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I am recapping a vintage power supply and I've done similar before but I'm a hobbyist non-expert. My understanding has always been that using a non-polarized cap is acceptable where an polarized capacitor is specified but that, essentially, that's uncommon because the NP/BP cap will be substantially larger and more expensive anyway.

I'm hoping someone can help me understand why it would be that the 250uF/100V part I'm trying to replace is available very inexpensively (and in a reasonable package size) as a non polar electrolytic apparently intended for speaker crossovers ($3.50 currently) but Mouser has nothing at all in that spec for less than like $12 a piece.

This suggests to me that something else is going on here and I'm wary of using the cheaper "spec-compliant" non polar part because I assume I'm missing something. Are caps in speaker crossovers different or special (or worse or unreliable thus cheaper)?

Thanks for any insight. I have framed this as this specific question but if you think this bears as a jumping off point to link to or gently explain other characteristics of capacitors that I'm apparently unaware of, I'm all ears!

BZo
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    So, it boils down to why are mouser more expensive than some low-reputation site that sells something that doesn't have a data sheet (implying that technical comparisons cannot be made) and, from a manufacturer that is unknown and therefore traceability to source is prevented as well as any verification of quality standards used in the build process. – Andy aka Apr 21 '20 at 15:59
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    Depending on the tolerance spec, the dissipation factor and the lead configuration, you can get 220uF/100V for less as a polarized capacitor than non-polarized. https://www.digikey.com/products/en/capacitors/aluminum-electrolytic-capacitors/58?k=&pkeyword=&sv=0&pv2049=u220µF&pv3=1340&pv2079=u100V&sf=0&FV=-8%7C58&quantity=&ColumnSort=0&page=1&pageSize=25 –  Apr 21 '20 at 16:12
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    You might be stuck on the "250uF" thing. That's an odd capacitance nowadays, and the high cost you're seeing is probably a result of narrowing your search. You didn't specify exactly what this cap is for, but it's very reasonable to expect a "vintage power supply" would not mind the use of a modern standard value, such as 220uF, in the filtering section, with no ill effect. The tolerances of those original caps were all over the place anyhow. The next standard size up is 470uF, which "might" present a problem due to a higher power-on surge current (tube rectifiers don't like that). – Kyle B Apr 21 '20 at 16:30
  • @Andyaka: Thanks. I sense you're being a bit sarcastic, but I think the point you're making is that it's actually not crazy to see a "no-name" part be notably cheaper than alternatives, even though the explicit specs meet or exceed, and that this may well be a case where you get what you pay for? – BZo Apr 21 '20 at 17:47
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    @KyleB: Thanks. Sounds like part of the issue is I'm trying to exact match some specs that are fairly obsolete meaning the options are going to necessarily be a bit over the place in availability and price. – BZo Apr 21 '20 at 17:49
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    No, not sarcastic (nor facetious, nor flippant either). Possibly despair mixed with pedanticism or nit-picking (all of which are usually normal traits for a senior engineer). – Andy aka Apr 21 '20 at 18:27
  • @Andyaka Fair. :-) – BZo Apr 21 '20 at 18:33

3 Answers3

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It's actually the polarity that you have to worry about more, non polarized caps can be used wherever polarized caps are used. But, there are other properties (such as max voltage, and ESR) of capacitors that need to be considered and what the application of the capacitor is.

In general if it's a power bypass capacitor, you need a higher voltage rating than the application (or existing capacitor) and a lower ESR (equivalent series resistance).

If the capacitor is for a filter, then if you don't want to calculate how it will alter the filter frequencies, then you will need to match the specs of the cap as close as possible.

I didn't look at the links, but as far as I know all electrolytic caps are polarized. (you can put them back to back, positive to positive, to make them unpolarized)

Voltage Spike
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  • Non-polarised electrolyics are available. Back to back doesn't protect against reverse polarity. You'll need some diodes too. – Transistor Apr 21 '20 at 16:30
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    If two, same-value, aluminum electrolytic capacitors are connected in series, back-to-back with the positive terminals or the negative terminals connected, the resulting single capacitor is a non-polar capacitor with half the capacitance. https://electronics.stackexchange.com/questions/21928/can-you-make-a-non-polar-electrolytic-capacitor-out-of-two-regular-electrolytic – Voltage Spike Apr 21 '20 at 17:42
  • Thanks. I have no present need to create non-polar caps, just trying to understand the landscape out there. Thanks for explaining the equivalences. – BZo Apr 21 '20 at 17:50
  • Related question @VoltageSpike: Candidly, I don't know what the functions "power bypass" and filtering really exactly are, though I do know that power supplies, and amps, have some fairly standard design patterns that use these things-- is there a known good resource I can go after to fill in some of these blanks? – BZo Apr 21 '20 at 17:57
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    https://macrofab.com/blog/bypass-caps-decouple-your-way-to-cleaner-power/ – Voltage Spike Apr 21 '20 at 18:11
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To answer your question in the title:
Basically, it is "inappropriate" when it is unnecessary, and it is unnecessary in the circuits where the voltage across it will not be changing in polarity.

