Greenhorn question

[Russlk]

In the interest of accuracy: The insulator in polarized caps is an oxide on the plate. The electrolyte is a conductive liquid or paste that fills the gap between plates. The process of building up the oxide layer by passing a current thru the cap is called "forming".

 

[DigiTan]

Is it the breakdown factor that makes electrolytics react so violently to reverse voltages; or it some a other mechanism at work?

 

[Russlk]

The electrolyte vaporizes, causing the cap to explode. most large caps have a sealed vent to avoid explosion.

 

[DigiTan]

How is the that the electrolyte doesn't react this way to a forward voltage? I seems to me that as long as the plates use identical materials, the voltage direction shouldn't matter (I know this isn't really the case though).

 

[Russlk]

It is not the direction of the current but the amount. The cap does not build up a charge in the reverse direction, so the current is limited only by the resistance in the circuit. In a power supply, the resistance is low and the voltage is high resulting in very high current.

 

[DigiTan]

But what would stop the capacitor from building a charge in the reversed direction? Every place I check describes electrolytics as two plates with an electrolyte sandwiched in the middle, but under that analogy, it should matter which way the voltage is applied. :?

 

[Nigel Goodwin]

But what would stop the capacitor from building a charge in the reversed direction? Every place I check describes electrolytics as two plates with an electrolyte sandwiched in the middle, but under that analogy, it should matter which way the voltage is applied. :?

The capacitor is 'formed', as Russelk mentioned a number of posts back. If you're repairing old valve radio, that haven't been used for decades, you may need to reform the capacitors, or they could explode when you plug them in!.

 

[DigiTan]

Okay, so this forming is always done in the manufacturing process? And it's repeated ever time the capacitor is used essentially?

 

[Nigel Goodwin]

Okay, so this forming is always done in the manufacturing process? And it's repeated ever time the capacitor is used essentially?

Yes, but if the capacitor has been unused for a number of decades, and you're applying 350V DC to it very quickly, it could well explode as it's lost it's forming over the years. In which case you reform it by feeding it via a suitable current limiting resistor - there have been a number of 'capacitor reforming' projects over the years.

 

[Oznog]

I understand that negative flows to positive, but sometimes like with capacitors and PNP transistors (a whole other beast) it seems to contradict this. I.E., why create a power supply that outputs + voltage? Can you 'output' positive voltage, and if so, what about the negative to positive thing? Along that lines, how can you use a capacitor in an AC circuit if it's polarized? Doesn't the alternating current ensure it's never always positive?

Current flowing from a positive voltage to a negative voltage means the component is sourcing power, like a 12v battery. Put the battery across a resistor and the resistor will have 12v across it but the current is flowing into its positive and out the negative. This means it's absorbing electrical energy. When a battery's being charged by another power source, current is reversed, going into its positive and out the negative, like a resistor. Thus it is consuming energy.

People have almost gotten into fistfights over conventional vs electron flow...