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    SMD Electrolytic Capacitor: Decoding Capacitance & Voltage Rating

    I have an old accelerator board used in the PDS slot of an Apple Macintosh SE/30. The board was manufactured around 1994. I want to replace the electrolytic capacitors but I'm not sure how to decode the numbers printed on the caps. Here's a photo:

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    As you can see the numbers are:

    22
    6A
    7C6


    I could take guess and assume that 22 means 22uF, but what about the voltage spec? Would 6A mean 6.3V? Do any of you know? There's no logo mark on the cap that I can see to indicate manufacturer.

    Capacitor dimensions are:

    Diameter = 4.0mm
    Height = 5.9mm (including plastic base)

    Thank you.

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    Re: SMD Electrolytic Capacitor: Decoding Capacitance & Voltage Rating

    Marking is vendor specific and can't be decoded unequivocally without knowing the manufacturer and possibly capacitor series. Marking schemes are however similar, in so far, it's almost clear that 22 designates the capacitance in ĩF. If you don't manage to find a capacitor series with obviously matching marking scheme, it's easier to choose the voltage according to the circuit properties.



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    Re: SMD Electrolytic Capacitor: Decoding Capacitance & Voltage Rating

    Thank you for sharing your thoughts.

    I'm pretty sure they are 6.3v rated in light of their size and in light of the fact their (+) side connects to the 5V rail. I now need to ponder suitable replacements. Tantalum are nice in that they won't leak, but the voltage derating is 50%, so I would need to spec them at 10V or even 16V to be safe, and they are rather large and expensive at those voltage ratings. Hmmm...



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    Re: SMD Electrolytic Capacitor: Decoding Capacitance & Voltage Rating

    Hi,

    why donīt you want to use electrolytics anymore? There should be plenty of replacements.

    ***
    The selection of a proper replacement capacitor depends on the application.
    * High ripple current --> Low ESR
    * high ambient temperature
    * low leakage current
    * low distortion (for audio..)
    and so on.
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    Re: SMD Electrolytic Capacitor: Decoding Capacitance & Voltage Rating

    I think you won't do wrong if replacing it by state-of-the-art low ESR aluminium electrolytic capacitors from a major manufacturer.



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    Re: SMD Electrolytic Capacitor: Decoding Capacitance & Voltage Rating

    In reply to your question, as I said in my previous post, the reason I have been thinking about Tantalum or even Niobium Oxide capacitors is because even the best electrolytic capacitors will leak at some point. Sure, it may take 15 years, but they will leak and need to be replaced. Solid tantalums on the other hand, will last longer (assuming they are properly voltage derated). Leaking caps is a major issue in vintage computing, not because the caps are old tech. Even new tech can and will leak at some point. Leaky caps often will eat through traces on the PCB. This happens often in vintage Macs. So again, that's why I'm pondering all my replacement options at this point. Physical size is a concern though, since the existing PCB pads are only 6.5mm, measured outer edge to outer edge.



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    Re: SMD Electrolytic Capacitor: Decoding Capacitance & Voltage Rating

    Lifetime of electrolytic caps with moderate stress is typically much more than 15 years. I saw occasional failure of tantal caps due to high current peaks, thus the circuit may need to be checked before changing to tantal. Using all ceramic caps can be alternative.



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    Re: SMD Electrolytic Capacitor: Decoding Capacitance & Voltage Rating

    Quote Originally Posted by FvM View Post
    Using all ceramic caps can be alternative.
    The problem with ceramic capacitors is that they lose capacitance sharply with applied bias voltage. Many datasheets are completely ridiculous in that they don’t even supply me with a curve to see exactly how much the capacitance falls in accordance with applied bias voltage! To get around that loss of capacitance you have to choose a much higher voltage rating than you normally would need in order to maintain the rated capacitance. That higher voltage rating makes the ceramic capacitors physically larger and more expensive too. Also, my board uses 5pcs of 22uF 6.3V electrolytic capacitors on the 5V rail. If I switch to ceramics, my ESR will fall to something like 8m-ohms. The aluminum electrolytic capacitors are probably 2 to 4 ohms. I’m wondering if the low ESR of ceramic capacitors would pose a problem in my circuit.



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    Re: SMD Electrolytic Capacitor: Decoding Capacitance & Voltage Rating

    Hi,

    In my eyes you overcomplicate things.
    Instead of writing a lot of words you should compare real values.
    A usual electolytic capacitor may have a tolerance if +/-20%.
    A 22uF/10V/X7R ceramics capacitor may have 10% tolerance in a 1206 or 1210 package.
    According an actual AVX datasheet the change in capacitance at 50% (10V rated, 5V applied) applied DC voltage at an X7R capacitor is just -2.5%.
    Thus we are well within 20% tolerance.

    ESR: most voltage regulators improve performance with decreasing ESR. Indeed I know only a single one linear regulator that has a problem with too low ESR.

    Don't get me wrong. It's good to have an eye on such things. Why not simply buy a couple if ceramics and use a scope to check performance in your circuit?

    Klaus
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    Re: SMD Electrolytic Capacitor: Decoding Capacitance & Voltage Rating

    The lowest cost SMD ceramic XR7 cap at Mouser that is suitable for my application, in the <10pc quantities I need, is US$0.85EA and 20% tolerance, and drops 30% in capacitance with applied 5V bias voltage. The 25V rated parts start at US$1.17EA and still have the same capacitance drop with applied bias voltage. Stepping up to 33uF to offset the capacitance loss at 5V bias yields a price of $1.66EA for a 20% tolerance part.

    I ultimately decided to purchase 5pcs of this niobium oxide part, since it is US$0.75EA and gives me a solid 22uF at any bias voltage, won't fail shorted or burn like tantalum, fits my existing pads perfectly, and has an ESR of 700m-ohm, which is much lower than the ESR of the stock electrolytic 22uf 6.3V caps.

    Thank you for the all the advice in this thread. I gave due consideration to all the thoughts presented.



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