Polarized capacitors?  - Page 1
Forum section: electronics / beginners

author: MarkS
author_rank: Unknown
timestamp: January 12, 2023, 01:52:29 am
content: Why do they exist? What's their purpose? I've never understood this. Given how capacitors work, it makes as much sense to me as a polarized resistor, two-way diode or non-amplifying transistor. 😳 Yet, they're used frequently... 🤷‍♂️

author: bdunham7
author_rank: Super Contributor
timestamp: January 12, 2023, 01:57:50 am
content: The general answer is that they can be used anywhere they will only be subject to a DC voltage in one direction. A simple example is the filter capacitor of a rectified DC power supply. You don't have an issue with polarized batteries, do you?

author: MarkS
author_rank: Unknown
timestamp: January 12, 2023, 02:00:21 am
content: Quote from: bdunham7 on January 12, 2023, 01:57:50 am🤣 Fair point!

author: BillyO
author_rank: Super Contributor
timestamp: January 12, 2023, 02:19:07 am
content: Polarized capacitors are generally electrolytic. They use an electrolyte to form the cathode such that the dielectric layer, usually formed through anodization, can be extremely thin thus increasing the capacitance of the capacitor several magnitudes over what could be done using other means. Unfortunately, this comes at the price of them being polarized. Putting them under reverse bias will cause the electrolyte and the dielectric to break down resulting in a short. To make capacitors of equal capacitance and voltage rating using thin film dielectrics would mean they would need to be very large and very expensive. A 10,000 uF capacitor made using polyester film would be the size of a breadbox (or larger). So, we put up with them being polarized to drastically reduce size and cost, but we have to make sure the anode stays positive WRT to the cathode or .. bang!

author: MarkS
author_rank: Unknown
timestamp: January 12, 2023, 02:23:15 am
content: Quote from: BillyO on January 12, 2023, 02:19:07 amThat makes a lot of sense! Thank you!

author: tooki
author_rank: Super Contributor
timestamp: January 13, 2023, 07:42:57 am
content: In short: Polarization in capacitors isn’t afeature, it’s alimitationwe tolerate in exchange for much higher volumetric efficiency.

author: tooki
author_rank: Super Contributor
timestamp: January 13, 2023, 07:47:52 am
content: FYI, nonpolarized electrolytic capacitors do exist.* But their comparative rarity is a good indication of how rarely being polarized is an actual problem.*Internally it’s two regular polarized electrolytics in series, but in opposite polarity, with each one double the capacitance of the pair.

author: JohanH
author_rank: Frequent Contributor
timestamp: January 13, 2023, 08:48:23 am
content: Quote from: bdunham7 on January 12, 2023, 01:57:50 amA capacitor wouldn't have much use in pure DC applications, except for battery applications like super capacitors. They have to withstand a small AC voltage, to be able to filter the ripple in the signal. That's why capacitor specifications also mention how much AC voltage they can withstand. Now you could think of it as a component that tolerates only a modest amount of AC voltage, but tolerates a high DC bias (but only in one direction; thus polarized). Some capacitors (e.g. some ceramic capacitors) do not tolerate high DC bias and their capacitance will suffer.

author: Zero999
author_rank: Super Contributor
timestamp: January 13, 2023, 09:24:52 am
content: Quote from: BillyO on January 12, 2023, 02:19:07 amFun fact: the electrolytic capacitor came from the electrolytic diode.Aluminium electrolytic capacitors behave like poor diodes, with a large capacitance and forward voltage of a couple of volts. Imagine a capacitor with a diode in parallel with it: anode pointing to the capacitor's terminal which is used as the negative. Obviously they're designed to be reverse biased and mustn't be forward biased for very long, otherwise it'll be damaged.A non-polarised capacitor can be made using two polarised capacitors of double the desired value, connected back-to-back.

author: armandine2
author_rank: Frequent Contributor
timestamp: January 13, 2023, 09:32:13 am
content: I believe that motor start capacitors (non polarized) are made from a pair of polarized electrolytics connected in series as shown above - but wonder if one of the two permutations is preferred?

author: PA0PBZ
author_rank: Super Contributor
timestamp: January 13, 2023, 11:14:19 am
content: I never understood how putting 2 polarized capacitors in series like that can work, in my eyes it's just an AC voltage divider meaning that the capacitor will have a reverse voltage half of the time (thinking sine wave here).

author: Zero999
author_rank: Super Contributor
timestamp: January 13, 2023, 11:24:05 am
content: Quote from: PA0PBZ on January 13, 2023, 11:14:19 amIt works because electrolytic capacitors act like poor diodes and conduct at DC in one direction only. Imagine the circuit above with a diode connected in reverse, across each capacitor.

author: PA0PBZ
author_rank: Super Contributor
timestamp: January 13, 2023, 11:38:10 am
content: Quote from: Zero999 on January 13, 2023, 11:24:05 amThank you, I was assuming that it had to be something like that, maybe I should have just built and test it

author: armandine2
author_rank: Frequent Contributor
timestamp: January 13, 2023, 03:00:16 pm
content: likewise