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banned on 5/26/2009 Registered Member #1877
Joined: Mon Dec 22 2008, 02:03AM
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Posts: 147
Thank you very much hvguy. i think i will buy one of those instead of getting a bunch of resistors. thanks all. although, i did read on the site "A high voltage probe that allows a multimeter to measure up to 40,000V respectively. Intended for low energy applications only" i'm especially interested in the "Intended for low energy application only". i wouldent call 20kj low energy. if you or someone else could clarify what this means exactly before i buy :) Nick, EDIT- To aonomous, i would like to monitor discharge patterns, but i don't have a scope or anything to even bring it down to a measurable voltage by my sound card. so my intent right now is just to check on the voltage and make sure it is charging properly.
Registered Member #1262
Joined: Fri Jan 25 2008, 05:22AM
Location: Maryland, USA
Posts: 451
If you do end up going for a string of resistors you need to keep in mind that in open air, resistors can arc over at as little as 500. Oil will certianly help, but its still a good idea to make sure that the voltage across each resistor isn't more than a few hundred volts, even under oil.
Registered Member #1062
Joined: Tue Oct 16 2007, 02:01AM
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If you want to look at the output waveform (current), you can easily make a rogowski coil for seeing measurements. Keep in mind it will be relative, unless you have the means to calibrate the coil. As far as i know, TDU,aono, and I have made one. TDU and aono (did it ever work?) did active integration, I did passive.
A new high voltage probe will be at least 100$, for the one posted. Try to find a local surplus store, i found one for 40$. There are resistors made for high voltage, so try and find those.
banned on 5/26/2009 Registered Member #1877
Joined: Mon Dec 22 2008, 02:03AM
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Posts: 147
wow, the only thing in my town in "surplus" is surplus stores.... not to mention princess auto. ill look around. i geuss anything is risky when you are stepping down 15kv into a sound card... lol
Registered Member #1917
Joined: Fri Jan 09 2009, 02:38AM
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I was surprised to see that someone actually bought one of those things. I saw two problems with this capacitor when I first stumbled across "Surplus Sales of Nebraska": 1. Rated voltage reversal of 10%, fault at 80%. 2. Rated peak current of 15kA, fault at 30kA.
For a big cap like that, 30kA is nothing. At this kind of voltage, it's going to be a pain to keep the current down so low for coilguns/railgun and "destroying stuff". Now, I'll have to assume that if you sunk over a thousand dollars into it, you already have a plan to combat this, or some alternate information about the ratings. So what's the deal?
As to the voltage monitoring, I used 105 (95 + 10) standard 1MOhm resistors as a voltage divider for anything up to 10kV (step-down ratio of 20:1, and my DMM goes to 500V). If you blow a homemade divider, you'll be a lot less unhappy than you would be at the destruction of a shiny new one that cost a hundred dollars or more.
banned on 5/26/2009 Registered Member #1877
Joined: Mon Dec 22 2008, 02:03AM
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Posts: 147
DYI wrote ...
I was surprised to see that someone actually bought one of those things. I saw two problems with this capacitor when I first stumbled across "Surplus Sales of Nebraska": 1. Rated voltage reversal of 10%, fault at 80%. 2. Rated peak current of 15kA, fault at 30kA.
For a big cap like that, 30kA is nothing. At this kind of voltage, it's going to be a pain to keep the current down so low for coilguns/railgun and "destroying stuff". Now, I'll have to assume that if you sunk over a thousand dollars into it, you already have a plan to combat this, or some alternate information about the ratings. So what's the deal?
As to the voltage monitoring, I used 105 (95 + 10) standard 1MOhm resistors as a voltage divider for anything up to 10kV (step-down ratio of 20:1, and my DMM goes to 500V). If you blow a homemade divider, you'll be a lot less unhappy than you would be at the destruction of a shiny new one that cost a hundred dollars or more.
I was considering either 1 ) getting a high voltage probe or 2) making a voltage divider. what do you mean by keeping current down? especially for rail guns and "destroying stuff". the more current the better is the way i see it. i just need a bigger switch and wires xD EDIT: do you mean keeping it down so it doesn't blow the capacitor? if you do... then iv got some thinking to do :/ EDIT EDIT" voltage reversal is not a problem. i plan on putting 15 of those diodes i mentioned earlier in series, then in parallel. yes, alot of soldering but it should work. or ill find some bigger diodes. (as easy as they are to find).
Registered Member #1917
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Yes, not blowing up the capacitor is the problem here. In a typical can-crushing scenario, without anything to limit the current and with good thick connectors and strong contacts, you'd be looking at currents in the >100kA range. Across a hard short, this capacitor would discharge just over 700kA.
Since you're (presumably) already stuck with it, you may want to play around with Barry's RLC calculator until you can come up with a combination of a (very low value, homemade) resistor and possibly an inductor that will bring the output values down to the rated levels.
Just to clarify, you spent $900 on something without even figuring out how to use it? I want your job
Registered Member #1062
Joined: Tue Oct 16 2007, 02:01AM
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Check out niel's post. He bought a 32kJ pulse capacitor brand new from GAEP (with a rogowski coil... cost him 12k total).
I really don't think that the current rating will be a problem. When capacitors have a pulse rating (from GAEP) those numbers are for specific rates, like 1 pulse a second, Though the voltage is high. I think it comes down to the condition of the capacitor. Your best bet is to take your load, and slowly increase the voltage with a rogowski coil on it, to look at the ringing.
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Monitoring voltage of 15kv pulse cap?
