# capacitors in series

Discussion in 'Electronic Basics' started by Johan Wagener, May 3, 2004.

1. ### Johan WagenerGuest

I need 75V 10,000uF capacitors for a four channel leach amplifier I am
building. I will have two 750va toroids. Voltage rails will be + and
- 58V dc

I cant seem to find 75V caps. I did find 10,000uF 50V caps at a very
cheap price thought. I am thinking - wiring two caps in series for
100V.

2. ### AndersGuest

The effect will be 5,000uF !
/Anders

3. ### happyhobitGuest

2 in series = twice the voltage and half the capacitance.

2 in series & 2 in parallel (4) = twice the voltage and the same capacitance.

4. ### CFoley1064Guest

Subject: capacitors in series
Hi, Johan. First, you'll need four of these caps to get one 10,000uF 100V cap
(view in fixed font or M\$ Notepad):

4 X 10000uF Caps

+#| +#|
.----#|-----#|----.
| #| #| |
+ o o -
| +#| +#| |
'----#|--o--#|----'
#| #|
V(a)

Figure series capacitance like you calculate parallel resistance, and figure
parallel capacitance like you'd calculate series resistance. (Sounds crazy,
but look it up!)

Ideally, the four caps will have identical capacitance, and equivalent ESR
(Equivalent Series Resistance), which is your main consideration in brute force
filter caps. In fact, they'll be different, and you'll end up with one cap
taking more DC voltage than the other. But also as a practical matter, you've
got so much safety margin, you should be O.K.

Since you have to buy 4 50V caps to equal one 100V cap, the cheapies might not
look so economical.

Good luck
Chris

5. ### Robert C MonsenGuest

This is probably an AC application, yes? Don't use electrolytic caps
unless they are always properly biased. If they are subjected to any
significant voltage in the reverse direction (meaning more than a few
volts), you can blow them up.

If you put two electrolytic caps in series (plus side to minus side),
and subject them to an AC voltage, the caps will leak in the reverse
direction, get hot, and thus blow up. If somehow they don't manage to
blow, they will still generate lots of distortion.

If you put them + to +, then the junction will go way positive (since
electrons can more easily flow from + to -), causing the junction to
charge up to near your max AC voltage. Thus, at opposite points in the
waveform, each cap will be subjected to the peak to peak AC voltage.
This may work for a while until they form hot spots, start to leak,
and maybe blow up.

In either case, you don't really get what you want.

Regards,
Bob Monsen

6. ### Rich GriseGuest

In addition to what the others have said, put a resistor in parallel
with each cap - say, 100k to 1 meg. This will help balance the voltage.
Actually, if your supply design has a bleeder resistor across the cap,
use two at half the value each.

Good LUck!
Rich

7. ### John PopelishGuest

The effective series resistance will be doubled and the capacitance
halved. Better to get the right capacitors.

10,000 uf @ 80 VDC
http://www.panasonic.com/industrial/components/pdf/pic_t-up_series_dne.pdf

Or parallel 2 of the 5,600uf 80VDC units for about the same money but
higher ripple current rating and lower ESR:
http://www.panasonic.com/industrial/components/pdf/pic_ts-up_series_dne.pdf

Digikey has these.

8. ### Pieter HoebenGuest

They will give in series a capacitance of 5000 uF! And more: leakeage
between capacitors vary. When one leaks 10 time that of the other one,
the best one will get most of the voltage (90%), so you must use some
resistors across the caps to cancel that out!

I have 8 nice elko's over here of Epcos: 33.000 uF, 100V. But I think
it will be expensive to send them to South-Africa...

Regards,

Pieter Hoeben
http://www.hoeben.com