# Quick Capacitor Question?

Discussion in 'Electronic Basics' started by Me, Mar 26, 2007.

1. ### MeGuest

Hi,
just wanted to know:

If I can't find a ceramic of the value I need, can I place two elects of half value in parallel
but reverse polls? Ie: + -
---¦ ¦---
---------¦ ¦---------
---¦ ¦---
- +

Like that. Or would it be equally ok to do it with like polls, (+ too +)?

My meter reads it as Double the value either way round.

Thanks

2. ### Stephen J. RushGuest

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

3. ### ehsjrGuest

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

4. ### Michael A. TerrellGuest

There are quite a few non polarized electrolytics available.

--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

5. ### Paul E. SchoenGuest

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

6. ### Paul E. SchoenGuest

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

7. ### EeyoreGuest

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

8. ### Michael BlackGuest

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

9. ### Michael A. TerrellGuest

capacitance is high enough that he can't use Ceramic, Mylar or other non
polarized capacitors.

--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

10. ### Michael BlackGuest

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

Michael

11. ### Rich GriseGuest

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

12. ### Michael A. TerrellGuest

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.

--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

13. ### EeyoreGuest

I agree.

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

Graham

14. ### EeyoreGuest

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

Graham

15. ### Ross HerbertGuest

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?

16. ### Michael A. TerrellGuest

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.

--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida