Electrolytic capacitor power handling

[crackyflipside]

On parts express, it says rated for 200w and I'm pumping in 250w is it ok?

edit: its in the crossover.

[Johnloudb]

Are you sure you're pushing 250 watts on the cap. it's in the low pass section I guess. I mean that would only be at musical peaks not RMS or anything, and that would be pretty damn loud if you speaker has any sensitivity at all.

I've never designed a speaker crossover, so I guess someone else should probably answer your question.

[Jed]

On parts express, it says rated for 200w and I'm pumping in 250w is it ok?

edit: its in the crossover.

You can parallel multiple caps to fix that problem. Use half the value of the original with 2 in parallel. For example if the cap is 100mfd, use two 50mfd caps in parallel to make 100mfd. That is if you really need to increase the power handling.

[crackyflipside]

Are you sure you're pushing 250 watts on the cap. it's in the low pass section I guess. I mean that would only be at musical peaks not RMS or anything, and that would be pretty damn loud if you speaker has any sensitivity at all.

I've never designed a speaker crossover, so I guess someone else should probably answer your question.

Yeah thats a really good point.

[Face]

How about paralleling the cap in question with one of a lower value but higher wattage? Say a 63v 230uf electrolytic and 200v 10uf mylar?

[ThomasW]

Caps are rated by voltage.

[Bent]

Caps are rated by voltage.

I held back on pointing this one out, but ^

[Johnloudb]

Caps are rated by voltage.

Yeah, I've never seen a power rating on a cap. ops: Blown up a few electrolytics by wiring them in the wrong polarity, but that's different of course.

On parts express, it says rated for 200w and I'm pumping in 250w is it ok?

edit: its in the crossover.

If you're putting more than 200 volts across a 200 volt cap that is a problem, though. Need a higher voltage cap.

[crackyflipside]

Caps are rated by voltage.

100V Non-Polarized Crossover Capacitors

These high quality Bi-polar (non-polarized) electrolytic capacitors are perfect for your next crossover design. They feature a 5% dissipation factor and are rated at 100V which are effective to approximately 200 watts.

[ThomasW]

If you're going to be flogging these speakers buy 2 caps that are 1/2 the value needed and wire them in parallel

[---k---]

The 100 and 80uf caps are all ready getting wired in parallel to get to a value of 180uf.

I just recently changed this BOM to include the electrolytic caps since the price was getting out of control. The poly caps are in the Upgraded BOM. I don't think either is an issue.

[crackyflipside]

If you're going to be flogging these speakers buy 2 caps that are 1/2 the value needed and wire them in parallel

]

Nah, it was in the surround speakers I'm also building, Mark K redesigned his rs225/28a MT with much less baffle step compensation, but the bad thing is he had an 80uf cap in there, and building four of them, the price adds up quick. I went ahead and ordered the electrolytics for the surround speakers.

On the khanspires I went with the poly for all caps, if I would have realized it, I would have gotten the electrolytic caps for the big values.

[JonP]

Yep, it's already covered that voltage is the major cap rating.

So, apply a little Ohm's Law and power formula:

250W @ 8ohms (higher is worse voltage case) = 44.7V @ 5.6A

250W @ 4ohms (lower is worse current case) = 31.6V @ 7.9A

Well, for these impedances at this power, the voltage is not even half of the 100V rating. (if that is what the cap is) The current might be an issue if it was long term continuous use. (which with music it won't be) Actually, every cap has a current rating, it's not usually published, since it's commonly not pushed. You will see it in filter caps, since they actually will have to conduct a lot of AC current. Big power supply caps will have ratings of only 4-6A tops, so I'd guess one like this is somewhat less. But those are 24/7 continuous current ratings, possibly the intermittent surge ratings are higher. It's mostly a heating effect, and momentary heating isn't as bad. If you are a DJ, and the speakers would have pro use, AND they are at a lower impedance in their operating range, doing the half values and paralelling them would be an idea. (each cap will carry half the current)

Now say instead of having a low impedance there, you have a high Z peak on your resonance... fairly common and sometimes quite high:

250W @ 32ohms (higher is worse voltage case) = 89.4V @ 2.9A

250W @ 40ohms (higher is worse voltage case) = 100.0V @ 2.5A

Current of course goes down, (W = V x I) the voltage goes up, but it takes 40 ohms to hit the 100V rating. Not sure how often there's that high of a peak R at Fs. A single RS225 has a 24 ohm peak, that's only going to raise it to 77V. Still good, and your amp will probably clip before going that high in voltage anyway.

Unlike overcurrent, exceeding the voltage rating even for very short peaks is a dangerous thing. Instead of heating, the cap would break down and probably short. Boom! :E To calm you on that, most electrolytic caps will probably handle 120% of their voltage rating without trouble, but don't bet on it!

Wow, wrote a short story on caps... :B Bottom line, looks like those caps should handle some heavy use without too much stress.

