electrolytic capacitors

[confounded]

Hi, i read in this 555 IC projects book;

''If electrolytic capacitors are used, the voltage rating should be about the same as the supply Vcc. An electrolytic capacitor does not become a capacitor until about 0.1 of its voltage rating.''

This is describing selection of Ct for a basic monostable 555.

My question is - is it true electrolytics dont become a capacitor until about 10% of its rated voltage?

 

[Sceadwian]

I'm going to say no. Maybe over .1 volts? Polarized electrolytics maintain their capacitance because of the oxide layer that exists when they're forward biased. They still act like capacitors but they may have some obscure linear equivalents maybe even diode like when VERY low voltage is applied. Reverse voltage on electrolytics causes the oxide layer to disolve into the electrolyte rapidly and destroys the capactor.

 

[smanches]

As a side question, what is the difference between polarized and non-polarized electrolytics? What's the advantage of making it polarized and living with that limitation?

 

[dknguyen]

As a side question, what is the difference between polarized and non-polarized electrolytics? What's the advantage of making it polarized and living with that limitation?

Polarized electrolytic means half the cost and half the space of a unpolarized electrolytic.

 

[smanches]

That's a pretty good reason to live with being polarized.

 

[Sceadwian]

Half is conservative too I think. The oxide layer that's in electrolytics is extremely thin. Especially in low voltage caps the capacity/space tradeoff is huge because the 'plates' are close together. To use a normal insulator that doesn't rely on the oxide layer polarization results in a device many times bigger.