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Charging a capacitor with induction

  1. Jul 26, 2013 #1
    Hello all,

    I am new here. So you see where my lack of understanding comes from; my background is as a Mechanical Engineer and i'm currently in my masters year. I haven't touched electronics since A levels but I was hoping somebody could point me in the right direction. These forums tend to have a policy of helping people figure things out by themselves and I welcome that with open arms so I'm just asking for pointers and a little patience :)

    I'm trying to come up with a simple circuit that charges a capacitor with kinetic energy, preferably using a magnet. My initial thoughts are using a wire coil with the magnet to create a voltage but if you can picture a magnet in a sealed tube with a coil inside it and you're shaking this thing you'll presumably be creating an AC current if the coil is connected to a circuit. So my first question is can you charge a capacitor with an AC current utilising other components and my second question is, if not, how would you create a DC current with the coil and the magnet scenario so as to charge the capacitor and get it to store the charge for a period of time.

    If what i'm saying is gibberish to you knowledgeable people I'll try and rephrase.

    Thank you kindly for any responses,

  2. jcsd
  3. Jul 26, 2013 #2

    jim hardy

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    At any given instant, AC is flowing one direction or the other. What you need to do is capture it.

    Find one of those flashlights that you shake and take it apart. You'll find a magnet, coil, capacitor and rectifier inside. It'll be obvious to you then.
  4. Jul 26, 2013 #3
    Ok so I went out and bought one of those shake to light torches with the induction coil and magnet and shenanigans. It's got a rectifying bridge built into it to charge the capacitor. It's, as far as I can tell, a 5.5V, 0.22F capacitor but its got 2 3v lithium cell batteries in the circuit as well for reasons I can't glean. So i've managed to figure out how the circuit would work.. how would I determine what Farad capacitor I would need to supply 1.5-2 amps of current to a heating coil for ~30 seconds. Is that even feasible?
    Currently the LED in the torch is pulling about 20 mA so it's not going to produce the current I need..

    Many thanks!

  5. Jul 26, 2013 #4
    Well the thing Jim hardy was trying to tell you is that a capacitor can store charge and that is what it does but AC is not that good for this because it periodically reverses direction and magnitude so you would have to unplug the capacitor when the sine or square or whatever wave is at it's peak because if you would do that while the power of the AC is at it's lowest you would get a pretty much empty capacitor like in the beginning.

    That's why you have to rectify to DC which is constant at it's potential and current flow which leaves he capacitor always charge at it's max after the cap itself has reached it's max charge value which usually happens soon after you start to charge it.

    now the batteries are there so that the energy one created while shaking the flash light wouldn't go away as a capacitor stores energy in the form of electric field which doesn't last as long as desired so a battery is added to store that energy chemically.
    the capacitors are probably there to smooth out the induced voltage etc.But for that look up wikipedia or some other site about how they work.
  6. Jul 26, 2013 #5
    I understand how the capacitors work now, I already understood that AC cant charge a capacitor!

    So I have my circuit here, I can shake the bugger and get me a charged capacitor using the rectifying bridge.
    What I need it to do, and the reason I'm on here is for it to deliver 4V over a 2 ohm resistor to give me 2A current for about 30 seconds. I want to link it up to a heating coil and a switch so I can heat the coil with the 2A current in short bursts of 2-3 seconds. I'm just not sure about what size of capacitor that requires, I've read something about supercapacitors being able to discharge slowly!

    I've read as much as I can about capacitors but I was hoping just for some direction on what to look for specifically for determining what size of type of capacitor I need to complete the job! The charging with the induction coil and the shaking might also get a bit ridiculous if the capacitor is large...

    I might need to take the battery idea and put it into my circuit if there aren't capacitors than can hold 30s of charge for a decent amount of time! :) I hope you can understand the scenario more now.

    Any guidance is appreciated!
  7. Jul 26, 2013 #6
    Ok so I need a 15 Farad capacitor to store 60 coulombs in order to suppler 4V and 2A over a 2 ohm resistance heating coil for 30 seconds. Do those things even exist, that aren't the size of a large mug?

  8. Jul 27, 2013 #7
    15 farad is extremely large capacitance , quite honestly I think you just need a small everyday capacitor say about 2200 uF 10v or something like that and a battery to keep the rest of the induced current on hold , trying to store large amounts of energy for quite long time periods in a capacitor is not the wise thing to do , capacitors are not made for that , capacitors can indeed store quite alot of energy but not for a very long time ,. as when you unplug the pins of it it slowly stars to fade away.Ofcourse this fading varies with different types of capacitors and some can even hold their charge for a very long time say several hours but those would normally be bigger and high voltage.
    So I would say a battery is a good thought.
  9. Jul 27, 2013 #8

    jim hardy

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    I've never run across one of those 'shake' flashlights with a battery inside. Interesting....

    Anyhow, you are getting there.

    A capacitor stores charge just as a tank holds air.

    The units seem strange because they have the names of people not anything physical.
    The Farad is analogous to volume,
    The Volt analogous to pressure,
    The Coulomb analogous to amount of air - number of molecules or mass. One Coulomb happens to be the amount of charge equivalent to ~6E18 electrons..
    The Amp is analogous to flow rate of air, ie how much of it moves past a particular point every second. One Amp is one Coulomb per second going past perhaps a spot on a wire or through an ammeter, or perhaps into a capacitor.

    One Farad will store one Coulomb for every Volt.
    Accordingly, a one Farad capacitor that is delivering one Amp will lose one Volt per second.

    So to get your two amps you'll have to decide what rate of voltage decay is acceptable and size your capacitor accordingly. Fifteen Farads delivering two amps will lose (2/15) volts per second.

    here's a 150 farad... around twenty bucks

    Also you are demonstrating that the equivalence of work and heat involves a large conversion constant - which is fortunate for us else automobile brakes would have to be HUGE.

    Have fun!

    old jim
  10. Jul 31, 2013 #9


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    A shake torch must have a diode and a battery in it for it to work after you've stopped shaking, surely???
    A capacitor will hold charge (as an alternative) but the volts vary so much that the bulb will either not glow or burn out when the volts on the C are outside its operating range.
    Capacitors are 'brilliant' for some applications but they really don't do this sort of job anything like as well as ordinary Chemical Technology, with steady Volts available.
  11. Jul 31, 2013 #10

    jim hardy

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    The only one I ever took apart had a diode and 'super-cap' and no battery. The cap was open so it lit only while being shaken. I don't remember the capacitor's value but it would deliver tens of milliamps for a few seconds. Bear in mind i'm old, when I was growing up you'd have needed a wheelbarrow to carry one farad.

    I assumed the little electronic part, almost too small to see, was some sort of a current regulator.

    A battery would make such a torch almost practical . Having done some spelunking , one wants energy for his light to be chemistry based not muscle.
    Ahh, nostalgia - the Calcium Carbide lamp! It doubles as a bug zapper.
    image courtesy these guys: http://www.chem.wisc.edu/deptfiles/...Gen_Chem_Pages/22organicpage/carbide_lamp.htm
  12. Jul 31, 2013 #11


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    Perhaps it's me who's being old fashioned as I haven't played for long on a shaky torch. I guess they don't stay on for long and, on second thoughts, a battery would pretty soon be well knackered if it spent most of its life discharged. But I have a wind up radio which surely must have a battery??? V. confusing but I like your picture.
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