Solen Fast caps have VDC and VAC ratings?

[schoolie]

Just wanted to clear this up.  The Solen capacitors are rated for 630VDC, but only 330VAC.  Even if it were a matter of RMS vs peak voltage for AC, it would still be lower than the DC rating.  I know people use these as filter caps, but are these rated for 460V rectified where the voltage is swinging from 0V to 460V.  Is this the equivalent of a 230VAC signal?  Thanks for any insight for the noob

[PRR]

> where the voltage is swinging from 0V to 460V

It's a smoothing cap. It's swinging 450V to 460V and back.

> if it were a matter of RMS vs peak voltage for AC

On DC there is no current flowing in the cap. On straight AC (put it across the wall outlet) there IS heavy current. For 8uFd at 60Hz and 330VAC, about one Ampere. If the cap were perfect, no effect... but in fact the cap has some stray resistance which is heated by this current.

Also DC circuits tend to be mellower and safer than AC circuits. All kinds of terrible things come through the utility wires but are moderated by transformer rectifier filter. The AC rating may be extra conservative.

We audio folks rarely see a big AC voltage on a capacitor. Sometimes we have caps across the AC wall-power, but 8uFd is far too big. What does use big AC-rated caps is some motors. Single-phase AC induction motors won't self-start, some trick is needed. Often a capacitor is used to shift phase and give a "2-phase effect" for starting or running.

[schoolie]

Thanks for your insight, PRR!   These caps have extremely low ESR.  The DF is like .0007 (@120hz?).  I haven't found any posts complaining about failures in power supplies.  I'll bet the failures would be pretty spectacular, though

[HotBluePlates]

Yeah, the a.c. rating likely won't apply to you.

It could apply if you were doing something like a parallel feed output stage, where there is a choke from plate to B+ and a large cap from plate to SE output transformer.

Or, if you were building an output-transformerless (OTL) amp, but then again, you'd probably have an output cap from the cathode to the speaker in that case.

In any event, that a.c. rating has more meaning when handling a 300+ vac signal, or were using them in a crossover or the like. 330vac implies there might be a case where you're blocking 330vdc on one side of the cap, with 0v on the other side (your speaker), and have a -330v signal; 330v *2 = 660v total across the cap.

I've used Solen caps in power supplies, and as long as you're below 630v total across the cap, you're perfectly fine.

Beware, these caps are enormous compared to similarly-rated e'lytic caps.

[Tiny_Daddy]

I had a low DF polypropylene cap failure, motor run cap, in the house A/C compressor. I cut it open to see what happened. Inside the metal can was the usual yellow axial lead cap. The aluminum foil had melted and fused and collected in a puddle in the bottom of the can. Good thing there was a can. But hopefully there's not that much current available in your amp.

[PRR]

> could apply if you were doing something like a parallel feed output stage

Even then, only if you drive it below bass cutoff. Scale your C-L for 40Hz, drive hard at 10Hz, you have voltage across the cap. For all in-band signals, the voltage across the cap must be small.

Shunt a small cap across OT primary to tame highs... that gets into large AC across the cap.

Caps directly across rectifier diodes (not the DC filter cap) will have a large AC component.

[HotBluePlates]

Seems then the ac voltage rating is pretty meaningless in this case. I know I've used Solen caps, and never was aware of a vac rating (only the 630vdc rating).