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The following circuit contains an unpolarized capacitor

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I only have access to polarized ones, with the same ratings as the unpolarized cap. If I were to implement the circuit, should I place the polarized one this sense

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or this sense

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? An explanation as to why one position, as opposed to the other, should be chosen would be highly appreciated.

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    \$\begingroup\$ Could you clarify how you're unable to find an appropriate 100n unpolarized cap? These are typically as common as sand ... \$\endgroup\$
    – brhans
    Feb 5, 2020 at 19:09
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    \$\begingroup\$ Never mind why Hilbert can't find unpolarized 100nF capacitors. I'd like to know where the 100nF polarized capacitors came from. I looked, and couldn't find any. \$\endgroup\$
    – JRE
    Feb 5, 2020 at 19:47
  • \$\begingroup\$ @JRE It's an old 0.1 uF capacitor. The constructor is Safco, it has a tolerance of 20% and a maximum voltage of 1kv. \$\endgroup\$
    – Hilbert
    Feb 5, 2020 at 19:53
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    \$\begingroup\$ @Hilbert that sounds like a pretty bad choice for this application. 1. old elcos are always a gamble, 2. an old 1 kV-rated capacitor is bound to have pretty high ESR for its capacity, so a very low quality factor, which might (will) dampen the oscillation you get out of this 3. really, 100 nF caps are < 5ct a piece. Do yourself a favor, and order some. Get better, i.e. Rail-to-Rail, opamps on the way. Like this, that oscillator won't reliably work – and you've been told exactly that in your previous question, so no excuse to not order. \$\endgroup\$
    – Marcus Müller
    Feb 5, 2020 at 19:59

2 Answers 2

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You can use polarized capacitors in place of non-polarized ones if you use the the correct technique. If your non-polarized requirement is for a 100nF capacitor then obtain two polarized 200nF capacitors and wire them back to back in series. Then the series combination would insert into the circuit like showing below.

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Equal sized capacitor in series act as an equivalent capacitor of half the size. "Size" of course referring to the capacitance value.

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    \$\begingroup\$ the high ESR of two in-series electrolytic caps might be problematic in this application (which looks like a somewhat stable square wave generator). Also, gut feeling, you'd want to high-ohmicly tie the "middle" point to a voltage that is guaranteed to be lower than any of the voltages present at the positive terminals. \$\endgroup\$
    – Marcus Müller
    Feb 5, 2020 at 19:56
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A capacitor value as low as 100nF is usually a non-polarized film type in your circuit. Your circuit places low frequency AC with both polarities across the capacitor.

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  • \$\begingroup\$ So the unpolarized cap cannot be switched with a polarized one? \$\endgroup\$
    – Hilbert
    Feb 5, 2020 at 19:15
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    \$\begingroup\$ @Hilbert yes, a polarized cap cannot be used. It's expected on this website that you research basic things like what kind of things polarized caps don't like (hint: it's practically in the name), and whether the answer you get describes exactly this. Mentally cooperating with your answerers isn't optional ;) \$\endgroup\$
    – Marcus Müller
    Feb 5, 2020 at 19:19

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