Quick Capacitor Question?

half value in parallel

Match the polarities. The parallel-crossed connection has one capacitor backwards. Electrolytics connected backward fail, sometimes loudly.

half value in parallel

When you put polarized electrolytics in parallel, keep the polarities the same. Whether you can use such an arrangement in place of a ceramic is a different issue. If a non-polarized capacitor is needed, then the two electros in parallel scheme is not the right thing to do.

Ed

half value in parallel

There are quite a few non polarized electrolytics available.

They are the best solution, but polarized electrolytics can be wired in series for half the capacitance value. I think the capacitor that is reverse biased just becomes leaky so the correctly biased one charges up. It would probably be better to add diodes across the caps so only about 0.7 volts reverse bias is ever applied.

Paul

"Paul E. Schoen" wrote in message news:46075d3d$0$30098$ snipped-for-privacy@news.coretel.net...

I did a quick LTSpice simulation to compare two 10 uF capacitors in series, with diodes, to a single 5 uF capacitor, into the same load, and there was virtually no difference. I was surprised to see that the diodes only conducted during the first few cycles, and then the two capacitors alternated charging and discharging, with correct polarity voltages on both. The ASCII file follows:

Paul

Version 4 SHEET 1 880 680 WIRE -112 144 -128 144 WIRE 64 144 -112 144 WIRE 144 144 64 144 WIRE 256 144 144 144 WIRE 352 144 320 144 WIRE -128 192 -128 144 WIRE 352 224 352 144 WIRE 64 240 64 208 WIRE 144 240 144 208 WIRE 144 240 64 240 WIRE 64 256 64 240 WIRE 144 256 144 240 WIRE 144 320 64 320 WIRE -128 384 -128 272 WIRE -80 384 -128 384 WIRE -48 384 -80 384 WIRE 64 384 64 320 WIRE 64 384 32 384 WIRE -80 432 -80 384 WIRE 352 432 352 304 WIRE 352 432 -80 432 FLAG -128 384 0 FLAG 64 384 Vout FLAG -112 144 Vin FLAG 352 144 Vout2 SYMBOL polcap 48 144 R0 WINDOW 3 -45 37 Left 0 SYMATTR InstName C1 SYMATTR Value 10µ SYMATTR Description Capacitor SYMATTR Type cap SYMATTR SpiceLine V=35 Irms=44m Rser=2.8 MTBF=2000 Lser=0 mfg="Nichicon" pn="UPR1V100MAH" type="Al electrolytic" ppPkg=1 SYMBOL polcap 80 320 R180 WINDOW 0 24 57 Left 0 WINDOW 3 24 8 Left 0 SYMATTR InstName C2 SYMATTR Value 10µ SYMATTR Description Capacitor SYMATTR Type cap SYMATTR SpiceLine V=16 Irms=35m Rser=2.8 MTBF=1000 Lser=0 ppPkg=1 SYMBOL diode 128 256 R0 SYMATTR InstName D1 SYMATTR Value MURS120 SYMBOL diode 160 208 R180 WINDOW 0 -37 37 Left 0 WINDOW 3 -101 0 Left 0 SYMATTR InstName D2 SYMATTR Value MURS120 SYMBOL voltage -128 176 R0 WINDOW 3 -151 134 Left 0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V1 SYMATTR Value SINE(0 12 60 0 0 0 100) SYMBOL res 48 368 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R1 SYMATTR Value 100 SYMBOL cap 320 128 R90 WINDOW 0 0 32 VBottom 0 WINDOW 3 32 32 VTop 0 SYMATTR InstName C3 SYMATTR Value 5µ SYMATTR SpiceLine V=50 Irms=0 Rser=0.004 MTBF=0 Lser=0 ppPkg=1 SYMBOL res 336 208 R0 SYMATTR InstName R2 SYMATTR Value 100 TEXT -280 506 Left 0 !.tran 100m startup

half value in parallel

No. You must place them in series, + to + or - to - (doesn't matter which way).

Electrolytics have *very* different characteristics to ceramics though. That's quite likely to be a problem. What's this capacitor for ?

You'd be far better off using a plastic film capacitor if you can't get the right value ceramic.

Graham

half value in parallel

But the question was incomplete. The poster is talking about not having a ceramic capacitor of the right value, but by the time you have the need for such large capacitances that you find in electrolytics, one would rarely be talking ceramic capacitor. Until we have a capacitance value from the original poster, his "solution" is irrelevant because there may be all kinds of other paths to follow to get to the end game.

Michael

The OP asked about using electrolytics, which implies that the capacitance is high enough that he can't use Ceramic, Mylar or other non polarized capacitors.

But he said "If I can't find a ceramic of the value I need..." which is ambiguous.

Michael

half value in parallel

I don't get your original point - ceramics and electrolytics are usually several orders of mangnitude different in value.

What value of cap do you need? Great big huge ceramics might go up to maybe 1 uF , and I've never seen an electrolytic that small.

What are your cap values, and what are you trying to accomplish?

And, yes, to make 'lytics work on AC, they go in series, back-to-back (i.e., + to + or - to -).

Thanks, Rich

Have you seen any of the high value ceramics? They are made, but are not easy to find. I've used a number of them above 1 µf. In that range the non polar electrolytic is probably a better choice. We needed 10 µf, in a unit built to last 20 years, or more so we used SMD ceramics in all of the AGC time constant filters.

I agree.

Not at all. Electrolytics are available with values as low as 0.1uF.

Graham

Electrolytics go as small as 0.1uF. Quite why you'd want one that small I don't know though.

Graham

not having

the need

would rarely

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Unless I am missing something I would have thought that the reason the OP enquired about "ceramic" caps was because of the characteristics these possess. His suggestion that using electrolytics might be a possible solution doesn't seem right to me since their characteristics are nowhere near the same. He appears to want a cap value of fairly large value hence the query about using electro's. His drawing shows them connected in par'l but with polarities opposing which could only be of use where very low voltage was applied, otherwise they would be destroyed when polarity was reversed to either of the electro's (with higher voltages).

If he genuinely needs a capacitor with the characteristics of a ceramic and can't find one large enough, then why not parallel them to get the desired value?

It is done in some designs, but some people don't really know what family to chose from, so they make wild guesses as to what type of component they need.