Capacitor Problems

  1. bjmiller1 New Member

    I found this forum through a web search and after reading some of the posts, I figured this would be a good place to get some help. I have a Lincoln SP 250. Its older and I bought it cheap because it was missing some parts. I got it home and hooked it up to find that the capacitor bank was sparking. Upon further investigation, I found some burn spots in the capaitor. I called Lincoln and they want nearly $600.00 bucks for a replacement bank. To my question, where can i find capacitors to build my own bank? It has 4 mepco/Centralab capacitors measuring 30,000 UF -10+75%, 50VDC 75 Surge, 85C Max.
    It has been a long time since I had any electrical classes so maybe someone can help. I know that 30,000 UF is capacitance , and voltage is obvious. What does the -10+75% mean? Can I maybe use different capacitors to eaqual the same capacitance? Please help. Thanks.
  2. rtbcomp

    rtbcomp Moderator Staff Member

    Welcome to the forum.

    The -10+75% means the actual capacitance is between 30,000 uF -10% (27,000) to 30,000 +75% (52,500). I'd be inclined to use good quality caps, perhaps increase the working voltage slightly.
  3. shenion

    shenion Tool Pack Rat

    Posts: 7,600
    Stone Mountain, GA USA , will sell caps. you will have to look for 30,000uf at at least 50V. Ones labeled "low ESR" or "computer grade" would be better.

    Check ebay. I picked up two 30,000uf/50V caps for $35.

    You can probably find the right value, but the size may be a bit different. So, may need a bit of tweaking to the mounting.
  4. R Kraft Member

    Posts: 810
    What is the physical size of the capacitors ?
    Mouser Part #: 539-CGS50V30000 is 50v but the surge rating is lower.
    All of the caps I see online are 63v with a 75v surge.
    Good grade capacitors that add up to your 4 x 30000uf should work.
    But they still add up to about $180.
  5. voipio Member

    You are correct on the capacitance value and the -10+75% value is the tolerance on the capacitance. Electrolytic capacitors generally have quite wide tolerances. Substituting with capacitors of different make but equal the capacitance should work OK, but it is important to check that the replacements have both low internal resistance and a high ripple current capability.

  6. bjmiller1 New Member

    What is "ESR"? What is a low internal resistance and high ripple current for a capacitor? An example please.
  7. shenion

    shenion Tool Pack Rat

    Posts: 7,600
    Stone Mountain, GA USA
    ESR - Equivalent Series Resistance

    Low ESR, Computer Grade, low internal resistance and high ripple current are basically the same. It is how much heat is generated due to internal losses to large currents.

    Here is one on Digikey.
  8. bjmiller1 New Member

    A couple more questions. Will it hurt anything to use one of a higher voltage rating? ie 63VDC 75 MAX or 75VDC 95 MAX. Why so many smaller caps? Is there a heat dissipation theory? Can I use one 120K UF cap or stay /w four 30K UF caps? You guys have been a tremendous help. Thanks alot.
  9. R Kraft Member

    Posts: 810
    Higher voltage won't hurt.
    There is a power dissipation factor with larger and smaller size capacitors, and the esr will change with capacitors in parallel.
    I would go with the same physical size caps.
  10. martinez

    martinez Member

    Posts: 77
    west mids uk
    you can also use lower voltage caps if you wire them in series, can't remember though if it lowers the uf rating
  11. rtbcomp

    rtbcomp Moderator Staff Member

    Lowers the uF value.

    1/Ct = 1/C1 + 1/C2 + 1/C3 etc.

    Two equal value caps in series gives half the capacitance of one.
  12. martinez

    martinez Member

    Posts: 77
    west mids uk
    thought so, but useful if they are free
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