Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Charging Capacitor with 12v battery

  1. Apr 25, 2008 #1
    How would I go about upping the voltage from 12V to 300-400V for charging capacitors
     
  2. jcsd
  3. Apr 25, 2008 #2
    The usual approach is to design an inverter from 12V to 300V. An inverter is basically an oscillator driving a step up transformer.
     
  4. Apr 25, 2008 #3
    Could you elaborate some, my knowledge of circuits is pretty limited
     
  5. Apr 25, 2008 #4

    mgb_phys

    User Avatar
    Science Advisor
    Homework Helper

    And you want to play with Caps charged to a few hundred volts - been nice knowing you!!
     
  6. Apr 25, 2008 #5

    rbj

    User Avatar

    yeah, Jack. you could be dead soon.
     
  7. Apr 28, 2008 #6
    Research!
     
  8. Apr 28, 2008 #7

    berkeman

    User Avatar

    Staff: Mentor

    As the others have said, you need to learn a bit more about electronics and safety, before you go messing with voltages above about 50-60V.

    That said, the straightforward way to convert 12Vdc to 300Vdc is with a DC-DC converter (hence the name). The topology of DC-DC converter for increasing the voltage is called the "boost converter", and the basics are described here:

    http://en.wikipedia.org/wiki/Boost_converter

    Please consider making your first boost converter one that boosts 12Vdc to about 30Vdc, to see how the basic circuit operates. Before you start working with voltages higher than that, you need to understand some of the safety rules regarding high voltages (because of the risk of shock and fires).
     
  9. Apr 29, 2008 #8

    rbj

    User Avatar

    at least higher voltages that are backed up with essentially unlimited current. some high voltage transformers that drive certain Xenon or similar light sources have enough internal resistance that the max current they dump out might not kill you. (i remember in undergraduate physics lab doing something with sepctroscopy and looking into some spectroscope with my other eye closed, reaching over both hands to adjust the placement of some neon light or similar and get knocked to the floor (a convergence or comedy of errors). my left hand touched one terminal and my right touched the other terminal. if the high voltage transformer had unlimited current, i think the school could be sued for exposing students to such danger.
     
  10. Apr 29, 2008 #9

    mgb_phys

    User Avatar
    Science Advisor
    Homework Helper

    I would like to thank (probably posthumously) the unknown lab tech that built the 'old-smoky death-omatic' 10Kv power supply for a photo multiplier I once used.
    They decided the safest connector for the 10Kv supply was an BNC 'cos it was shielded - until you unplugged the lead and touched the centre pin.
     
    Last edited: Apr 29, 2008
  11. Dec 11, 2009 #10
    I am sorry to have to say that a limited-current supply won't make this safe: once charged, the capacitor may well be able to deliver a dangerous current, albeit for a short time.

    Personally, I nearly didn't make it out of my teens because of just such a setup. The capacitor took a minute or two to charge up, but an awful long time to go down by itself. There was still plenty of juice left when I touched it, hours after turning it off.

    I think that anyone who is really ready to mess about with little inverters would find a blindingly obvious place to get them from, but let's not spell this out so some kid can go and hurt him/herself.

    BTW, why do so many people want to charge capacitors up to dodgy voltages?
     
  12. Dec 11, 2009 #11
    I have no idea as well. Don't people know high voltages could be dangerous to their health?

    I recall a time at work I opened up an instrument to work on it from the inside. I made the mistake of wearing my metalic watch. Typically nothing is exposed, but I had the shields off so I can monitor some lines with my scope. My watch hit a live line and gave me a jolt. From that experience on, I don't dare mess with high voltages.
     
  13. Dec 11, 2009 #12
    A boost converter would probably be the easiest approach, but I must agree with the others in that you must be very careful when working with voltages above 50V. Don't be misled by the size of capacitors your using either, a typical photo flash capacitor for example, is about the size of a "C" battery, yet holds 16.5J of energy when it only takes 16J to kill you. You can also get electrical burns to your inside tissue from current passing through muscle and skin. A good habit to get into when working on circuits like this is to keep one hand in your pocket to reduce the chance of passing current through your heart, use a voltmeter to verify that everything is discharged before you touch it, and put bleeder resistors on all high voltage capacitors as they can regain some charge even after they have been discharged. Here is a good site about electrical safety, you should read it and understand it before you ask anymore questions about how to build something like that.
    http://ehs.okstate.edu/modules/electric/index.htm
     
  14. Dec 11, 2009 #13
    Hey guys I think this post is a little old
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?