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DIY - Stuff about caps...

Discussion in 'reference' started by Andrew L Weekes, Mar 10, 2004.

  1. Andrew L Weekes

    Andrew L Weekes Senior Reverse Engineer

    It's worth understanding why caps (and other components) may sound different.

    So to start, capacitors are on the menu so look here: -

    www.newson-consulting.com/emi-capacitors.htm

    at figure 3 - this shows some of the non-ideal characteristics. As you can see our perfect cap now has inductance (ESL) and resistance (ESR).

    In addition there's another resistor in parallel with the capacitor - this models leakage current, in addition to this add another RC compination in parallel with the C bit and you have now modelled dielectric absorption. In real capacitors the DA element is actually multiple sets of RC components, bringing non-linearities and circuit response effects that can totally ruin fast and slow dynamic performance.

    Complicated isn't it!

    Here's some options, with their respective characteristics: -

    Polystyrene: -

    Ultra low DA, good stability (circa 120ppm/deg C), high inductance (wound) - no longer available readily and high temp performance poor (85 deg C max operating)

    Polyphenylsulphide (PPS): -

    Almost as good in most parameters as above (Temp is better IIRC), but available in stacked plate form, so lower inductance. Wider temp range.

    Polypropylene: -

    Low DA, stable (200ppm), large, high inductance (wound)

    Teflon: -

    Low DA available, v. stable, good at high temp, inductive, expensive.

    Polycarbonate: -

    Stable, cheap, wide temp range, large, inductive, DA limits operation to 8 bit apps.

    Polyester: -

    Moderate stability, cheap, wide temp, low inductance (stacked film types), smaller than poly types above, but still large for value. Again limited to 8 bit app's

    NP0 ceramic: -

    Small, cheap, v.stable (30ppm), low inductance, DA generally low, but rarely specced, max value limited.

    Aluminium electrolytic: -

    Large values, high currents, high V and small size, High leakage, polarized, v. poor stability, inductive.

    Tantalum: -

    Small, large values (but low V) moderate inductance, high leakage, polarized, expensive, poor stability, don't take abuse readily

    Then there's the time factor - many of the above (particularly electrolytic) change over time...

    Have fun!
     

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