lightbulb cap bank charger / Electromagnetic Projectile Accelerators / Forums

Forums

4hv.org :: Forums :: Electromagnetic Projectile Accelerators

« Previous topic | Next topic »

lightbulb cap bank charger

1 2 next

Move Thread

LAN_403

JoeDarkon

Mon Nov 19 2007, 11:51PM

Registered Member #485 Joined: Sun Jul 09 2006, 01:14AM
Location: San Francisco
Posts: 36

I have build a cap charger before using a transformer light bulb and a rectifier but I just cant remember how to do it again and I dont want to mess up when dealing with the mains so can someone post a diagram I cant seem to find one

Simon

Tue Nov 20 2007, 12:14AM

Registered Member #32 Joined: Sat Feb 04 2006, 08:58AM
Location: Australia
Posts: 549

It's good to be wary of mains power where you don't know what you're doing. On the other hand, using some schematic you've grabbed from the web is still not much good if you don't know what you're doing.

As a general site rule, don't post for schematics. It's better to post an idea you have in mind and ask members for opinions. If you like, I'll unlock this thread for you to post a schematic of your own. Just PM me.

Edit: After PM conversation, thread has been unlocked.

JoeDarkon

Wed Nov 21 2007, 12:20AM

Registered Member #485 Joined: Sun Jul 09 2006, 01:14AM
Location: San Francisco
Posts: 36

Sorry about not puting my idea of a schem up...hows this one look... currently I have a 200W light bulb and im not sure about which transformer to use

Shaun

Wed Nov 21 2007, 03:31AM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

You would need a transformer that puts out a peak voltage about 10V less than your capacitors rated voltage, as a safety margin. If you have a 100V capacitor and charge it with a rectified 100VAC transformer, you are gonna be in big trouble, since 100VAC*1.4=140VDC peak.

What are the ratings of the capacitors you will be charging?

JoeDarkon

Wed Nov 21 2007, 05:30AM

Registered Member #485 Joined: Sun Jul 09 2006, 01:14AM
Location: San Francisco
Posts: 36

Aside from no ratings did I get the circuit correct? Also if I have no transformer will this give me 120v?(im in the US)
well I have several types of caps I want to charge (this is so I can charge them quickly for experimenting) one cap will be 450v 330uf another is 350v 6100uf (is there a difference between V and WVDC?) and another thats 420V 8000uf all of which will be in banks of various sizes (of same caps)

Shaun

Wed Nov 21 2007, 06:05AM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

Those are some big caps. Once you get this all figured out, they'll make a nice coilgun (if thats what you're going for).

But yes, your circuit is correct. However, on 120VAC mains, this will charge a capacitor to 170V. When you have an AC voltage rating, like in the US this is 120V, you must multiply this by the square root of 2 (~1.414) to get the peak voltage. This is the voltage a capacitor will charge to.

This is because 120V is the average voltage of the 60Hz sine wave that is used on the mains. If you think about a sine wave, it shows that the voltage will be zero once every half-cycle. To have a 120V AVERAGE, it must a have a higher peak voltage to compensate for the zero crossing.

JoeDarkon

Wed Nov 21 2007, 07:17AM

Registered Member #485 Joined: Sun Jul 09 2006, 01:14AM
Location: San Francisco
Posts: 36

In addition to or instead of a transformer can I use something like light dimmer to vary the voltage so I can charge at different voltages?
and yes I am going for a coilgun at first I was making single stage with a large cap now I am trying to make a smaller multistage one with the smaller caps...if I can only get over my switching problems... but then after I get a smaller one I have 4 of the 420v 8000uf caps and I think 6 350v 6100uf which can make a nice and big coil gun... more like coil cannon

Shaun

Wed Nov 21 2007, 07:30AM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

I think a light dimmer would work, as it does reduce the voltage by chopping the sine wave. However, you could also just shut off your charger when the caps reach the desired voltage.

JoeDarkon

Wed Nov 21 2007, 06:14PM

Registered Member #485 Joined: Sun Jul 09 2006, 01:14AM
Location: San Francisco
Posts: 36

If I put a transformer that ups the voltage to something around 500v wont that damage the light bulb? Shouldn't I have to put the transformer after the bulb? Also I want to double check to see if it is ok to charge a cap with a source that is beyond what it is rated as long as I stop the charging before the cap hits its rated voltage... and one final thing is how do I calculate the max amperage a certain watt bulb will pull so I can know which transformer to get

Dr. Slack

Fri Nov 23 2007, 01:00PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Yes, you should put the 120v bulb on the 120v side of the transformer.

The current it pulls when hot is just given by rated power/volts, so a 60W bulb will pull 500mA. But, the metal filament has a very strong positive tempco, and to a surprisingly good first hand-waving approximation it draws a constant current, the resistance dropping as it cools, as long as the rate of change of load is slow enough for the temperature to follow.

OTOH, when you switch on from cold, the filament can draw a very large current, 20x rated easily (you can check by measuring the cold resistance of the bulb). In the first few mains half cycles, before the bulb has had time to warm up, the current draw will be high. The transformer does not need to be rated for this surge, only the continuous, but your fuse will blow unless it is generously rated or is a time delay type. Your charging diodes will also need to survive this transient, but most are inrush rated to deal with capacitively loaded bridges.

Yes, it's OK to charge a cap with a source which has the *potential* to output more than the cap rated voltage, as long as it doesn't *actually* charge to higher than their rated. If you either arrange to switch off before that's exceeded, or shunt your caps with some beefy Zeners when prevent overcharging, then there's no problem. Of course if your source *can't* put out excess volts, and you have Zeners or a switch, then more things have to go wrong simultaneously to put you or your gear in danger. Do you feel lucky, punk? (Eastwood, Harry, Dirty).

1 2 next

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob