If you need assistance, please send an email to forum at 4hv dot org. To ensure your email is not marked as spam, please include the phrase "4hv help" in the subject line. You can also find assistance via IRC, at irc.shadowworld.net, room #hvcomm.
Support 4hv.org!
Donate:
4hv.org is hosted on a dedicated server. Unfortunately, this server costs and we rely on the help of site members to keep 4hv.org running. Please consider donating. We will place your name on the thanks list and you'll be helping to keep 4hv.org alive and free for everyone. Members whose names appear in red bold have donated recently. Green bold denotes those who have recently donated to keep the server carbon neutral.
Special Thanks To:
Aaron Holmes
Aaron Wheeler
Adam Horden
Alan Scrimgeour
Andre
Andrew Haynes
Anonymous000
asabase
Austin Weil
barney
Barry
Bert Hickman
Bill Kukowski
Blitzorn
Brandon Paradelas
Bruce Bowling
BubeeMike
Byong Park
Cesiumsponge
Chris F.
Chris Hooper
Corey Worthington
Derek Woodroffe
Dalus
Dan Strother
Daniel Davis
Daniel Uhrenholt
datasheetarchive
Dave Billington
Dave Marshall
David F.
Dennis Rogers
drelectrix
Dr. John Gudenas
Dr. Spark
E.TexasTesla
eastvoltresearch
Eirik Taylor
Erik Dyakov
Erlend^SE
Finn Hammer
Firebug24k
GalliumMan
Gary Peterson
George Slade
GhostNull
Gordon Mcknight
Graham Armitage
Grant
GreySoul
Henry H
IamSmooth
In memory of Leo Powning
Jacob Cash
James Howells
James Pawson
Jeff Greenfield
Jeff Thomas
Jesse Frost
Jim Mitchell
jlr134
Joe Mastroianni
John Forcina
John Oberg
John Willcutt
Jon Newcomb
klugesmith
Leslie Wright
Lutz Hoffman
Mads Barnkob
Martin King
Mats Karlsson
Matt Gibson
Matthew Guidry
mbd
Michael D'Angelo
Mikkel
mileswaldron
mister_rf
Neil Foster
Nick de Smith
Nick Soroka
nicklenorp
Nik
Norman Stanley
Patrick Coleman
Paul Brodie
Paul Jordan
Paul Montgomery
Ped
Peter Krogen
Peter Terren
PhilGood
Richard Feldman
Robert Bush
Royce Bailey
Scott Fusare
Scott Newman
smiffy
Stella
Steven Busic
Steve Conner
Steve Jones
Steve Ward
Sulaiman
Thomas Coyle
Thomas A. Wallace
Thomas W
Timo
Torch
Ulf Jonsson
vasil
Vaxian
vladi mazzilli
wastehl
Weston
William Kim
William N.
William Stehl
Wesley Venis
The aforementioned have contributed financially to the continuing triumph of 4hv.org. They are deserving of my most heartfelt thanks.
Registered Member #3323
Joined: Sun Oct 17 2010, 03:19PM
Location: West Midlands, UK
Posts: 116
I will use your 'fancy' method as it seems more reliable. I will have to find the tables later, but for now, I might use my school's equipment which should be better. Is there a particular way to measure capacitors ESR ? Forgive me if its just attaching a multimeter to the 2 terminals
Registered Member #27
Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058
The fancy way to measure milliohms is to force a known DC current, say 1 amp, and measure the voltage between the coil ends.
Consider ramping the current up and down. Depending on the coil you might end up hurting yourself and the instrument if you just connect/disconnect it instantly.
I thought it would be best to use a big wire, then add resistance with resistors as needed. Also, I have 3 450V 1.8mF capacitors and 105 330V 160uF photoflash capacitors (16.8mF overall). Where did the 350V come from?
Oh forgot this part.
No, it isn't a good idea at all to put resistors. They will kill and bury your efficiency, and they will cost a freaking fortune, since you need incredibly high powered power resistors.
The idea is doing the math before buying anything... But I learnt by hitting my head to the walls aswell gosh... Burnt a 555 timer, replaced two times an inductor, burnt 5 resistors and 2 pots... All on the same board LOL. They say we aren't born knowing everything
350V is just a round-up from 330V. Don't know really, but the thing is that the pulse will behave mostly the same way, since both capacitance and inductance are the same, and voltage isn't that different.
Registered Member #90
Joined: Thu Feb 09 2006, 02:44PM
Location: Seattle, Washington
Posts: 301
Bjørn wrote ...
The fancy way to measure milliohms is to force a known DC current, say 1 amp, and measure the voltage between the coil ends.
Consider ramping the current up and down. Depending on the coil you might end up hurting yourself and the instrument if you just connect/disconnect it instantly.
A reasonable setup is to use a car battery connected in series with a large 8-ohm (10-watt) resistor and your coil. Measure the voltage across the coil. Either measure the current directly, or measure the voltage across the known resistance to find the current. From the current and the coil voltage you can calculate the coil's d.c. resistance.
Don't have a big 8-ohm resistor? Try a 60w light bulb. Or two or three. You'll have to measure current with an ammeter since the bulb resistance depends on temperature and is unpredictable.
I don't see a need to ramp up current with coils like this. The inductance is pretty small, so the total stored magnetic energy is pretty low: 1/2 L I^2.
Navrit Bal wrote ...
I will use your 'fancy' method as it seems more reliable. I will have to find the tables later, but for now, I might use my school's equipment which should be better. Is there a particular way to measure capacitors ESR ? Forgive me if its just attaching a multimeter to the 2 terminals
No. The best we can do without special equipment is read the manufacturer's datasheet.
