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Hi, To charge my railgun capacitor (6 4700uF 400V low esr) I have a variac connected to a standart 230V circuit (live in france), then there is a microwave oven transformer and finally a bridge diode.Both the variac and transformer are about 800W. Without the bridge diode ,when the transformer is connected to nothing the variac is getting hot slowly but surely,but with the bridge it starts burning (smoking) very quickly.Its supposed to be 3A max,so i guess this is the prob.To correct this should I add resistor before the variac or after?before the transformer or after?What value and what watt do I need for the resistor?I thought about a lighbulb as the resistance varies but I'm afraid it will slow too much the charge. And I read several times that I need an isolation transformer,but I dont see the point as touching the capacitor is lethal anyway ,right?So I don't see how it makes things safer. Merry christmas to all Sebastien
Registered Member #1062
Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529
Hello, 3A is way too low. You need to put the resistor before the variac, so it is constant. A light bulb will actually work fine, try that. For my railgun capacitor bank (400v .15F, 12kJ), I use a voltage tripler connected to 120 volt mains. The voltage tripler is limited by 4 225W 50ohm resistors in parallel, I do not use any variac. If your interested in switching methods, here is my bank/tripler and current limiting resistors. the tripler is on the lower right.
Try using a lightbulb, and if it is too slow, parallel two lightbulbs and so on so forth.
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716
magictomcat wrote ... Without the bridge diode ,when the transformer is connected to nothing the variac is getting hot slowly but surely,
Welcome, Sebastian. Merry Christmas! It's normal for variacs & comparable transformers to slowly get hot when operated at their rating. MOT's will get very hot, even if unloaded, without forced air cooling. I've seen magnetizing current of 6 A in a 120V MOT primary (would be higher if measured with a true-RMS meter), and core loss is independent of output loading.
Heating should not be an issue if your capacitors can be charged within a few minutes. A MOT should be able to charge them in seconds. Less than a minute if the C's are in parallel, which would be a terribly mismatched application of a MOT.
but with the bridge it starts burning (smoking) very quickly.
That would happen if MOT secondary were connected to the wrong terminals of the bridge, creating a DC short circuit in one direction. Or if one side of bridge output and one side of MOT secondary are grounded. If your bridge voltage were exceeded, the smoke would come from the diodes.
An AC current meter would serve you well in this hobby. The clip-on kind is extremely convenient, though not cheap. [edit] Even more importantly, have a proper fuse in circuit between variac and MOT. Replacing a fuse is much easier than fixing a burned transformer winding.
Please be careful. It would be simpler and a bit safer if one side of capacitor bank and MOT secondary are well connected to earth ground, and you use a half-wave rectifier. In any case the charger should be disconnected (or isolated by a series resistor) before firing your railgun.
rp wrote... 3A is way too low.
Remember that's in Europe, so variac and MOT primary currents will be half of what we're used to un USA. THe 50-Hz-rated transformers will use more iron. Note that variac ampere ratings apply to both input and output currents.
I've just received my new multimeter so I'll check. I have 4 5W resistor rater at 47Ohm,putting them in // will be enough? And I don't see how grounding the circuit makes things safer,can you detail? And finally the bridge is well connected and I dont know why but when I used a 1600V 35A rectifier it burns quickly,with the 500V 12A its much slower.
Thanks anyway for your help guys,I really appreciate
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Capacitor charger:variac burning
