Paul E. Schoen said:
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
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