How to calculate filter Capacitor for 24VDC 6.5A rectifier.

Discussion in 'The Projects Forum' started by Pravinbs, Mar 14, 2014.

  1. Pravinbs

    Thread Starter New Member

    Mar 12, 2014
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    Hi,

    I want to use 24VDC supply to brake application whose current is 6.5A. I considered transformer of 415/18VAC & diode bridge mono block of 12A.
    pl. suggest how to find out the filter capacitor value which i use in output of bridge rectifier.
    also is it ok if i use diode rectifier or i need to use selenium rectifier for brake.

    Regards
    Pravin
     
  2. #12

    Expert

    Nov 30, 2010
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    What kind of brake? We need to learn how good the filter needs to be.
     
  3. Pravinbs

    Thread Starter New Member

    Mar 12, 2014
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    it's 24VDC parking brake used for Uncoiler , Recoiler AC motors in Rolling mill applications.
     
  4. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Silicon diode bridge - yes.
    Selenium rectifiers - no.

    After the bridge you will just barely have 24VDC at the peaks of the rectified sine wave. Capacitor size depends on how much ripple your brake device (solenoid?) can handle before it starts to lose some of its gripping power.

    The official equation for this has an exponential in it. If you don't want to deal with that, here is a first order approximation that is good enough to give you a feel for the capacitor size:

    C (filter capacitor) = i (load current) x t (period of the rectified AC) x 1,000,000 (conversion from farads to uF) / E (ripple voltage)

    So for you, assuming 50 Hz AC:

    C = 6.5 x .01 x 1000000 / your ripple voltage

    If you think your brake can stand 4 volts of ripple, the capacitor is 16,250 uF.

    ak
     
  5. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Capacitors are rarely ever used or needed for Brake and clutch applications, the Collapsing field in conjunction with a reverse diode is usually sufficiently large to carry the armature through the zero transition time.
    Max.
     
    Last edited: Mar 14, 2014
  6. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    The diode bridge will need some serious heatsinking. It will probably be dissipating 15 - 20W.

    The capacitor is easily calculated:

    I = C dV/dt

    I is the current which is 6.5A, dt is .0083 because the rectifier ripple will be 120 Hz (if the AC power is 60 Hz), so you can calculate the "C" if you put in the allowable ripple voltage on the cap.....
     
  7. Experimentonomen

    Member

    Feb 16, 2011
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    In the world of audio, its said that 1000µF per Ampere is a decent figure.
     
  8. wayneh

    Expert

    Sep 9, 2010
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    That's a handy rule of thumb, but can you reconcile it with the calculation from AnalogKid, that shows a 4 volt ripple (out of 24V) with >2A per 1000µF?
     
  9. inwo

    Well-Known Member

    Nov 7, 2013
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    I agree with Max. No capacitor needed. It will only lead to switching problems.

    Selenium is better but hard/costly to source.

    The last ones that I needed were for a generators rotary exciter. They were >$800 each.:eek: And I needed 4.
    They were in the circuit just to protect the silicon.

    I found the original manufacturer in Canada.:) "ICTPOWER"

    The brake circuit transformer is powered from motor leads. With secondary circuit thru a contactor to insure dropout during coast.
     
    Last edited: Mar 15, 2014
  10. MaxHeadRoom

    Expert

    Jul 18, 2013
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    I haven't seen Selenium for some years now, one down side to them is the larger volt drop, not to mention the smell when they blow.:eek:
    On larger coils the drop out time is often visible when a BEMF diode is used, in some case it can cause a problem due to the delay.
    So if it can be visible, the effect of voltage supply drop to zero every 120th/sec will be a non-issue.
    Max.
     
  11. inwo

    Well-Known Member

    Nov 7, 2013
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    Not something I do every day.:eek:

    "Dave, As per our phone conversation, we respond as follows to your RFQ:
    Selenium Suppressor, 4” x 4”, 8 plates … 4 each $ 995 each"
     
  12. MaxHeadRoom

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    Jul 18, 2013
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  13. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Did you ever smell an overloaded selenium cell?
    It smells like rotten eggs.

    Bertus
     
  14. MaxHeadRoom

    Expert

    Jul 18, 2013
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    My sense was cats pee!:eek:
    But if you smell it, it's shot.!!
    Max.
     
  15. #12

    Expert

    Nov 30, 2010
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    We have at least 2 antiques running today. Don't miss magnet's "300V DC supply" in Chat. It's nothing less than beautiful.
     
  16. Experimentonomen

    Member

    Feb 16, 2011
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    Selenium rectifiers also release toxic gases when they go.

    Naaaaasty stuff.
     
  17. Pravinbs

    Thread Starter New Member

    Mar 12, 2014
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    Dear All,

    Thanks..

    Pravin
     
  18. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Well, I think that if you put 4 volts of 120 Hz across a speaker, you'd hear it. Isn't that "audio"?

    ak
     
  19. wayneh

    Expert

    Sep 9, 2010
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    Sorry, I screwed up the question. AnalogKid's example shows a 4V ripple using 2500µF per amp of current. That's a lot more capacitance than your rule of thumb but it's still producing, as you noted, a very large ripple.
     
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