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Capacitor Voltage Divider

Discussion in 'General Electronics Discussion' started by fatman57, Oct 22, 2016.

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  1. fatman57

    fatman57

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    May 27, 2013
    I am trying to make a capacitor voltage divider. I used an online tool to check the values before purchasing any parts. Upon connection it does not behave as I expected. Across positive (+) and negative (-) of 22µF capacitor a very low voltage was measured and I was hoping for 8v.

    My second attempt pictured below, I added a load, in this case a Peltier and still got the same results. I have never built one of these before, might anyone be able to advise as to where I am going wrong?

    [​IMG]

    Thanking you in advance.
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Is this for DC? If so, capacitors won't work.

    What are you trying to do?
     
  3. fatman57

    fatman57

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    May 27, 2013
  4. Alec_t

    Alec_t

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    That circuit works only if you draw no current from it. Here's what happens as soon as a load draws current :-
    CapDivider.PNG
     
    fatman57 likes this.
  5. fatman57

    fatman57

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    May 27, 2013
    It would have been good if it was stated in the article! Thanks Alec_t - much appreciated.

    So is the only 'simple' way with DC to use resistors, I didn't want to do that. Is there another?
     
  6. fatman57

    fatman57

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    May 27, 2013
    Just to confirm, I am trying to get a 8v output to power Peltier (a max draw of about 6 amps).
     
  7. Alec_t

    Alec_t

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    Resistors would waste 42W of power if you need to drop from 15V to 8V at 6A. What you need is a switched-mode power supply (buck converter). Cheaper to buy one than build one.
     
    davenn and BobK like this.
  8. Chemelec

    Chemelec

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    Jul 12, 2016
    Or build a PWM Circuit to drive the Peltier.
     
  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Or a switch mode regulator.
     
  10. Kabelsalat

    Kabelsalat

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    In this realm, we can't fly without complex machinery around us - nor can't capacitors be used for DC like transformers are used for AC. If it was possible however, any power supply would be ridicilous cheap to buy.
     
  11. fatman57

    fatman57

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    May 27, 2013
    Thanks, in its most simple form wouldn't this be a transistor and capacitor - reading the voltage level in cap and keeping it up to required voltage by pulsing a higher voltage in?

    Do you have any links to some basic circuits?
     
  12. fatman57

    fatman57

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    May 27, 2013
    Thanks, do you have any links to simple circuits?

    Thing is in my current project I need to power 6 Peltiers, I was going to power each 3 on their own equipment. I fear this will get pricey and very time consuming if I have to make everything myself, but don't mind...
     
  13. Alec_t

    Alec_t

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    What is the maximum rated voltage of the Peltiers?
     
  14. Chemelec

    Chemelec

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    Jul 12, 2016
  15. fatman57

    fatman57

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    May 27, 2013
    About 18V I think...
     
  16. fatman57

    fatman57

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    May 27, 2013
    Hey, I live in London UK. I have used them before to play about with - my current project uses them and this will be my first proper application. I am an amateur in electronics but have a strong interest for many years, I am a programmer by trade.

    Great link, thanks! Will have a look at that next week.

    Got some step down buck converters today, it seems the current demanded by Peltiers is not directly proportional to the voltage, so even though 12V chomps through 5A, 5V takes much less amps then it should. I'm hoping 8V (in middle of 12v & 5V) will be lower than half, so a 3 amp buck converter can be easily purchased. Also looking into changing the pot to a digital one to be controlled by Arduino. Should be able to do some testing next week to see how the buck converter plays out for my requirements...

    There is a pattern there if you hadn't seen it: 12v, 8v, 5v ...my three planned power settings. Got a computer PSU and that does 3v, 5v and 12v so something in the middle of the two largest values would be great.
     
  17. Alec_t

    Alec_t

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    A PWM circuit running from 15V with a 8/15 mark/space ratio (duty-cycle) would give you an average of 8V. The long thermal time constant of a Peltier does the averaging. It's a doddle to control the duty-cycle (and hence average voltage) with the Arduino.
     
  18. fatman57

    fatman57

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    May 27, 2013
    Thanks, 12v is such an easier thing to find these days so thats what I usually try to aim for. Would be great to build one some day (always wanted to, power control always a barrier I seem to encounter) but just to get this thing finished I will see what parts I can buy.

    Thinking out loud (I know mains is not something to mess with) but a pair of capacitors and diode rectifier to AC source sounds a simple (and apparently efficient) way to get a fixed DC voltage. Maybe that could be an easy way to get 15V...? If it works of course!
     
    Last edited: Oct 23, 2016
  19. Chemelec

    Chemelec

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    Jul 12, 2016
    QUOTE: Thinking out loud (I know mains is not something to mess with) but a pair of capacitors and diode rectifier to AC source sounds a simple (and apparently efficient) way to get a fixed DC voltage. Maybe that could be an easy way to get 15V...? If it works of course!

    NOT VERY PRACTICAL at That Current.

    Peltier devices work most efficiently when operated at rated current.
    I would recommend running them at the 12 Volts from a Battery or a good 12 volt supply.
    Ideally with a Control Circuit to turn it On and Off, based on the Cold or Hot side Temperatures.
     
  20. fatman57

    fatman57

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    May 27, 2013
    I thought it could supply as much current as required, capacitors will be able to cope with these high currents in this application, no?

    Does this mean they are also best at their rated voltage? Or is 5amps @ 5V as efficient as 5amps @ 12V?
     
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