understanding capacitor measurement

Discussion in 'General Electronics Chat' started by electronis whiz, Mar 5, 2012.

  1. electronis whiz

    Thread Starter Well-Known Member

    Jul 29, 2010
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    i have been confused by this for a while now hoping somone can clerify.
    i have always thought of a capacitor similar to a water tower the higher the voltage the higher the tower, the larger the value of the cap the biger the pipe to the tank, and the phisical size as the caacity. but you take a 1000 uf cap and charge it to 12 V you get a good size spark but if you charge a super cap when shorted out it makes no spark. (but this could be bacause most super caps i have are aprox 5v. but still would think somthing would be seen.) so this makes it seem like the value and the size of the cap are kind of the same thing. so is my anologie wrong?
    also is value and phisical size important? because i have replaced some caps i find one of the same value as the bad one but it is phisicaly slightly biger or smaller. can these be used as long as the values are the same or do they need to be the same in phisical size as well?:confused:
     
  2. buzaiandras

    Member

    Jul 18, 2011
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    I may be wrong about this (so use this information at your own risk :) . Maybe someone with more knowledge will help us on this - I am also a beginner in electricity/electronics) but in the case of the capacitor at 5V I think there is not enough energy for the charges to jump from one lead to another when they are not in a perfect contact, so there are no sparks.

    The capcitor will always charge at the voltage level of the power source used to charge it, even if the capacitor is rated at a much higher voltage.
    The capacity of the capcitor indicates how much charge the capacitor can hold.
     
  3. buzaiandras

    Member

    Jul 18, 2011
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    Could someone with more knowledge shed light on this? :)

    Thank you
     
  4. crutschow

    Expert

    Mar 14, 2008
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    For a water analogy you can look at a capacitor as a filled tank with a rubber bladder seal across the middle, with two inlets, one on either side of the bladder. The size of the tank bladder is the capacity. The more pressure (voltage) you apply, the more water (charge) is transferred into one inlet and out the other, as determined by the back pressure exerted by the bladder. Notice that a capacitor does not actually "store" charge, it just transfers it into one lead with a corresponding charge leaving the other lead. But it does store energy in the form of the amount of transferred charge and the voltage of the transferred charge.

    How similar the replacement capacitor needs to be compared to the original depends upon the application. If it is used for power supply filtering or DC blocking in an AC amplifier then an equal or larger capacitor (for capacity and/or voltage) of the same type should be ok. If it is used as a frequency determining component in an AC signal filter circuit, then it should be of the same type and capacity with equal or greater voltage rating. It either case they don't have to be the same physical size.
     
  5. MrChips

    Moderator

    Oct 2, 2009
    12,449
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    The energy stored in a capacitor is equal to

     \frac{1}{2}CV^2

    If you wish to use the analogy of a water tank, capacitance is equivalent to the size of the tank. Voltage would be equivalent to the height of the water tower.

    The reason you see a spark with high voltages and not low voltages is a bit more complicated and not related so much with the stored energy. But do note that the energy increases as the square of the voltage.

    When two charged conductors approach each other there is an electric field that builds up and gets stronger as the two conductors get closer. Note that the voltage at the conductors do not change but the field gets stronger. The magnitude of the field is inversely proportional to the square of the distance of separation.

    Even before the conductors touch, the molecules of the air become ionized and eventually the insulating properties of the air breaks down leading to a discharge spark. You need extremely high fields for this to occur. Hence this is more likely to occur at higher voltage. The amount of energy does not have to be large. This is why you can get a spark as you bring your door key close to a door knob. The energy is not high enough to electrocute yourself.
     
  6. Brownout

    Well-Known Member

    Jan 10, 2012
    2,375
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    Your water tower would be better if you thought of the voltage as the height above the ground, and the capacitance as the capacity in volumn of the tank. The voltage is related to potential energy ( and so is the height of the water ) and the capacity is related to an amount of the energy that can be stored at a particular voltage. So, the amount of energy that a capacitor can store is a product of both voltage and capacitance, just as the amount of energy a water tower can store is a product of both the capacity of the tank and the height above the ground.
     
