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Barry

Sun Jan 30 2011, 06:29AM

Registered Member #90 Joined: Thu Feb 09 2006, 02:44PM
Location: Seattle, Washington
Posts: 301

Navrit Bal wrote ...

I haven't actually tried yet, but my supervisor brought up the point of when the capacitors have reached their target voltage, and I turn off the PSU which is charging it, she thinks the capacitors would discharge into the PSU.

Would this actually happen? I'm somewhat doubtful of whether it will because so far I haven't seen anything about this on any other coilgun logs.

All power supplies necessarily have diodes to generate a positive voltage by rectifying the AC mains. As a side effect they will prevent the circuit from discharging back into the power supply.

If you somehow manage to encounter a problem (although I can barely imagine how) with the capacitors discharging back into the power supply, then insert a series switch to isolate the capacitors from the p.s. after they're charged.

As for arcing voltage with respect to distance, it is generally accepted for safety planning that in the right circumstances, air may be able to breakdown and arc at a rate as low as 800v per inch. Most often, i.e. at less than 100% humidity, the insulating capability is much higher than this.

Current has no effect on arcing, because an "arc" is the response when two different voltages are physically brought close enough together that the insulating material between them can no longer withstand the electromotive force. If the insulator is air, then under sufficient voltage potential, the atoms and molecules accept (or are stripped of) electrons and form a conductive plasma.

Cheers, Barry
I believe in reincarnation, so when I die, my gravestone will say BRB instead of RIP.

Navrit Bal

Sun Jan 30 2011, 11:34AM

Registered Member #3323 Joined: Sun Oct 17 2010, 03:19PM
Location: West Midlands, UK
Posts: 116

Thanks for that Barry, I knew there would be something like that. Unfortunately my supervisor doesn't really know what she is talking about outside of the syllabus.

Navrit Bal

Sat Feb 05 2011, 12:56PM

Registered Member #3323 Joined: Sun Oct 17 2010, 03:19PM
Location: West Midlands, UK
Posts: 116

At the moment I am waiting for this 'High Voltage Variable Voltage Multiplier'

This is for charging the capacitors by the way. I have been asked to remotely fire this as another teacher made my supervisor aware of the potential EMP effects, therefore I will be firing this outdoors, in a field.

I have been asked to remotely fire the coilgun, I was thinking about using long wires from the SCR triggering wires and all other wires as short and fat as possible and reasonable.

I am going to have completely separate charging and firing circuits. I'll make a first go and upload it soon so everyone can point out problems.

Barry

Sat Feb 05 2011, 03:18PM

Registered Member #90 Joined: Thu Feb 09 2006, 02:44PM
Location: Seattle, Washington
Posts: 301

Navrit Bal wrote ...

I have been asked to remotely fire this as another teacher made my supervisor aware of the potential EMP effects, therefore I will be firing this outdoors, in a field.

I suppose it's good that they have safety in mind. But they're clueless to the scale of things. Here, the EMP effect will be virtually undetectable at a distance of a meter or more from the coil.

The fearsome EMP from a nuclear device comes from high broadband energy at short wavelengths. It can couple into (be received by) short wires of similar lengths thereby generating significant voltage and current in printed circuit boards and other small devices. This is what damages transistors and ICs. The conductors are acting as receiving antennas.

Your coilgun's wavelength is thousands of times longer. So if your coilgun fires its one-half of a sine wave at a frequency of, for example, 2 kHz then it is transmitting a signal at a wavelength of 150,000 meters. This could be picked up by antennas and random wires of 1/4 wavelength or longer, which is still kilometers long. So rest assured your cellphone, watch, computer and any printed circuit board with traces less than a few kilometers long (haha) is completely safe from internal voltages generated by your EMP.

You'll want to keep people and valuables more than a meter away from your coilgun anyway, for lots of good reasons. But its EMP is not one of them.

Cheers, Barry
This JUST in! There's a _tasty_ way to enjoy whole grains and leafy greens... Feed them to a cow and they'll turn into a delicious, satisfying steak.

klugesmith

Sat Feb 05 2011, 05:19PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Barry wrote ...

