Capacitive Voltage Divider - Downsides?

Discussion in 'General Electronics Chat' started by jkenny, Oct 17, 2009.

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  1. jkenny

    Thread Starter New Member

    Aug 15, 2009
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    So recently in my EE class, we were taught you can use capacitors in series as a voltage divider. When I thought about it, it seemed like a way better option than resistive voltage dividers, since resistor based ones are inherently lossy, and can only deliver a limited current based on the resistance of the elements you use. I read on Wikipedia that capacitive voltage dividers are current limited by the values of the capacitors. Does anyone know how to calculate this limit? I'm having trouble finding more information about it.

    Why aren't capacitive voltage dividers more common? It seems like they would be a much better solution than resistive voltage dividers with almost no loss, and I'm thinking of using it in place of a linear regulator in one of my electronics projects since the output voltage doesn't need to be very exact.
     
  2. SgtWookie

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    Jul 17, 2007
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    Capacitive voltage dividers are really only usable for AC circuits.

    Capacitors block DC, and pass the effects of AC.

    However, attempting to use capacitors in the place of transformers to provide isolation from the mains power is extremely dangerous, as capacitors can (and often do) fail. You must use a transformer to safely isolate yourself from the mains power. Discussion of hazardous/dangerous practices is against the policy of the Board.

    If you attempt to use capacitors in series to derive a DC voltage level for a supply, you will be disappointed.
     
  3. jkenny

    Thread Starter New Member

    Aug 15, 2009
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    Ah, missed that note about capacitive dividers, thanks for the input. I was just looking for an easy way to divide down DC voltage. I don't know much about AC so I stay away from it.
     
  4. SgtWookie

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    A resistive divider network is a passive device; the output voltage will be dependent upon the load. If the load is a fixed resistance, the voltage won't vary much. However, resistive dividers are generally a poor choice for such things as inductive loads (motors in particular) and microcontrollers, as the load current will vary widely.

    Linear voltage regulators such as the LM317 are really pretty easy to use for low power applications.
     
  5. Thav

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    Oct 13, 2009
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    I actually do think you can use capacitors as a DC voltage divider. It's a problem you run into in CMOS integrated circuit design. The charge shares between the two capacitors. This is the reason you can stack say 50V capacitors in series to be able to store charge at voltages about 50V. I forget how you can calculate it.

    You will have a lot of trouble drawing any current from it at the voltage you expect. You might be able to buffer it immediately to use it as a voltage reference. It's a little less practical than using resistors from a cost perspective too. 1% resistors are really common now. You can get 1% capacitors but they will just be more costly than any 1% resistor you can get.
     
  6. SgtWookie

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    Jul 17, 2007
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    Well, theoretically I suppose you could use a couple of capacitors as a voltage divider for a CMOS input. However, if one capacitor has a higher leakage rate than the other, or you have some contamination causing current drain somewhere, the capacitors will eventually become charged or discharged.

    All capacitors have SOME leakage.

    CMOS inputs have extremely high impedance, but it is not infinitely high.

    A good compromise is to use fairly high-value resistors to minimize current as a voltage divider (perhaps 100uA to 1mA, depending on load) with a small capacitor connected from the junction to ground to help eliminate noise.

    Plain carbon resistors are pretty noisy. Carbon film resistors are somewhat better. Metal film are more precise and less noisy.
     
  7. Brother_of_the_wind

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    Dec 7, 2009
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    I used the capacitve divider to drop 120vac to approx 12vac. I then used a 317 to get my desired 3.75vdc. It worked great but my load was only 100ma. I did alter my design to use a xformer to expedite the ul/csa approval.
     
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