capacitor value

Discussion in 'General Electronics Chat' started by sampinoy, Nov 14, 2004.

  1. sampinoy

    Thread Starter Active Member

    Sep 24, 2004
    31
    0
    hello everybody,

    good morning to all. :)

    i would to make a power supply for my recently assembled AM radio and it is now powered by 4 D cell batteries for a total of 6v.

    i know i could just buy an adaptor but i would prefer making one so i can have a hands on expereince in making one. what i would to know is how would compute how much value for the electrolytic capacitor for me to use.

    your help would be greatly appreciated :D :D
     
  2. vineethbs

    Well-Known Member

    Nov 14, 2004
    56
    0
    first of all , r u planning to use ,(well ,i don't remember what u call it exactly , but i think it) is trickle charging , meaning => will the battery along with the adaptor wud to connected to the ckt while u r using it ?

    i think that is dumb :p radio is meant to be portable !

    well , in that case , u hav to see how much charging current the battery needs.

    but battery chargers don't need that much clean dc .

    i think u can put in a 4700uF capacitor
     
  3. mozikluv

    AAC Fanatic!

    Jan 22, 2004
    1,437
    1
    hi sam

    should i call you "jigger"? as i understand it's your nickname?

    it's good that you decided to make a power supply for your am radio. you were asking on how to compute for the capacitor value that you would use to smooth out the ripple voltage.

    theoritically capacitance would be computed in this manner:

    C = (Iload / Ripple voltage ) x Voltage Freq x 1000uf

    Ripple voltage = 2v p-p
    Voltage Freq. = 120hz

    if you are going to use this formula, you will find out that you would need a very capacitance value for just a 6v supply.

    but in your case you can use a 500ma to 750ma 6v transformer and use 2 470uf/16v and connect them in parallel configuration.

    as to your query on ESR, it stands for Equivalent Series Resistance when applied to electro caps.

    ESR rating of an electro cap is a form of quality rating. in dreamland they have a perfect electro caps with no loss, a zero ESR and has no inphase AC resistance. but on this world of ours there is no such thing, all caps will have its own share of ESR. that's why the lower the ESR the better the quality, hence as i have said it's a form of quality rating.

    there is also such a term ESL w/c means Equivalent Series Inductance. this is cause by indauctance of the electrodes and leads of the cap. this thing sets the limit of how fast a cap can decouple noise from power lines. likewise it also sets the resonate -point because the inductance shows in series with the cap hence forming a tank circuit.

    now how does ESR evolve, when the dielectric material is thickened the ESR increases. when the plate area increases and the plate thickness remains the same the ESR decreases.

    this ESR sets the operational reliability of the cap. the lower the ESR the better. but this ESR cannot be tested by the usual method of testing of capacitors. you need an ESR meter.

    well so much of this for now :)
     
  4. mozikluv

    AAC Fanatic!

    Jan 22, 2004
    1,437
    1
    hi sam

    should i call you "jigger"? as i understand it's your nickname?

    it's good that you decided to make a power supply for your am radio. you were asking on how to compute for the capacitor value that you would use to smooth out the ripple voltage.

    theoritically capacitance would be computed in this manner:

    C = (Iload / Ripple voltage ) x Voltage Freq x 1000uf

    Ripple voltage = 2v p-p
    Voltage Freq. = 120hz

    if you are going to use this formula, you will find out that you would need a very capacitance value for just a 6v supply.

    but in your case you can use a 500ma to 750ma 6v transformer and use 2 470uf/16v and connect them in parallel configuration.

    as to your query on ESR, it stands for Equivalent Series Resistance when applied to electro caps.

    ESR rating of an electro cap is a form of quality rating. in dreamland they have a perfect electro caps with no loss, a zero ESR and has no inphase AC resistance. but on this world of ours there is no such thing, all caps will have its own share of ESR. that's why the lower the ESR the better the quality, hence as i have said it's a form of quality rating.

    there is also such a term ESL w/c means Equivalent Series Inductance. this is cause by indauctance of the electrodes and leads of the cap. this thing sets the limit of how fast a cap can decouple noise from power lines. likewise it also sets the resonate -point because the inductance shows in series with the cap hence forming a tank circuit.

    now how does ESR evolve, when the dielectric material is thickened the ESR increases. when the plate area increases and the plate thickness remains the same the ESR decreases.

    this ESR sets the operational reliability of the cap. the lower the ESR the better. but this ESR cannot be tested by the usual method of testing of capacitors. you need an ESR meter.

    well so much of this for now :)
     
  5. pebe

    AAC Fanatic!

    Oct 11, 2004
    628
    3
    I'd just like to add a bit more on ESR and ESL, if I may.

    ESL only affects capacitors used at higher frequencies and as has been pointed out is due to lead inductance. Which is why caps used for RF decoupling etc should have short leads and mounted as close as possible to the device they are being used with, whether that is an inductor in a tuned circuit or the supply pins of an IC.

    All medium sized caps using modern 'poly' plastics as dielectrics will cause no problems with either ESR or RSL at mid and low frequencies.

    ESR is generally only a problem with large electrolytic caps. Their construction is an aluminium anode with an anodic film 'formed' on it, in contact with an electrolyte which is the cathode. An aluminum 'collector' strip is in contact with the electrolyte to enable current to move in out of the capacitor freely. That aluminium strip is connected to the negative terminal of the cap. The liquid electrolyte has an inherent resistance through which the charge/discharge current must pass and that is its ESR.

    Any electrolytic will have a certain impedance depending on its capacitance and the frequency it is being used at, and the ESR should be small compared to that. The ratio of ESR to its impedance is its 'power factor'. This feature explains why a small poycarbonate capacitor is often used in parallel with an electrolytic in filter circuits.

    The tantalum capacitor does not suffer from that problem, and is ideal where a large capacity is required at low and medium frequencies
     
  6. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Hi,

    Since you're using D cells, I would impagine current draw ahs to be no more than 50 milliamps. Find an AC adapter that is rated 6 VDC at a higher current rating - say 100 - 200 mills. They're not well regulated, so figure on adding about 100 microfarads for a filter capacitor. Make sure the voltage rating is 16 volts so it will be unstressed. The reason for the extra rating is that most AC adapters are load-dependant, and the actual voltage will most likely be 9 VDC. Probably won't hurt the radio.
     
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