Navrit Bal wrote ...
... my supervisor brought up the point of when the capacitors have reached their target voltage, and I turn off the PSU which is charging it, she thinks the capacitors would discharge into the PSU.

I can attest that there is cause for concern, at least with low-voltage regulated power supplies.
And many voltage regulator IC datasheets warn of damage if the input voltage is brought to zero while the output remains connected to a large, charged capacitor. It's about the pass element of regulator operating in an unexpected quadrant.

Long ago I used my employer's Hewlett-Packard benchtop PS to charge a 6V battery. When I switched off the PS (or maybe turned down the voltage setting) with battery still connected, the PS let out some smoke. I opened the PS, figured out the problem, and fixed it. Started over with a series diode to prevent back-flow. Promptly blew up the PS again by connecting the battery backwards.

If in doubt, or to relieve your instructor's anxiety, put a diode in series.

Navrit Bal

Sat Feb 05 2011, 10:16PM

Registered Member #3323 Joined: Sun Oct 17 2010, 03:19PM
Location: West Midlands, UK
Posts: 116

When my PSU arrives, I can check whether it has a diode which would stop this sort of thing, the supplier told me that it would.

I completely agree about how she really seems to have a very small understanding of Physics, even in general, but as you said, it is good about her safety concerns, but they can be overkill.

I did think that the EMPs effects would die off very quickly, correct me if I am wrong but wouldn't it decrease by the inverse square of the distance? I think I'll stand 2-3 meters away.
With electrolytic capacitors, is the dot next to the terminals a safety release valve? Also, is there any way to tell whether capacitors are rated for horizontal or vertical installation?

Barry

Sun Feb 06 2011, 02:10AM

Registered Member #90 Joined: Thu Feb 09 2006, 02:44PM
Location: Seattle, Washington
Posts: 301

Navrit Bal wrote ...

I did think that the EMPs effects would die off very quickly, correct me if I am wrong but wouldn't it decrease by the inverse square of the distance? I think I'll stand 2-3 meters away.

An EMP is energy radiated like light rays and radio waves; they do decrease by inverse square law. Actually I don't exactly call that "die off quickly" because on amateur radio I can talk around the world with no more transmitting power than a 100w light bulb.

I think 2-3 meters away is a good distance.

By the way, the magnetic field around a solenoid is not "radiated energy" and falls off much more quickly with respect to distance.

Navrit Bal wrote ...

With electrolytic capacitors, is the dot next to the terminals a safety release valve? Also, is there any way to tell whether capacitors are rated for horizontal or vertical installation?

The dot is almost certainly the safety pressure release. They design a weak spot to break first in case of overheating. This is supposed to let it leak and dribble out acidic electrolyte before building up too much steam pressure and exploding all at once. But I wouldn't trust it, especially in a big capacitor bank where a bunch of other cans are able to instantly dump their energy into one failed capacitor that has an internal short. Always a good idea to build the capacitor bank into an enclosure.

As far as I know, the only way to learn the ratings for orientation and other details is from the manufacturer's data sheet.

Cheers, Barry
You might as well sleep with me because Iâ€™m going to tell everyone we did anyway

Saz43

Sun Feb 06 2011, 05:30AM

Registered Member #1525 Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294

See it work here:

Navrit Bal

Sun Feb 06 2011, 02:25PM

Registered Member #3323 Joined: Sun Oct 17 2010, 03:19PM
Location: West Midlands, UK
Posts: 116

Thanks for the information and links, I have just e-mailed the person who I got these capacitors from about which way they should be installed.

Are there any coilgun templates for FEMM? I'm asking because I want to estimate a few things using FEMM but this is my first time using it.

Barry

Mon Feb 07 2011, 12:09AM

Registered Member #90 Joined: Thu Feb 09 2006, 02:44PM
Location: Seattle, Washington
Posts: 301

I have a couple examples here: www.coilgun.info/femm/home.htm for starters. Check around for others. FEM takes some study to get the hang of it.

Cheers, Barry
This space for rent.

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