Can I use nose to nose electrolytic caps to make a large a.c cap? 1

[martindossor]

I need to use a capacitor for implementing part of a design in a mains circuit and a colleague has suggested using two electrolytic capacitors nose to nose instead of an expensive a.c. capacitor - is this possible? Need to get a capaitance of around 10uF

 

[dspDad]

Yes and no.

Electrolytics conduct and eventually self-destruct if a reverse polarity is applied across them. Since the leakage currents in electrolytics is significant, it is inevitable that you will have an amount of reverse current flow. If you put diodes across each electrolytic you can prevent this, noting that the equivalent capacitance does not change (doesn't add like capacitors in series) So two 10 uf caps will give one 10 uf ac cap plus a little diode drop.

The rub is that the internal impedance of electrolytics is high which is one of the reasons we bypass with both electrolytics (for energy storage) and non-electrolytic (for low high-frequency impedance).

So if you plan to have high ac currents, the electrolytics will either perform poorly or will overheat.

 

[Skogsgurra]

Absolutely so, dspDad!

And, if you are going to use the bipolar capacitor in any filter for audio, then the 0.7 - 1.2 V diode drop will introduce a lot of distorsion. I have heard what it sounds like in filters for loudspeakers. Terrible.

AC capacitors are expensive, but they are worth the dough if you ask me.