Electronic – Is it safe to use series polarized capacitors to form non-polarized capacitors to be used for higher rated voltage

analogcapacitorelectrolytic-capacitorpower electronics

schematic

simulate this circuit – Schematic created using CircuitLab

It is known that we can use series capacitors to increase rated voltage. But I want to know how far we can push this feature to? Suppose, each of this capacitors are identical, 200V rated and polarized. We also know that polarized capacitors can be turned into nonpolarized by connecting their negative terminals as in the scheme above.

So, in the circuit above, can I safely say that I have formed a 25 mF – 400V rated capacitor? If I charge them up to 400V, then discharge them down to -400V, should I be expecting some problems? I suspect, over time the voltage of each capacitors will be varied and after a some point, one or two of the capacitors will be holding more than 200V (or be holding negative voltage) and blow up. If so, is there a way to prevent that and safely form a higher voltage rated, non polarized capacitors by polarized capacitors?

edit: I editted the question to be more clear. Two series polarized caps forming a nonpolarized cap has the same rated voltage.

edit2: I'm editting the question just to address the diferences between this other question on stack. Firstly, the questions are entirely different. In my question, I am aware of that we can form a non polarized capacitor out of two polarized capacitors. Secondly, I am asking for a safe design guide. I provided a schematic to demonstrate my particular problem.

Best Answer

I think the question is, how can it be guaranteed that the +/- 400 V is properly distributed between all capacitors.

When two identical polarized capacitors are combined to form one non-polarized capacitor, when charged only one capacitor will take care of the voltage, the one that has the right polarity for that voltage and is charged. We assume that the capacitor with the "wrong" voltage will sort of act like a diode and not charge but also will not be damaged. I have my doubts if this will be the case for all types of polarized capacitors.

Now your case adds that also the voltage must be shared. So 400 V is shared equally between two 200 V capacitors. Both should be charged equally so that no voltage difference will occur.

In theory that could work as the capacitors will behave in an identical way, they have exactly the same capacitance etc.

However in the real world no two capacitors are exactly the same. Also the behavior of a capacitor will change over time. If one capacitor has slightly less capacitance, it will be charged to more than 200 V. Suppose that slight overvoltage causes more aging and loss of capacitance. Then this difference would get progressively worse over time.

I would not rely on these capacitors to "sort themselves out". I would add balancing resistors and perhaps even diodes (protecting the reverse biased capacitors) so that the effect of mismatched capacitors is minimized.

This is what I mean:

schematic

simulate this circuit – Schematic created using CircuitLab

This would result in a 400 V non-polarized capacitor of 50 mF (\$\frac{1}{2}C\$) where \$C\$ is the nominal capacitance of each of the 4 capacitors.