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Monitoring voltage of 15kv pulse cap?

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hotcrazyfruit

Tue Mar 24 2009, 02:26AM

banned on 5/26/2009
Registered Member #1877 Joined: Mon Dec 22 2008, 02:03AM
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I recently bought a 15 KV * 225 uf pulse rated capacitor from

(prime only, no finish) , rectification wont be a problem. i have 1000, 1 amp diodes that are rated for 1000 volts each. I plan on charging it with 4 parralell NST's at 15kv. the only problem i can think of facing is telling when its charged. other than that it will stop drawing current, i want to know if it is getting to the desired voltage. any help is greatly appreciated.
Nick,
P.S.
ill be using it for coilguns/railgun and destroying stuff :)

Backyard Skunkworks

Tue Mar 24 2009, 02:48AM

Registered Member #1262 Joined: Fri Jan 25 2008, 05:22AM
Location: Maryland, USA
Posts: 451

Why not just build a massive resistive voltage divider and monitor it with a DMM?

You just need to be careful that each resistor has very little current through it and is well below its arc over voltage.

hotcrazyfruit

Tue Mar 24 2009, 02:59AM

banned on 5/26/2009
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Backyard Skunkworks wrote ...

Why not just build a massive resistive voltage divider and monitor it with a DMM?

You just need to be careful that each resistor has very little current through it and is well below its arc over voltage.

do you have a schematic for one? or a place to find one? thanks :)

aonomus

Tue Mar 24 2009, 03:12AM

Registered Member #1497 Joined: Thu May 22 2008, 05:24AM
Location: Toronto, Ontario, Canada
Posts: 801

Resistive dividers don't behave well with AC iirc, you have to add compensating capacitors?

rp181

Tue Mar 24 2009, 03:14AM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
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A resistive voltage divider is the most basic of principles. All it consists of is 2 resistor's in series, with a resistance ratio.

Keep in mind what backyard said. Get some high voltage resistors, and keep the resistance high. If it draws too much current, they could easily fail and cause a catastrophic event.
I hope you understand the danger of that high energy, not to mention that high energy at that high voltage. Have you ever used a smaller bank? Take it slow, and don't charge to 15kV for your first charge.

EDIT: Why would AC be a problem? Its being rectified. If I get what you mean, they i second the compensating capacitors.
EDIT EDIT: noticed the link you posted. Sorry about that =)

Backyard Skunkworks

Tue Mar 24 2009, 03:17AM

Registered Member #1262 Joined: Fri Jan 25 2008, 05:22AM
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Posts: 451

aonomus wrote ...

Resistive dividers don't behave well with AC iirc, you have to add compensating capacitors?

Interesting point. But the cap is obviously not seeing AC. He'll be monitoring (from a large distance) DC voltage across the cap after the rectification diodes.

hotcrazyfruit

Tue Mar 24 2009, 03:35AM

banned on 5/26/2009
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ok thanks
rp- iv got a 5 kilojoule bank of the same caps you've got.(except 16 instead of 18) i made the 400v boost converter. worked perfectly. got some good shots off with a coil gun. never made a rail gun . thinking about a mortar cannon. but i got bored of that bank. so instead of 5 KJ i wanted something bigger xD like say 20KJ ?? . and yes i do understand the risks and dangers :) i hadent made or ever had the need to use a resistive voltage divider before (at least for monitoring voltage) . thanks all.
Nick,

Proud Mary

Tue Mar 24 2009, 12:24PM

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

I suggest a 1G resistor in series with 1M1. With a 10M DMM, the voltage across the 1M1, will represent 1000:1 across the whole series i.e. 15kV will read as 15V.

If you have no 1G resistor of suitable voltage and wattage, you could solder 100 * 10M resistors etc in series. Wind the resistors round a plastic rod, and submerge in a tube of oil.

Most TV service EHT probes use a 1000:1 voltage divider.

aonomus

Tue Mar 24 2009, 02:19PM

Registered Member #1497 Joined: Thu May 22 2008, 05:24AM
Location: Toronto, Ontario, Canada
Posts: 801

Sorry, are you trying to monitor charging voltage, or are you trying to capture the discharge waveform? I may have read the wrong thing...

If you are just trying to monitor charging voltage, resistive divider works, but if you are trying to view discharge on a scope and capture I don't think it will work too well....

hvguy

Tue Mar 24 2009, 08:31PM

Registered Member #289 Joined: Mon Mar 06 2006, 10:45AM
Location: Conroe, TX
Posts: 154

If you can afford one of those caps chances are you can save yourself the hassle and just buy on of these

off of ebay. I use them all the time for monitoring caps up to 100kv (more than twice the rating, NOT recommended!)

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