[---k---]

On the khanspires I went with the poly for all caps, if I would have realized it, I would have gotten the electrolytic caps for the big values.

Sorry about that. I never really gotten into the cap wars. I just used what is best in my price range to avoid any long term regrets. When I built mine, they were probably about $100 /ea less than what they are now.

[Amphiprion]

Capacitors, theoretically, have zero power dissipation. They merely store and release energy. Same thing for inductors.

Now, because of non-ideal behavior, they do have some losses. But I seriously doubt anyone would run into an electrolytic in a crossover that would give out before they went deaf as a stump. As mentioned above, just keep the voltage rating well above what is necessary (250WVDC is plenty).

[Johnloudb]

Thanks for clarifying that Mark ... that definitely cleared that up for me.

Just want to mention, I haven't ever seen a 250v bi-polar electrolytic cap, although they're probably out there. I usually see a lot of 100v ones. You could wire a couple in series though for more voltage handing.

If you don't need the higher voltage handling, paralleling caps has the added benefit of reducing the dynamic impedance. Probably doesn't matter though, in a low-pass woofer crossover.

[Davey]

A bi-polar electrolytic cap is just two polarized caps wired in series (one reversed) inside the same package. You could series-connect a couple for more voltage handling, but then total capacitance drops. It's kind of a catch-22.

Also, anytime capacitors are connected in series there is a voltage sharing concern. The better one will always have a higher voltage across it. Over time a 50/50 split can deviate by quite a bit.

Shunting resistor dividers across series capacitors is a technique used commonly in days of yore (in equipment power supplies usually) but you don't see it much anymore.

Cheers,

Dave.

[Dennis H]

So, apply a little Ohm's Law and power formula:

Keep in mind that peak voltage is 1.4x the RMS voltage.

[Amphiprion]

A bi-polar electrolytic cap is just two polarized caps wired in series (one reversed) inside the same package.

Fearing that I may be contradicting someone who has corrected me many times before , Cyril Bateman's series of articles on capacitors in Electronics World (specifically the 5th installment) addressed this common misconception (of which I too believed before reading his work). Polar cap construction constists of very different anode and cathode foils inside one capacitor; bipolar caps completely eliminate the cathode foil and use two identical anode foils in one cap. The measurements he took showed that bipolar caps constructed this way had lower distortion than polar capacitors back-to-back (although the distortion measured was very arguably inaudible at the levels he recorded in all cases).

The PDF is available here (capsound5.pdf):

http://uk.geocities.com/cyrilb2@btinternet.com/downloads_4.html

Shunting resistor dividers across series capacitors is a technique used commonly in days of yore (in equipment power supplies usually) but you don't see it much anymore.

It's still used, we were looking at "split cap" technology for single ended PWM amplifiers when I was at Cirrus. Some folks use resistors to bias it correctly at DC, others just depend on the relatively low speaker impedance tied to the half-bridge output to take care of it. The biggest advantage is huge PSRR improvements (around -60dB) for single ended, no feedback amplifers:

http://www.planetanalog.com/article/printableArticle.jhtml?articleID=198500286

[JonP]

Keep in mind that peak voltage is 1.4x the RMS voltage.

True! It was a bit late when I went off into EE land... That's for pure sine waves, music peaks probably have a higher crest factor, some of the time.

So, the peak voltage situation will be that much worse, with the current situation actually diminishing a proportionate amount during those peaks, for the same power level.

I still wonder about the current rating of those bipolars, probably lower than your average single layer electrolytic. As well as wondering how quickly you have problems going over the "rated" current capacity. Probably heating, and low duty cycle music peaks aren't going to add up to that much average heat increase.

Still, this is 250W going into a speaker. Unless its a sub, how often does that happen, other than on a very momentary basis? A 250W continuous RMS power level, would smoke just about every residential class speaker, as well as damaging hearing. So you're likely never really running the cap at that level.

[Mark K]

]

Nah, it was in the surround speakers I'm also building, Mark K redesigned his rs225/28a MT with much less baffle step compensation, but the bad thing is he had an 80uf cap in there, and building four of them, the price adds up quick. I went ahead and ordered the electrolytics for the surround speakers.

On the khanspires I went with the poly for all caps, if I would have realized it, I would have gotten the electrolytic caps for the big values.

http://www.audioheuristics.org/projects_gallery/RS225_RS28A_updates/dayton_reference_rs225.htm

This is for the 4dB version, not the 3dB one.

Did I really send you the final version ? I've fiddled with it so many times I'm not sure what is the final version anyway...

Did you order all the parts yet? It should work as is with 3dB bafflestep even on the ceiling although it might not be perfect as a sort of soffitt. Still, it worked ok for the Ushers...

Honestly, for the price of electrolytics, you might as well use them. No way you'd hear a difference in the first 5-10 years. They might change enough over time to have an effect. If you decide you really like them after 5 years, then buy some Solen's!