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Help with capacitor bank charging please

Discussion in 'General Electronics Discussion' started by hyudryu, Feb 24, 2010.

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  1. hyudryu

    hyudryu

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    Feb 24, 2010
    I want to use transformers and transform my outlet(120V AC) to around 330V AC then i can rectify it and make it 330V dc. Which transformer should i use? and where can i get that?
     
  2. Mitchekj

    Mitchekj

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    Jan 24, 2010
    If you rectified 330Vac you'd get ~467Vdc... this sounds dangerous, especially if you're not 100% on working with mains. There are a number of transformers which will get you ~233Vac (which would rectify to ~330Vdc.) You may end up needing to smooth that voltage out, regulate it, etc. What kind of current are you looking at? What are you trying to do here exactly? That will help you decide which transformer to use: the end goal.
     
  3. hyudryu

    hyudryu

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    Feb 24, 2010
    I would like a high current because i will be charging a LOT of capacitors connected parallel. Which adds up to about 1.8 Farads
    Ok then i would just like to transform 120VAC to 230VAC and then i can rectify it with my high voltage high current diodes and a large capacitor
    And im making a coilgun for fun. I used capacitors from a camera and i made a big bank of it. So the faster it charges, the better. I decided not to use the circuitry that comes with the camera because its too slow and is too energy consuming
     
    Last edited: Feb 24, 2010
  4. 55pilot

    55pilot

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    3
    Feb 23, 2010
    I have a baaaaad feeling about this.

    What capacitors are you using to make a 1.8F 330V capacitor bank?

    Do you know that capacitors have a voltage rating?

    Do you realize that you are looking at storing about 10KW of energy in the bank?

    What do you plan to do with the energy when the caps are charged up?

    ---55p
     
  5. hyudryu

    hyudryu

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    Feb 24, 2010
    Really? the voltage ratings are standard for camera flash. 330Volts
    and i plan to use a darlington power transistor at 400V and 20A and short it on a coil then it creates a magnetic field and pulling the ferromagnet like a bb bullet or a ball of steel wool and it will shoot out and hit a gel target
    Are you sure about 10000 watts? because there is resistance on my coil so it wont pass that fast so only about 700-1000 watt is going through the coil maximum
    And dont worry!!! i will insulate all the "hot" wires and the capacitor bank with a plastic box and electrical tape

    And i wont shoot anyone or any living things dont worry. only gel targets
     
    Last edited: Feb 25, 2010
  6. Resqueline

    Resqueline

    2,848
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    Jul 31, 2009
    A little math & clarification: stored energy is measured in Joules, in your case 98000J. (Capacitance / 2 * Voltage squared). Charging that bank with 2kW of constant power will take 50 seconds. Charging it with a constant current of 18 Amps is going to make the voltage rise with 10V/second, making for a 33 second charge.
    Now, the challenge is, how are you going to go about hooking that big short-circuit to the mains w/o blowing fuses or burning transformers?
    And I don't think that 20A transistor is going to hold up for many milliseconds.. At that current level you could also just as well have operated straight from the mains. The point with such a capacitor bank is to be able to pulse out thousands of Amps..
     
  7. hyudryu

    hyudryu

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    Feb 24, 2010
    I dont really care about the charging speed as long as its under 1 min. I need the DC voltage to be 330V and the current should be around 5A-10A
    And i will solder 2-4 power transistors parallel so the current will be divided and i will be operating those transistors on a 9V battery as a trigger and i will only send 1 pulse every time i click the trigger with my 555 circuitry
     
  8. Resqueline

    Resqueline

    2,848
    2
    Jul 31, 2009
    The point is not wether you care about the charging time or not, but how you intend to make a charging circuit that is up to the task at all.. It's not as easy as using a transformer and perhaps a resistor.
    Use 4 such transistors if you will, they'll still be going blam.. If they don't then your gun isn't work very well anyway.
    What are your resources?
     
  9. hyudryu

    hyudryu

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    Feb 24, 2010
    Oh ok so its not as easy as i think?
    Because i was going to use the outlet's AC as my power source for a voltage ladder and i can simply double it by using a 330V 120uF capacitor and a bunch of high voltage diodes.then i would get 240VAC which turns out to be around 340VDC and then i can use high current resistors and get it down to 330VDC and then connect that to my capacitor bank. I lessened the capacity on my bank to around 1F because 2F makes the projectile shoot out but then shoot back because there is still a magnetic field and it pulls it back and then runs out of power so it ends up shooting backwards -_-
    OR i can make it dual stage and attach another coil to divide the current and it will have double the magnetism
    Well, i have to get more magnet wire and a bunch of stuff
     
    Last edited: Feb 26, 2010
  10. 55pilot

    55pilot

    434
    3
    Feb 23, 2010
    The 10KW came out of the fact that I somehow though you were planning to discharge the caps over 10 seconds (100KJ over10 seconds). Looking over what you wrote before my post, I have no idea where I got that impression. My bad.

    What you are trying to built is called a coil gun. You can find a number of designs and discussions on the web. Some are downright dangerous; far more so for the person building and firing it than for whatever is being shot at.

    You are dealing with some fairly dangerous stuff with minimal appreciation for the risk. I really do not feel comfortable with this. So I will suggest that you not do what you are thinking of doing because it is dangerous and bow out of the discussion.

