Depolarising electrolytic capacitors

[alec_t]

Here's an asc file if you want to try the sim.

 

[Sceadwian]

I'd like to be proven wrong (cause I'd like to see it myself) but I'm not so sure simulation will pick this up because it's an electro-chemical process not a behavioral one, polarized capacitors simply aren't modeled under reverse conditions. Try any polarized cap in a non-polarized schematic using LTSpice of capacitors with equivalent values... It will simulate perfectly fine. Kind of like a transistor will simulate just fine with a few dozen gigawatts of power showing up in the wattage graph even though if that occurred in the real world a small thermonuclear reaction would be taking place =)

Try the experiment on a breadboard with a scope or a good meter, the diode drop should show up there, as long as the currents are kept within a (derated) value (I'd say close to 50%) of the original cap though the oxide layer will never be hurt, at least as long as it's actually used in an AC environment, as alec said assuming leakage currents are minimal the oxide layer won't dissipate into the electrolyte and the bulk of it will reform during the following positive cycle (especially at the typical frequencies electrolytic's are used at)

So basically as long as you're not using it for pulsed sine wave cycles of large current and you derate it to 50% (75% might be safe) it should work out.

 

[BrownOut]

Here you go.

 

[Sceadwian]

Ummm.. <cough> The diode is rectifying not the cap.

You're starting to actually model the electrolytic caps in reverse which DOES NOT OCCUR with the electrolytic cap model by itself. Because there ISN'T one. It's a schematic change not a model change.

 

[BrownOut]

Um, cough, the diode is part of the capacitor model. Try reading the data posted, as a smart member would do. As I said in the other thread, guessing doesn't cut the mustard.

 

[clel miller]

I do not have have Anywhere Near The Knowledge you guys do......but there seems to be a lot of confusion between caps in series and a "non polarized" elytic.
Series caps are very common.....to solve a voltage concern (twice the capacitance and at least half the voltage required), the caps are connected in series and act as a normal elytic.
A non-polarized elytic sees very limited and intermittent use......as in a motor start.
Is that not correct.?
Thank You

 

[BrownOut]

Hi cel, a couple things: capacitors in series are 1/2 the capacitance, and twice the votalge, as opposed to what you wrote ( this assumes the caps are of the same values ) Also, motor start caps can be made for intermittant use, but that has nothing to do with them being non-polarized. A non-polarized cap can be made for continuous use, just as any other capacitor.

 

[alec_t]

@Brownout
1uF is far too low a value for AC coupling of 60Hz. Try running your sim at 10kHz and you'll find the 'rectifying' effect magically vanishes.

 

[Nigel Goodwin]

I do not have have Anywhere Near The Knowledge you guys do......but there seems to be a lot of confusion between caps in series and a "non polarized" elytic.
Series caps are very common.....to solve a voltage concern (twice the capacitance and at least half the voltage required), the caps are connected in series and act as a normal elytic.
A non-polarized elytic sees very limited and intermittent use......as in a motor start.
Is that not correct.?

Not really

The classic place is in passive crossovers, and non-polarised electrolytics are just two polarised ones in series in a single encapsulation.

 

[Ratchit]

To the Ineffable All,

I believe there are some facts being ignored in this thread.

1) First, manufactured NP (non-polarized) are different than two regular capacitors connected back to back. This is stated in https://en.wikipedia.org/wiki/Electrolytic_capacitor under the section "Polarity", which reads "Special capacitors designed for AC operation are available, usually referred to as "non-polarized" or "NP" types. In these, full-thickness oxide layers are formed on both the aluminum foil strips prior to assembly. On the alternate halves of the AC cycles, one of the foil strips acts as a blocking diode, preventing reverse current from damaging the electrolyte of the other one." That tells you that the manufacturing process is different between the two types.

The same section quoted above also says, "Most electrolytic capacitors are polarized and require one of the electrodes to be positive relative to the other; they may catastrophically fail if voltage is reversed. This is because a reverse-bias voltage above 1 to 1.5 V will destroy the center layer of dielectric material via electrochemical reduction (see redox reactions). Following the loss of the dielectric material, the capacitor will short circuit, and with sufficient short circuit current, the electrolyte will rapidly heat up and either leak or cause the capacitor to burst, often in a spectacularly dramatic fashion."

So, that tells me that any scheme to connect ordinary caps in a back-to-back reverse configuration had better make sure the voltage does not exceed the above (1 to 1.5 volts), and then only for a short time, or there will be trouble.

2) The model of a capacitor shows a Zener diode which represents the capacitor working voltage, and the small capacitor reverse voltage (1 to 1.5 volts) allowed for a short time. It must be remembered that this is not a real diode, but only a model that sort of describes the cap's performance characteristic. The model does not tell you that you cannot put a reverse working voltage across the capacitor. So if you use something like that, either don't exceed the 1 to 1.5 volts limit, or insert real diodes in parallel or in series with the caps. Just remember, without diodes, all the forward current existing in one capacitor must also exist in the other series capacitor in the reverse direction.

3) Although the model of a back to back reverse connection will show two caps in series. In reality only one one capacitor at a time is functioning as a capacitor, so the capacitance of the unit will be the value of whatever each cap is by itself. It will not be one-half of the capacitance.

Ratch

 

[BrownOut]

@Brownout
1uF is far too low a value for AC coupling of 60Hz. Try running your sim at 10kHz and you'll find the 'rectifying' effect magically vanishes.

It doesn't disappear. The signal is attenuated due to the low impeadance of the capacitor, but it remains rectified. I showed it at 60hz because the rectification shows up better when the signal is not attenuated, but it remains rectified all the same.

 

[BrownOut]

"Special capacitors designed for AC operation are available, usually referred to as "non-polarized" or "NP" types. In these, full-thickness oxide layers are formed on both the aluminum foil strips prior to assembly. On the alternate halves of the AC cycles, one of the foil strips acts as a blocking diode, preventing reverse current from damaging

Wikipedia gets this wrong. I checked the references, and not one supports this. I'm satisfied that an actual capacitor manufacturer says it's OK to use two caps back-to-back. I had my doubts for a long time until I read the app note. All concerns have been addressed. The diode that appears in the model is real. No they don't insert a discreet diode inside the capacitor package, instead the diode is formed within the capacitor as part of the manufacturing process. It might be considered a parasitic diode, but it's real just the same. The other concerns about reverse voltage slowly breaking down the insulation have also been addressed. The reason electrolytic capacitors must be connected in the correct polarity is to prevent reverse voltage from damaging the dielectric through chemical reduction, but the back-to-back configuration prevents that from happening, because both capacitors see an alternating current, which neither reduces or reinforces the dielectric. In other words, the alternating voltage has no effect on the dielectric. This is all confirmed by the capacitor manufacturer. So go ahead and use this configuration. But remember, the manufacutrer states that both capacitors must be the same value.

 

[clel miller]

Hi cel, a couple things: capacitors in series are 1/2 the capacitance, and twice the votalge, as opposed to what you wrote ( this assumes the caps are of the same values ) Also, motor start caps can be made for intermittant use, but that has nothing to do with them being non-polarized. A non-polarized cap can be made for continuous use, just as any other capacitor.

1. What I am saying is.....to replace a given value cap, with two caps in series, you would use twice the capacitance and half the voltage to equal the cap in question.
2. I did not realize a non-polarized elytic could see continuous use.
Thank You