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Introduction and help w/ quenching coil

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BlakFyre

Wed Jun 25 2008, 05:58AM

Registered Member #1563 Joined: Wed Jun 25 2008, 03:55AM
Location: Wimer Oregon, Wewt for sticks!
Posts: 30

Hi! Quite obviously my first post so I'll try desperately to not mess it up. First of all, I'm currently a college student enrolled in my last term of physics which is dealing with electric and magnetic fields. One of the assignments is a full blown lab project, and was last reason I needed to build a coilgun. =D I had many of the components I needed already and only required my instructors approval (acquired through a couple rounds of pleading and his own curiosity). I worked all night that night and had a working project by morning, lacking cosmetics and proper measuring devices. So here are the specs:
Caps:
Two 400v 2200uf electrolytics in series.
(can you believe I have NO idea where I got these)
Coil currently installed:
200 ft 14 gauge wrapped around a pen tube.
(well, 60 meters to make math friendly)
Power Supply:
150w Computer PSU (12v rail) and a flyback. Output unknown except for I know its over 800v
Projectile:
Generally a 8cm in length and .6 cm in diameter steel rod.
Goes through two sides of a cardboard box so far... untuned and at 250v.

Anyways, my first shots were at 250-300v (and worked), unfortunately all I had was a protection diode (most likely under rated :/) that was going into reverse breakdown as the coil was trying to discharge, pesky little buggers. Got close to 50v back into my caps so I'm not happy about that. I'm now trying to quench it with a parallel diode and resistor so that the current loops back through. This worked great at 250v with noticeable increase in power and no reverse voltage. Actually, my caps never fully discharge. However, at 300v I again got reverse voltage into my caps, I adjusted my resister value a couple times until it again, acted like before. So, I'm guessing that the difference between the charge in the coil and the voltage drop from the resistor forward bias the quenching diode so it conducts, however there is a point where the diode no longer is forward biased and so the rest of the charge relieves itself once again through my protection diode thus recharging my caps, and not in a good way. Is there a way to calculate the needed resistance for the quenching resistor... or am I going to just have to change if I want to fire at different voltages?

rp181

Wed Jun 25 2008, 01:21PM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529

nice coilgun, just 2 things :
200ft of wire is way too much, use like 50 turns.
How are you switching it?

Be carefull with the flyback, thats probably more then 10KV, make sure you dont over charge the caps.

BlakFyre

Wed Jun 25 2008, 01:34PM

Registered Member #1563 Joined: Wed Jun 25 2008, 03:55AM
Location: Wimer Oregon, Wewt for sticks!
Posts: 30

I'm aware of the possibility of over charging... although I would have to let it sit for like 10 minutes for it to reach 800v... and I also know that 200 ft of wire is way too much, but I'm studying the effectiveness of many things and coil size is one of them, I only had about 300ft total and didn't want to unwind and rewind a new coil every time I wanted to change sizes.
my switching method is quite crude, is a manual switch made with an insulated solenoid and copper bus bar (ie. flattened copper pipe)

Oh, and I'm starting to be able to guestimate the charge in the caps by the pitch of the transistor and flyback which is neat =]
Anything I can do to charge this thing better that's fairly easy?

Bust_A_Cap

Sat Jun 28 2008, 09:51PM

Registered Member #1561 Joined: Mon Jun 23 2008, 07:58PM
Location:
Posts: 25

So your just plugging the capacitors into your flyback and then hold the multimeter on it until it reads a voltage you want?

BlakFyre

Mon Jun 30 2008, 10:22PM

Registered Member #1563 Joined: Wed Jun 25 2008, 03:55AM
Location: Wimer Oregon, Wewt for sticks!
Posts: 30

Essentially. Is there a problem with this?

rp181

Tue Jul 01 2008, 12:33AM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529

naa, its fine, i do it all the time.

BlakFyre

Tue Jul 01 2008, 01:20AM

Registered Member #1563 Joined: Wed Jun 25 2008, 03:55AM
Location: Wimer Oregon, Wewt for sticks!
Posts: 30

That's good. However, I'm still having issues with the opposite charge on my caps and I've pretty much stopped firing it until I have a better idea on what to do. My diodes are rated at well over 300v which is the highest I've gone so far. I think I'm using my RURG5060 diodes currently.

rp181

Tue Jul 01 2008, 01:46AM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529

since it is a mechanical switch, wire a diode antiparrallel with the coil AND use one in series with the switch.

BlakFyre

Tue Jul 01 2008, 03:09AM

Registered Member #1563 Joined: Wed Jun 25 2008, 03:55AM
Location: Wimer Oregon, Wewt for sticks!
Posts: 30

I have done this previously , and it does indeed work fine. This only makes it so the coil dies through its own resistance. My goal was to place a resistor in series with the diode that is in antiparallel with the coil so that it has added resistance when the field is collapsing, thus shortening the drop-off time considerably. This does indeed work to an extent... but as I said earlier, I have had to adjust this value (through very unscientific means) when I fire at different voltages. I was hoping for a helpful hint for calibrating this resistance, as I would like to refrain from accidentally blowing or damaging my caps.

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