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1
Sorry, I had to correct myself. I meant 20 consecutive can crushes, not coin shrinks. I corrected my original post. My mind was elsewhere. For coin shrinking, I use 12 AWG double-build wire but the coil is sacrificial.
2
Electronic Circuits / Re: Want to build HV HF oscilloscope probe.
« Last post by Twospoons on Today at 05:08:46 AM »
There is an argument to be made for putting a cable-driving buffer in your probe. That way you can reduce the capacitance of your probe and extend the frequency range without unduly loading whatever you are testing.  It also means you can correctly terminate your cable to avoid ringing.

What upper frequency are you aiming for?
3
Quote
I have been using 10 AWG double-build magnet wire with no failure yet after about 20 coin shrinks. The maximum voltage I have used is 5500 volts which corresponds to about 1500 Joules with my Aerovox capacitor.
I can see that making sense.  I doubt any of the magnet wire I tested had double-build insulation.  More important, I'd guess, is your mechanical containment.  Probably prevents wire from fracturing during discharge.  Also, most of my coils were double-layer to match higher voltage and lower capacitance, and more compact for coin shrinking rather than can crushing.  I did play a tiny bit with can crushing, both aluminum and steel, but not with a range of wire for that.
4
Induction Launchers, Coil Guns and Rails guns / Re: Ring Launcher AC vs DC?
« Last post by FPS on Today at 03:55:37 AM »
Which would launch further, just a coil and plate on top, or one with another aluminium plate on the bottom? It's just a meandering thought. I would assume the bottom plate will suck the energy out of the system and result in a lower velocity, but I'll pose the question as I don't really know.

Theory relies on a a few things. 1. That most of the magnetic pulse is lost out the bottom and not used to accelerate the plate, 2. the plate only has a few millimetres to accelerate, and 3. increasing the time the plate is in the magnetic field will have a significant result even if in the order of microseconds.

So placing a plate underneath will make the coil jump with the projected plate and lengthen the time it's under its influence thus resulting in higher velocity.

5
Electronic Circuits / Re: Want to build HV HF oscilloscope probe.
« Last post by klugesmith on Today at 02:48:22 AM »
Also be sure to mind the voltage rating of your resistors.   Look for ones that are called "high voltage resistors".

Most ordinary half-watt axial lead resistors have maximum rated voltage of 350 V.
https://www.seielect.com/catalog/sei-cf_cfm.pdf
A typical resistor product series has a wide range of ohm values, all with the same max power and max voltage.
For R values near the high end,  the voltage limit gets you before the power limit.
Large voltage overloads don't typically cause sparks or smoke, but they can cause resistance change (reversible or not) and long term degradation of the ohmic material.

[edit] your low side resistor is shown as 1Meg + 15.5k, for DC attenuation of about 1/2000 instead of 1/1000.
I guess that's what you want.   The simulation without capacitors shows RMS voltage of 10 kV in, 5 V out.
Lets see if your AC analysis, with 66.7 pF capacitor in parallel with low side R, gives you a 2.35 kHz lowpass filter.
At 10 kHz most of the current goes through the capacitor.

As twospoons said, to get flat frequency response you need a capacitive divider that matches the resistive divider.
Then at 10 kHz, the probe's input capacitance will draw a lot more current than its input resistance.
6
Electronic Circuits / Re: Want to build HV HF oscilloscope probe.
« Last post by Domo on Today at 02:05:05 AM »
Very good information on attachment.. Thank you.
7
I've had a small 1 liter xenon globe on my bench for a week or so now. It tend to be
self-starting ie nothing is needed other than turning the power supply on.

Today I moved it off my bench and to another location and the self-starting went away.
My thought was to add a ground plane underneath the coil and things got worse.

The factor that was important was the bench was wood, the new location was the surface
of a table saw which had a inch thick piece of wood over a metal ground (?).

It finally dawned on me (because I'm slow sometimes)...

...this is an RF circuit with all the weirdness that entails. Your circuit can and does include objects
nearby. It's like the issues with metal objects near a DRSSTC but even worse.

I am going to be very curious when the Chinese toroids start arriving here. Will they be as weird, interacting with
everything around them?

Time will tell.

Cheers.
8
Electronic Circuits / Re: Want to build HV HF oscilloscope probe.
« Last post by Twospoons on May 18, 2024, 11:30:11 PM »
Short answer is you need to add capacitance across your 2Gohm resistor to form a capacitive divider than matches your resistive divider, which in your example will be 66fF, or 1.3pF across each 100Mohm resistor . 

Also I suggest you run your sim as an AC analysis - then you will see the frequency response of your probe, and why the capacitive compensation is necessary.

Here's a couple of resources you should find useful :

Oscilloscope Probe Circuits (tektronix) https://w140.com/tekwiki/images/6/62/062-1146-00.pdf   (I would attach the file but it exceeds the file size limit.)
* 100kv_probe.pdf
9
I have been using 10 AWG double-build magnet wire with no failure yet after about 20 can crushes. The maximum voltage I have used is 5500 volts which corresponds to about 1500 Joules with my Aerovox capacitor. This will cut a can completely in half. More typically I use 4500 volts, which is about 1000 Joules,  to get a very good theta pinch on a can. I have been using my G10 coil holder.
10
Electronic Circuits / Want to build HV HF oscilloscope probe.
« Last post by Domo on May 18, 2024, 09:46:17 PM »
Hello all..

I would like to build a HV and HF probe, that can measure up to around 30kv (TV flyback output).
Since the output  (by calculation) of the flyback is around 3-4 mA, i didn't want the load of probe to draw much current, so I decided to use 20 100MOhm resistors (total of 2 G Ohm). This also was influenced by use of 1/2 watt resistors. Output would be 1000:1, so measuring 10kV would show up as 10V.  By calculations, probe would draw around 15 uA.


I have not yet build it, it is only in simulation, but i am already running into a problem. From the probe to oscilloscope, I would be using coax cable, and I am aware that it has average parasitic capacitance of 66.7pF per meter. When i add that to the simulation, sine wave does not rise more than 3Vpp. Changing values between output of probe and ground does not increase the sinewave amplitude, How can I compensate this?



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