I have checked Mouser and you are correct that the specific capacitance of 250uF is very expensive at 100V, because they seem like a whole another beast, so to speak.
I have also checked 100uF/100V non-polarized caps at Mouser, and they are many times cheaper, so it would be better to make a parallel combination to get the desired capacitance because the specific value you need is unusual and expensive.
Personally, I would go to Digi-Key because they have larger selection, better filters for specifying parameters, and lower prices.
Here is a link to bi-polar electrolytic capacitors for 100V, 47-220uF capacitance range:
https://www.digikey.com/products/en/capacitors/aluminum-electrolytic-capacitors/58?k=bi-polar&k=&pkeyword=bi-polar&sv=0&umin2049=47&umax2049=300&rfu2049=%C2%B5F&pv2079=u100V&sf=1&FV=-8%7C58&quantity=&ColumnSort=0&page=1&pageSize=500
You can select specific capacitors based on the required ESR, ripple current, temperaure, etc.
Just check the datasheet (if there is one) for the capacitor you're replacing to see the specific requirements.
Also personally, I would not bother replacing polarized capacitors with non-polarized ones if they are not going to give me any useful advantage in a specific circuit.

Edin Fifić
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  • Thanks for the multiple tips here. To the final point, I would be willing replace a polarized with an NP cap if that was significantly cheaper, all else being equal, regardless of circuit design-- that was sort of the gist of the question. But I take your point and it looks like I'm comparing on a strange spec combo which is skewing the results. Appreciate the DigiKey advice. – BZo Apr 21 '20 at 17:53
  • As far as I know, NP electrolytic caps are usually more expensive than the standard, polarized electrolytics, as well as physically larger for the same capacitance and voltage ratings. – Edin Fifić Apr 21 '20 at 19:09
  • Yes, that's the conventional wisdom. What triggered my question about is that I was looking at ones that were actually far cheaper (and for the record, not much different in size either)-- and was trying to understand what I was missing – BZo Apr 21 '20 at 19:11
  • Some newer capacitors will be smaller than the old models/technologies and thus cheaper because they used less material. But sometimes the smaller ones could be of a lower quality in some aspect, you just need to check and research it as much as you can. – Edin Fifić Apr 21 '20 at 19:14
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No! Do not change from Electrolytic to a non-polarized capacitor. This will not improofe the design, but cost more. It might even add instability. The ESR of the E-Cap will also dampen oscillations! You can even recreate this in LTspice. Just try to build a SMPS in simulation. To the ideal output capacitor just add a resistor and an inductor in series to the capactitor. The inductance will create resonance with the capacity. That is, why many differenc capacitors often are combined for high speed decoupling on modern electronics.

Just follow the rules:

  • Y/X capacitor for safety, when coupling an isolated PSU to the earth or the input (absolutely required to use a ultra high grade capacitor - otherwhise you could put live voltage on an USB charger, if the cap fails - which definitely happens sometimes - even with Y capacitors this can happen - but much less likelyhood.
  • Electrolytics for high capacities and lower frequencies
  • Non-polar caps in parallel to the E-Cap, to imrpoofe resonance stability and decouple high frequencies
  • For audio it usually does not matter - only remember: E-Cap for audio only, when decoupling a DC signal to AC or the other way around. Negative voltages instantly harm the capacitor. This is a matter of ms, till this effect starts. It will slowly degrade your isolation surface on the Al-foil and lower the voltage rating - and possibly worse. The longer, the voltage is inverted, the more severe the damage
  • For high speed decoupling combine caps like 1uF and 120nF. The high capacity will stabilize the "lower" frequencies, however this cap will not react to multy hundrets of MHz. The 120nF likely does. For even higher frequencies, use even smaller caps (yes, also build size affects the frequencie response.

So what to learn for your case: Just replace the capacitors with resonable quality capacitors of the same rating. Higher voltage or lower ESR rating are welcome, but not required. They likely will not help you at all. If you want absolute quality for your AL-Foil capacitors: Look at the WIMA sortiment. It is more expensive, but has very, very high grade capacitors.

Should you use mouser? Well, not at all! Mouser makes sense for high volume orders with higher reliability. Often however, even Mouser or Digikey screws up and accidentally sells either fake or wrong parts (for high end ICs this can be a huge problem).

Go to Ebay or Aliexpress. The shipping takes some time, but you will not loose like 100 bucks for just some parts. I did this mistake once. Just remember: When buying E-Caps, look out for name brands like Nippon Chemicon. Also try to weight them, if they weight the same as the data sheet says. There where huge E-Caps filled with air and a tiny E-Cap on all markets. Of course this will fail in your product. So make sure, it weights the proper ammount. Like 18650 cells: If they weight half as much as a Samsung INR 25 for example. the cell is just the lowest grade, money can buy. (And also a huge fire hazard)

Long story short: Buy cheaper stuff for 1-2 capacitors. Mouser is kinda expensive and more than often not required at all. Just use the same capacitor as it was before. Nothing lasts forever, even if you spend 100 times the money on a component.

P.S.: You can DIY a bipolar cap by just putting 2 E-Caps in series with one same pole together

like so: + C1 -- C2 + or so: - C1 ++ C2 - Remember: Twice the ESR, half the Capacity. But it absolutely works a treat.

HeartOfGermany
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