Oh yeah, I recommend against adding resistors to the high-current path of a coilgun. It kills performance. In fact, I'd say to do everything possible to reduce wiring resistance. Use fat and short wires, keep all connections very tight or better yet soldered.
Cheers, Barry Centrifuges: They're what separates the men from the boys.
Registered Member #3411
Joined: Sat Nov 13 2010, 08:25PM
Location:
Posts: 33
Why do you want a critically dampened circuit anyhow?? Any resistance that you bring into the circuit is eating away your precious current, and magnetic field strength is directly coupled to that same current. Take a look at this wiki diagram, it plots the current in a LCR circuit -
If im allowed to quote Barrys site :
A critically-damped RLC circuit will deliver a current peak that is only 36% of the maximum possible peak from an undamped circuit. This is a very heavy penalty for the convenience of a simple circuit!
64% less than a perfect circuit with 0 resistance. Much weaker field, much weaker force on the armature compared to the ideal circuit..
Artikbot wrote ...
50V is pretty low, so dampening is not as critical (back EMF is nowhere as destructive unless you're talking about several dozens of amsp) as if you use higher voltages.
While the maximal possible voltage generated would seem to be dependant on the rate of change in the current that created the magnetic field, I remember reading some paper about inductors used as power supplies for railguns mentioning the "desirable" feature of inductors to supply whatever voltage is necessary to force the current into the load. You provide a way for the voltage to flow, it wont reach its peak value?
Why do you want a critically dampened circuit anyhow?? Any resistance that you bring into the circuit is eating away your precious current, and magnetic field strength is directly coupled to that same current. Take a look at this wiki diagram, it plots the current in a LCR circuit -
If im allowed to quote Barrys site :
A critically-damped RLC circuit will deliver a current peak that is only 36% of the maximum possible peak from an undamped circuit. This is a very heavy penalty for the convenience of a simple circuit!
64% less than a perfect circuit with 0 resistance. Much weaker field, much weaker force on the armature compared to the ideal circuit..
It's the balance between pulse lenght and pulse power what makes you dampen more or less the pulse.. It's completely useless to have a 2MW pulse if it lasts for a millisecond and you need at least ten for your projectile to acquire the needed acceleration. It also puts a lot less stress on the dampening equipment needed to ensure the integrity of your cap bank.
So it's not always that you need to critically dampen, what you always need is to avoid secondary pulses due to underdampened coils since they can cause suckback.
Registered Member #3411
Joined: Sat Nov 13 2010, 08:25PM
Location:
Posts: 33
Ah, but a higher field strength equals more force, equals more acceleration, equals the need for a shorter pulse :).
How about using the diodes to rectify the back pulse into the capacitors? Its what im planing to do (waiting on my scrs tho, just now saw that paypal hasnt even completed the transaction for some reason... more waiting :( )
PS. I am more interested into inductance coilguns however, so suckback doesnt really concern me for my own little experiment
Registered Member #3323
Joined: Sun Oct 17 2010, 03:19PM
Location: West Midlands, UK
Posts: 116
I didn't expect so many responses then
Is this what can be used for DC rectification ? Would it rectify the voltage? If not, please tell me what would be used.
Is the most desired ζ value 1, I am referring to the diagram linked before.
Which wire gauge would be best for the following capacitor banks: 330V 16.8mF, 450V 1.8mF, 450v 5.4mF ? (just before I waste more money)
I am probably going to buy a 12v 7.2Ah SLA battery for something completely unrelated, so could it be used in the same way which you (Barry) suggested. Although I doubt I will measure the resistance of the coil as it looks like I am going to switch to a higher gauge wire.
Since, I can't find the datasheet for the capacitors I am going to use, I guess I will have to leave that alone.. Could I use the 2mm wire as very fat wiring connecting components? (since I can't think of a good use for it anymore)
Is 'B' usually used for magnetic field strength? Finally, is a large change in inductance the most wanted or a small/large inductance?
Recitfying the voltage... From where? It mostly depends on the type of charger. If you're using a flyback core based one, you'll need a full rectifyer otherwise you'll be wasting half of the current (all that's on the negative side of the wave will go trash if you just let the positive side pass). If it's a boost converter, a simple Schottky rectifier diode after the inductor will do, since it's pulsed DC and all you need to do is avoid current from the HV side go to the LV side of your board (logics part, the HV current would make everything on that side go BAM and so on, not really desirable).
For the huge wire/coil? Save it bro... You'll say "I WANT MOAR POWAH" in less than you think, and then you'll regret not having heavy gauge wire
Wire is dependant on the needed coil, and that can be found by using Barry's (thank God Barry is here with us, I can't imagine my life without his sims and his awesomesaucely clear explanations) RLC sim. There you get your inductance, and with it, you can play with his air core inductor sim and get the desired wire gauge.
Just make sure not to get too thin wire or it will instantaneously melt down when you apply a heavy pulse to it Talking from experience... Had two meters of very thin wire (I believe it's 30AWG or so... or better say it WAS lol) lying around and wanted to do an experiment... Wound it around a plastic tube, applied all the energy from my 3.5mF 330V bank... and guess what? It cut the tube in three halves!! Not to mention the wire literally 'vapourised' LOL.
This site is powered by e107, which is released under the GNU GPL License. All work on this site, except where otherwise noted, is licensed under a Creative Commons Attribution-ShareAlike 2.5 License. By submitting any information to this site, you agree that anything submitted will be so licensed. Please read our Disclaimer and Policies page for information on your rights and responsibilities regarding this site.