  7. electronis whiz

    Thread Starter Well-Known Member

    Jul 29, 2010
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    ok thanks that helps a bit. so a 35 V 1000 UF cap that is 1/4inch would have equal capacity to a 35V 1000UF cap that is 1/2inch? am i understanding this corectly.

    i kind of thought about the voltage diffrence later between the super caps with low voltage and electrolitics with more aprox 12ish V. still was suprised to see nothing though.
     
  8. exscape

    New Member

    Jan 19, 2012
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  9. buzaiandras

    Member

    Jul 18, 2011
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    I think the answer is yes.

    But they both can be charged at different voltage levels even if they have the same voltage ratings.

    The voltage rating of the capacitor is the maximum voltage it can withstand before it gets damaged. Say you have a 35V 1000uF capacitor. This can be charged at 5V, 12V ... or any other voltage; but if it gets charged at a higher voltage than 35V it will get damaged. It does not mean that the capacitor will be always charged at 35V. If your capacitor is charged from a 5V power supply then is will get charged at 5V even if it has the rating of 35V.
    If I understand correctly you should always use a capacitor with a higher rating than the actual voltage of the circuit. Ex: if you use the cap in a 12V circuit you should use a capacitor rated at something like 35V or bigger.
    I think that at voltages above the rated voltage of the capacitor the dielectric will break down (because of the electrochemical redox reactions - in the case of the electrolytic capacitors ) and the capacitor is damaged.

    But as I said before my understanding may not be correct. So feedback from more experienced users is welcomed :).

    Thank you
     
  10. debjit625

    Well-Known Member

    Apr 17, 2010
    790
    186
    Yes they could store same amount of charge, and their break-down for their dielectric are same 35V ,but their size differs because of other reasons ,like as you are talking about 1000mdf 35V caps, I guess they are electrolytic types ,generally the taller and thinner type will have LOW ESR (Equivalent Series Resistance) ratings.They have better temperature coefficients and higher tolerance.

    Not always the taller and thinner type will have LOW ESR ,some times the bigger ones also have LOW ESR ratings, it really depends on the manufacturer, but the size is always related to the ESR (other factors also counts) of the capacitor it may be high or low.

    Good Luck
     
  11. Veracohr

    Well-Known Member

    Jan 3, 2011
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    I'm not sure what you mean by 'capacity', but the rated capacitance and voltage are what they are. A lot of that size is likely packaging.

    True.

    electronis whiz: look up the equation for capacitance. You will see it is proportional to the area and separation distance of the two plates. So the size of the capacitor is determined by the capacitance, as well as the type of dielectric (different types having different dielectric constants, resulting in different required area & separation distance for a desired capacitance). I'm not sure how different voltage ratings are made, but if it also has an effect on the physical size, it's probably less than the effect of the capacitance requirements.
     
  12. electronis whiz

    Thread Starter Well-Known Member

    Jul 29, 2010
    519
    27
    ok thanks that is what always confused me how one value could be said to be ho much it could hold. but to me size could alwo mean this. this helps alot. i rember a few months back i was given a broken LCD monitor because it wouln't work. took it apart found a bad cap and o dug thorugh my bins of capacitors looking for one of the same voltage and value and phisical size. it seemed to me any size would have worked if the calues were the same but figured to be safe i'd use as identiclay sized as i could. if i have a similar isue again i should be able to find a replacement a bit teaiser since i know now that phisical size isn't all theat important except for esr porpuses.
     
  13. debjit625

    Well-Known Member

    Apr 17, 2010
    790
    186
    High ESR is the biggest threat in high power high frequency circuits, like SMPS if you don’t use Low ESR types capacitors in SMPS circuits then you will repeatedly destroy your capacitors like in computers motherboard in VRM section we use LOW ESR types as its a high power and high frequency circuit and in many more application like in SMPS of LCD TVs.

    So the tip is that, when their is high power and high frequency involved in a circuit and if any capacitor is damaged, while replacing consider about its ESR.

    Good Luck
     
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