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Does a capacitor have to have a dielectric?

  1. Dec 21, 2014 #1
    Suppose AC current is shooting into a wire, and the wire is attached to a bulk piece of metal off to the side of it, kind of like a ground, but not as deep as a ground. Wouldn't the bulk piece of metal work kind of like a capacitor and smooth out the voltage a bit? Everywhere I Google, I see the same definition of capacitor, which says it has to have a dielectric between two plates.

    I want a 1000uf 50V non-polarized capacity, but can't find one for sale anywhere. So I want to build my own. I'm having trouble finding any equations for electron electron repulsion in a fixed area of Culombs, stuck in the area by a voltage.

    I'm designing a battery charger, and want to use some capacitors to help smooth the voltage.
    Last edited: Dec 21, 2014
  2. jcsd
  3. Dec 21, 2014 #2
    Maybe I just need to buy a 36V charger, open the transformer, and change the number of winds.

    Do cheap battery chargers that convert AC to DC all have capacitors in them to smooth the current already?

    I found some 1000uf capacitors, but they are all bipolar, and many of them also are rated 35V or less.
  4. Dec 21, 2014 #3
    Well, I found some ac to dc circuit diagrams online. They converted to 9 volts, not 36, but used capacitors with half the capacitance I calculated. I feel good being in the ball park. They also put a resister from positive to negative before the battery, as an overflow relief. Clever. Will copy. And they used bipolar capacitors, which are lower quality but much cheaper than non polar. So maybe they can work.

    Does this mean that the wall outlet an forcefully push, but can't forcefully pull current from the capacitors, and that is why the bipolar ones don't break?

    I discovered there is an electrical engineering forum. I wonder if I can move this thread.

    As for modifying an existing charger, I suspect they too have bipolar capacitors. Also, most are rated 1.5A or at most 2A. Maybe that is OK. Maybe I might need different capacitor capacity if I change the voltage. I'll have to work that out. I think if you up the voltage, more current overflows, but the capacitor can take more current at higher voltage.
  5. Dec 21, 2014 #4

    Philip Wood

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    Battery chargers have to be dc. When being charged, batteries need current going through them in the direction positive terminal to negative terminal, that is in the opposite direction to the way it flows through them when they are supplying energy. For this to happen, the positive terminal of the charger needs to be connected to the positive terminal of the battery, and negative to negative. What's more the emf (open circuit voltage) of the charger has to be somewhat greater than the emf of the battery. You also need to include a series resistor to limit the current. These are the basics. Details depend on exactly what batteries you wish to charge.

    You will need to include a correctly rated diode or, better still, bridge rectifier in the circuit on the low voltage side of the transformer. This will give you unsmoothed dc. Connecting a largish capacitor across the unsmoothed supply will make it smoother. Since the voltage never changes direction you can use a polarised (electrolytic) capacitor. Do get someone to check your circuit before powering up.

    On the question of dielectrics, all capacitors need an insulator between their two plates (or conductors). A vacuum would do (though it usually isn't practicable – or desirable). Many insulators (e.g. polymers) are also dielectrics: their presence gives a higher capacitance than a vacuum would, because of polarisation of their molecules. Air at ordinary densities is an insulator and it increases the capacitance very slightly, so strictly it's a dielectric.
  6. Dec 21, 2014 #5


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    Hi stargazer. You raise a few interesting questions in your dialogue. The "bipolar" capacitors you keep discovering are actually quite the opposite---they are not bipolar. The polarities you see marked on their case indicate the polarity that must be observed in operation to preserve their dielectric because it is an oxide film produced and maintained by electrolytic action in a conductive electrolyte between the plates. Search on 'electrolytic capacitors'. Battery charging filters do not need bipolar capacitors. A common use for a bipolar aka unpolarized capacitor is in the starter circuit of the long fluro light tube fittings, where the capacitor must withstand AC with its two polarities.

    You are correct in thinking that an isolated lump of metal without an associated parallel companion will still exhibit capacitance. But the large capacitances needed for filter capacitors are impractical unless you use parallel plates. Wikipedia says that a single conductive sphere (it can be hollow to save weight) the size of the Earth has a capacitance of just 710µF. http://en.m.wikipedia.org/wiki/Capacitance#Self-capacitance
    I like an enquirying mind. Good luck with your continued investigations. http://thumbnails112.imagebam.com/37333/0363e9373324851.jpg [Broken]
    Last edited by a moderator: May 7, 2017
  7. Dec 21, 2014 #6
    Thank you for the hollow sphere info. That answers that question. It makes sense too since the surface is much smaller than the volume, so the electrons would be pushing up against each other.

    Are you sure the battery can handle voltage spikes? This is for a lithium ion battery.

    If I were to smooth the voltage with a capacitor, how soon does one wave of electrons exit one side of a battery after a wave enters the other side? Is the lag anywhere close to 1/60 of a second for a ten cell battery?
  8. Dec 21, 2014 #7


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    I am concerned about how competent you are to be contemplating this project - judging from the wording you are using in your questions.
    The battery behaves somewhat like a large Capacitor, in that it will have a particular voltage across it until sufficient current enters it - and charges it up. You would need a vast Capacitor to achieve what you want and, even then, the smoothing would only be there if there were an appropriate series source resistance (you're talking RC Low pass filter). If you connect a battery to a supply with a very low source resistance (i.e. a true Voltage Source) you could blow up the battery if you exceed the required charging voltage. (Infinite energy source and as much current as you could ever demand). If the (safe) charging circuit consists of a simple voltage source in series with a 'sensible' value of resistor, any voltage spikes will produce current through this series resistor and there will be a voltage drop, limiting the volts on the battery terminal. This is not a'smart' way to charge a battery if you want to charge it as fast as it will accept the charge. You can get away with a simple voltage source and high series resistor for charging many types of battery. NiCads can charge for ever at the 10 hour rate, I believe. Same with trickle charging Lead Acid accumulators (subject to topping up with water now and again.)
    You need to be quite careful with DIY battery charging, though and it's hardly the best thing to cut your electronic design teeth on. Dismantling and rewinding transformers is not for the squeamish either. The only Capacitors you should need in battery charger would be in the control circuit electronics - so you can save yourself some money in that direction.
    Look at the particular battery that you want to charge and then see what the manufacturers recommend for charging it. I would not leave the room if I built a battery charger that was not built on that basis. (Second thoughts: I might leave the room rather quickly!) Please do this thing properly.
  9. Dec 21, 2014 #8


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    The lag is related to the finite speed of light, so for a typical battery it is of the order of a nanosecond (1/billion second). Completely negligible for batteries.
  10. Dec 21, 2014 #9


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    Please read the following regarding charging of LI-ion batteries.

    http://www.digikey.com/en/articles/techzone/2012/sep/a-designers-guide-to-lithium-battery-charging [Broken]

    You do not just connect a supply and resistor to a Li-ion battery and expect it to charge. You can expect it to explode.

    There are many IC's to control Li-ion battery charging. You will need to use one.

    LI-ion chargers require a 3 phase charging prosess. Pre-charge, constant current, and then contant voltage. They require precision voltage measurements to maintain the contant current phase.
    Last edited by a moderator: May 7, 2017
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