I measure the impedance of this uAmpmeter?

Put it in series with a small resistor and put a small known voltage across the pair and measure the voltage across the meter with your DMM.

radhoo

Fri Dec 10 2010, 05:36PM

Registered Member #1938 Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 699

Sounds good so far!

I suspected static charge myself. I wonder how to prevent it.

I would like to understand more on how the uA's are "converted" to kVs in this setup. Some formulas would do. I just want to make sure that my measurements will be as accurate as possible, so doing it right is important.

What about Tesladownunder's design with the bridge?

Extra problems with my bipolar supply , but besides that, would it bring any benefit?

Pinky's Brain

Fri Dec 10 2010, 06:14PM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
Location:
Posts: 837

In theory you could put the meter under oil, in practice it would be easier to just keep it at ground. Build a second probe if you want to measure your +HV and -HV at the same time.

The bridge doesn't do anything if you aren't measuring AC, I don't think the low pass is necessary, the mechanical inertia of the meter will already do that, the two diodes across the meter are a nice touch though, clamps pulses (don't literally put them across the meter though, keep the diode leads short and pinch the wires near together where you solder on the diodes).

PS. the formula for your measurement is just Ohm's law ... I=U/R, and the only R of significance is the 1 GOhm probe.

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