# Placing a polarized capacitor in place of an unpolarized one

The following circuit contains an unpolarized capacitor

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

or this sense

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

• Could you clarify how you're unable to find an appropriate 100n unpolarized cap? These are typically as common as sand ... Feb 5, 2020 at 19:09
• 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.
– JRE
Feb 5, 2020 at 19:47
• @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. Feb 5, 2020 at 19:53
• @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. Feb 5, 2020 at 19:59

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.

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

• 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. Feb 5, 2020 at 19:56

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.

• So the unpolarized cap cannot be switched with a polarized one? Feb 5, 2020 at 19:15
• @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 ;) Feb 5, 2020 at 19:19