substitute bipolar electrolytic caps

Discussion in 'General Electronics Chat' started by ross, Aug 22, 2010.

  1. ross

    Thread Starter Member

    Jul 30, 2010
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    I have done a search but cant find anything relevant to my question which is! Can I use polypropylene,mylar ect.. caps instead of 3.3uf,25v and 47uf 6.3v,bipolar electrolytics in the pre-amp section of my sansui amp? I am asking this because I am in the middle of nowhere in west australia and cant get the right electros here. Thanks in advance for any help.
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    I don't know for sure myself.

    You do know you can put two polarized electrolytic caps back to back to make them unpolarized? The capacitance is halved, and the voltage is doubled.

    [​IMG]
     
  3. SgtWookie

    Expert

    Jul 17, 2007
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    Sure, you can use polypropylene caps instead of electrolytics. They will be considerably more expensive, and will take up a lot more room than their electrolytic counterparts. That can be a big problem if you're repairing something rather than designing from scratch.

    Like Bill said, you might use two 100uF polarized electrolytic caps back-to-back to get a 50uF cap. However, the ESR (equivalent series resistance) is additive, and electrolytics tend to have a high ESR to begin with, so your filtering/coupling will be that much less efficient.
     
  4. hgmjr

    Moderator

    Jan 28, 2005
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    sgtwookie,

    I thought that when you use two electrolytics as recommended by Bill M. that the capacitance actually stays about the same value since the electrolytic that is reverse biased at any given time would not act like a true capacitor.

    hgmjr
     
  5. marshallf3

    Well-Known Member

    Jul 26, 2010
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    You could always order them but shipping to Australia has turned from easy and inexpensive into tougher and costly.

    The smaller one will be easy to find, a 47 uF will be difficult. As noted though they are quite a bit larger as regular caps - quite a bit.

    Are you 100% sure they're non-polar and that they actually need to be? I realize Sansui did some strange stuff but I can't imagine them forking out the extra money unless they absolutely had to be or they just had some sitting around.

    If it still operates take voltage readings on both sides of the caps, if one side is predominately much more positive than the other I'd just use regular electrolytics as the AC component is going to be very small.
     
  6. Wendy

    Moderator

    Mar 24, 2008
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    I'm pretty sure capacitance is halved, though I'm not sure. With a DVM it is easy to verify though, just measure it.

    I've seen the technique recommended as long as I've been into electronics, but have never used it. The vast majority of applications can use a polarized capacitor. There are some simple oscillator (CMOS types) that need nonpolarized types, so I'll probably verify it for my own applications over time.

    Strikes me as voodoo though, letting one cap take up the strain while the other relaxes.
     
  7. marshallf3

    Well-Known Member

    Jul 26, 2010
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    The idea was that the electrolytic that faced the reversed polarity would just leak the DC component through and in practice there was no way to reliably predict what the end capacitance would be. Since even as reverse biased, under small voltages they will still present themselves as 47 uF caps so you'd end up with having to use two 100 uF back to back to get 47 uF.

    Regardless of the theory I still view the solution as a band-aid as opposed to a true NP electrolytic which has formed oxide layers on both plates.
     
  8. Wendy

    Moderator

    Mar 24, 2008
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    What works, works.
     
  9. ross

    Thread Starter Member

    Jul 30, 2010
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    thanks for the advice everyone. looks like I will have to order the correct ones from overseas.
     
  10. SgtWookie

    Expert

    Jul 17, 2007
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    Anything worth fixing, is worth fixing CORRECTLY, ONCE.

    Unfortunately, we have seen time and time again in here, people substituting parts even against advice, and dozens of pages later, they finally admitted the substitution.

    If the repairs are not performed correctly the first time, odds are very high that the time and money spent will be solely to learn how NOT to do things.

    Polarized electrolytic capacitors can often be re-formed if their electrolyte has not leaked or dried out. The process involves slowly charging the cap via a high impedance until the electrolyte is re-formed enough to have an acceptable leakage rate; which can be derived mathematically. Search the forum for "re-forming" for other entries I've made about this in the past.

    I frankly do not know the correct procedure for re-forming the dielectric on non-polarized aluminum electrolytic capacitors. It may be as simple as re-forming them in both polarities.

    If the device was in storage for an extended period of time and then failed suddenly when power was applied, the caps may be internally shorted; such damage can only be repaired by replacing the caps.
     
  11. AlexR

    Well-Known Member

    Jan 16, 2008
    735
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    Why?
    There are plenty of WEB bases mail order suppliers based in Aus and their shipping charges are going to be significantly less than overseas sites. The list below links to a few of the more popular sites, a google of Australian pages will bring up plenty more.

    http://www.jaycar.com.au/
    http://www.altronics.com.au/
    http://au.farnell.com/jsp/home/homepage.jsp
     
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