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First coil gun, 6 cameras, which charge circuit?

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Rustycan

Sat Apr 02 2011, 11:36PM

Registered Member #3805 Joined: Sat Apr 02 2011, 09:06PM
Location: Jakarta, Indonesia
Posts: 14

doesn't the caps overcharged if you close the charger switch all the time?

guitarlord66

Sun Apr 03 2011, 03:48AM

Registered Member #1805 Joined: Sat Nov 08 2008, 06:29AM
Location:
Posts: 67

I've only ever had a faulty cap blow, the 330v caps usually sit around 380v but I don't leave it going, if I'm gonna shoot, I charge and shoot.
I don't know what would happen if you charged and left it for 5 minutes or so, I'd consider it a waste of battery life because it has to regulate the voltage due to the capacitors leaking...

Rustycan

Sun Apr 03 2011, 06:09PM

Registered Member #3805 Joined: Sat Apr 02 2011, 09:06PM
Location: Jakarta, Indonesia
Posts: 14

i'm totally noob about these stuff @_@
what makes a cap blow anyway?

Barry

Mon Apr 04 2011, 03:40PM

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

Rustycan wrote ...

what makes a cap blow anyway?

The electrolytic capacitor uses aluminum-foil plates with a conducting semi-liquid chemical compound between them. The actual dielectric is a very thin film of insulating material that forms on one set of plates through electro-chemical action when a dc voltage is applied. The capacitance obtained with a given plate area in an electrolytic capacitor is very large, because the film is so thin. Much thinner than anything practical with a solid dielectric or any sort of physical separation between the plates. However, this also causes the breakdown voltage to be much lower than with solid dielectrics. The electrolyte is necessarily an acid; therefore it is extremely dangerous if heat builds up inside an electrolytic capacitor. The heating effect (think of current through a resistance) can make it bulge, leak and even explode from steam pressure. Look carefully at your higher quality electrolytic capacitors, and you may see a scoring mark 'X' at the terminal end which is designed to rupture and leak before the can explodes.

Cheers, Barry
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Rustycan

Mon Apr 04 2011, 04:58PM

Registered Member #3805 Joined: Sat Apr 02 2011, 09:06PM
Location: Jakarta, Indonesia
Posts: 14

Barry wrote ...

The electrolyte is necessarily an acid; therefore it is extremely dangerous if heat builds up inside an electrolytic capacitor. The heating effect (think of current through a resistance) can make it bulge, leak and even explode from steam pressure. Look carefully at your higher quality electrolytic capacitors, and you may see a scoring mark 'X' at the terminal end which is designed to rupture and leak before the can explodes.

so it's the amps make it blown?

Barry

Tue Apr 05 2011, 12:36PM

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

Rustycan wrote ...

so it's the amps make it blown?

Caps won't ever blow unless (a) you charge it to a higher voltage than their rating, or (b) their internal insulating layer has problems. A huge amount of current during a discharge is not a problem, it simply takes the energy from the electric field between the plates and sends it out the terminals. The beauty of capacitors is that it can dump all that energy extremely quickly, much faster than batteries or devices that depend on chemical reaction to provide current.

So, for (a), if you charge it to a higher voltage than its rating, the capacitor's insulating layer cannot resist the voltage potential, and it will punch through. This will suddenly discharge the capacitor's energy internally which causes heat, which in turn can boil the acid.

And, for (b), if something happens to damage the internal insulating layer, then again all that energy stored in the capacitor is discharged internally. Yeah, bad things will then happen. But things damage the insulator? Our biggest concern is a reverse voltage on the capacitor. If you charge an electrolytic capacitor in reverse then the chemical reaction that forms its insulating oxide layer can be un-done. Therefore most coilguns do something in their design to avoid getting a negative voltage on the capacitor's terminals.

Hope this helps! Barry
If you try to fail, and succeed, what have you done?

James

Tue Apr 05 2011, 06:13PM

Registered Member #3610 Joined: Thu Jan 13 2011, 03:29AM
Location: Seattle, WA
Posts: 506

When my friend worked at an electronics repair shop, one of the favorite gags to play on co-workers was to turn off the power strip on someone's bench and stick an old electrolytic capacitor in one of the outlets. They'd come in the next morning, flip the switch, and BANG! Just don't have the can pointing towards anyone's face.

The same thing will happen sometimes if you accidentally install one backwards. It's quite startling to have one pop while you're poking around inside a piece of equipment. The can shoots off and you get a poof of shredded cardboard bits from the jelly roll.

guitarlord66

Wed Apr 06 2011, 05:27AM

Registered Member #1805 Joined: Sat Nov 08 2008, 06:29AM
Location:
Posts: 67

Just so you don't make the same mistake as me, Put your capacitors INSIDE the coilguns case, I decided for aesthetic reasons I'd put them poking out, see the picture below...

One time when I was firing the gun, one of the capacitors blew and smacked me in the face...

I don't know the reason why it blew, I'm assuming its because its was faulty in some way, because its the only one to blow, maybe it couldn't handle the same voltage as the others? Their all the exact same capacitors scavenged from the same type of disposable cameras though.

Barry

Wed Apr 06 2011, 01:54PM

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

I love how you have a guitar in the background to go with your screen name!

guitarlord66

Thu Apr 07 2011, 01:07AM

Registered Member #1805 Joined: Sat Nov 08 2008, 06:29AM
Location:
Posts: 67

Haha I didn't even notice that! I got about 5 guitars so I guess where ever I take a picture I'll get a guitar, lol.

BTW the black thing my gun is sitting on, thats my guitar amp head :)

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