    ---55p
     
  11. hyudryu

    hyudryu

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    Feb 24, 2010
    I know my design is a coilgun. I have a lot of saftey precautions and i know exactly what i am doing. I wont shoot it at any living things. and i tested it today and it went around 30 feet at about the speed a nerf gun would shoot at. I am putting that inside a PVC tube and attaching a handle. I know that this is dangerous so i will not really use it... the main purpose is to put it on my shelf as an achievement.
     
  12. Mitchekj

    Mitchekj

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    Jan 24, 2010
    That's not what everyone's worried about. The high voltages and stored energy is far more dangerous than any projectile you'll be firing in this case. That's what we're worried about, and in that respect it doesn't sound like you know exactly what you're doing. It can, and will kill you if given the chance.
     
  13. 55pilot

    55pilot

    434
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    Feb 23, 2010
    Mitchekj clearly articulated both his and my concern. With the level of technology you have, I am not too worried about the well-being of your target, even though you can still kill or injure someone if you are careless or irresponsible. I am much more worried about your well-being.

    You are dealing with A LOT OF ENERGY in those caps with absolutely no precautions. Those caps can EXPLODE severely hurting or killing you or those around you. You are dealing with high enough voltages that can cause FATAL ELECTROCUTION. At those voltages, with that much stored energy, when something goes wrong, it will go wrong in a spectacular way and you seem to have no concept of those risks.

    Bottom line is that you have no appreciation of the amount of energy you are handling and the risk it poses to you and those around you. My advice once again is to quit before you or someone else gets killed or seriously hurt.

    ---55p

    Edited to add a data point that may get your attention. Your fully charged caps have 100KJ of energy. That is the same energy found in 0.8 ounces of TNT.
     
    Last edited: Feb 27, 2010
  14. hyudryu

    hyudryu

    64
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    Feb 24, 2010
    Oh. But my coil has resistance and it will charge up flux causing a magnetic field right? which acts like a resistor so there wont be thousands of amps going past the coil.
    When i tried to shoot it by 2 wires contacting, there was a big spark and the tips of the copper wire melted and fused together. I dont think thats 1000 amps because the outlet is 20A and it can vaporize copper so 1000A can cause the surrounding air to form plasma right? i would say that my current may be 40-60 Amps I can upload pictures in a few days...
     
  15. 55pilot

    55pilot

    434
    3
    Feb 23, 2010
    You are just reinforcing what has already been posted: You do not know what you are doing. Quit before you kill or seriously injure yourself or someone else.

    ---55p
     
  16. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,497
    2,838
    Jan 21, 2010
    OK, a couple of observations.

    He mentioned camera flash units. If he got them out of a studio flash unit (the 3000Ws things) he would already have a charger for them. My guess is that he has them from many disposable cameras.

    If so, they are typically 330V 160uF. I would not be planning to charge them beyond 300V so as to leave a little safety margin. These devices can easily discharge at over 100A, so they're not the sort of thing you want to be playing with when fully charged (11J of energy is easily enough to kill you if applied correctly).

    If he is talking about 1.5F then he must have 10,000 of them -- maybe he's calculated that incorrectly or he has something I'm not expecting.

    I have charged 16 of these in parallel using an isolation transformer with a light bulb (100W) in series with the primary to limit the current. Even that takes a while and the resulting bank of charged capacitors were seriously dangerous.

    To discharge, I'd actually consider using a xenon flash tube -- they're cheap and if you got them with the capacitors they'll handle the energy from a single capacitor easily. Trigger the xenon flash in the usual manner. You'll be left with 20 to 30 volts across the capacitor as that is the voltage that they drop out at.

    If you're discharging many capacitors at a time, then look at a device like a SKT 760/12D to initiate the discharge. These are very rugged SCRs and can handle surge currents in the thousands of amps.

    Unless your rail gun is a single stage, timing will be a major issue, and one I wouldn't be able to assist you with.

    The really great thing about a rail gun is that it provides so many opportunities to win a Darwin award.
     
  17. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,497
    2,838
    Jan 21, 2010
    A single 160uF capacitor charged to 300V will vaporise bits of a screwdriver used to short it out. The estimated current through a xenon flash tube from one of these is in the order of 50 to 100 amps.

    If you had 100 of these in parallel (about 1% of your claimed capacitance) you could easily get 5000 amps from them.

    Do you have any idea of the resistance of your coil an/or the inductance? Do you know at what current it will saturate? What will you do to prevent the capacitors from being charged in the reverse direction due to the inductance of your coil? How will you absorb this energy?
     
  18. davenn

    davenn Moderator

    13,837
    1,952
    Sep 5, 2009
    well I guess if we dont hear from him again then we know he has won the award :rolleyes:

    the mind boggles with some of the things I read on the forums from some ppl :eek:

    cheers
    Dave
     
  19. neon

    neon

    1,325
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    Oct 21, 2006
    You are messing with lethal stuff 1.8 FARAD is huge if that gets old of you you will be history i cannot believe anybody don't tell you that. if shorted the flash will blind you for sure. GIVE IT UP
     
  20. Mitchekj

    Mitchekj

    288
    0
    Jan 24, 2010
    There's about 5 posts above that say that exact thing, btw. He's dead (pun) set on doing it aparently.
     
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