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caps back to back

D

Deefoo

Jan 1, 1970
0
Probably a silly question, but when you connect two electrolytic capacitors
back to back to make a non-polarised capacitor does it matter if you connect
the positive plates together or the negative plates? If so, why?

Thanks
-- DF
 
Deefoo said:
Probably a silly question, but when you connect two electrolytic capacitors
back to back to make a non-polarised capacitor does it matter if you connect
the positive plates together or the negative plates? If so, why?

Thanks
-- DF

No not really, it will fail just as quick either way.
 
the classic arangement of doing that uses two diodes, one in series
with each cap. this adds a few vulgarities to the circuit, and I
wouldnt use it for audio!

Steve Roberts
 
M

Mark

Jan 1, 1970
0
that can't be correct, one of the fews rules I know that is ALWAYS
true,,,,,



a cap and a diode in series (with nothing else) is always wrong...



Mark
 
J

john

Jan 1, 1970
0
ive done this in a old tv years ago
to replace a cap, i think i put the 2
negitves to the centre.
 
F

frontline@nospam

Jan 1, 1970
0
No it does not matter although I seem to always put the positives
together.
I would like to know why other responders think this will not work
properly or will fail?
I have done this many times with no problems in AC and DC circuits and
as long as the
voltage rating is not exceeded this will work the same as a NP cap.
Jeff
 
R

Roger Hamlett

Jan 1, 1970
0
frontline@nospam said:
No it does not matter although I seem to always put the positives
together.
I would like to know why other responders think this will not work
properly or will fail?
I have done this many times with no problems in AC and DC circuits and
as long as the
voltage rating is not exceeded this will work the same as a NP cap.
Jeff
An aluminium electrolyte capacitor, depends on the forward bias, to keep
the aluminium oxide layer on the plates. If you reverse bias such a
capacitor, the oxide dissolves into the electrolyte. Depending on the
current flowing, and the duration, this can cause the capacitor to go
short circuit. You have probably 'got away' with it, because with AC, with
the current limited by the other capacitor, there is not time for the
layer to be completely destroyed. However you are 'living on borrowed
time', especially if voltages are present for longer periods...
It will also shorten the overall life of the capacitor.

Best Wishes
 
M

Mark

Jan 1, 1970
0
the interesting question that we should get to is.....

if you place two caps back to back.....+ to + or - to -

is the value of the combination 1/2 each or not.....


i.e. take 2 10 uF caps back to back, is it 10uF or 5 uF

its not a simple question as it seems...

Mark
 
K

Kevin White

Jan 1, 1970
0
Mark said:
the interesting question that we should get to is.....

if you place two caps back to back.....+ to + or - to -

is the value of the combination 1/2 each or not.....


i.e. take 2 10 uF caps back to back, is it 10uF or 5 uF

its not a simple question as it seems...

Mark

There are a couple of ways of connecting up polarized caps to make a
bi-polar one:

1) Just put them in series back to back (doesn't matter which way).
Usually an electrolytic cap will have a higher leakage in the reverse
direction and act a bit like a diode (they used to have electrolytic
rectifiers that relied on this action). This will tend to give a DC
bias on the capacitor so both caps are biased in the correct direction.
The effective capacitance will be that of two caps in series i.e. 5uF
in your example.

2) As someone suggested you can augment the basic circuit with a couple
of diodes to ensure that the caps don't get reverse biased - the diodes
are not just put in series with the capacitors though, they are put
across each cap in the appropriate direction - this improves the
rectification to guarantee the bias on the capacitor. The effective
capacitance will be the same as above.

kevin
 
2) As someone suggested you can augment the basic circuit with a couple
of diodes to ensure that the caps don't get reverse biased - the diodes
are not just put in series with the capacitors though, they are put
across each cap in the appropriate direction - this improves the
rectification to guarantee the bias on the capacitor. The effective
capacitance will be the same as above.

kevin

When you use the diodes, two 10 uF caps in series will give you the
equivalent capacitance of 10 uF, not 5 uF.

Jim
 
A

Al

Jan 1, 1970
0
When you use the diodes, two 10 uF caps in series will give you the
equivalent capacitance of 10 uF, not 5 uF.

Jim

What happenes to the capacitance if the voltage applied is below the
forward voltage threshold of the diodes?

Does the capacitance jump from 10 to 5 to 10 as the diodes conduct and
then reverse bias?

Al
 
J

John Popelish

Jan 1, 1970
0
Al said:
What happenes to the capacitance if the voltage applied is below the
forward voltage threshold of the diodes?

Does the capacitance jump from 10 to 5 to 10 as the diodes conduct and
then reverse bias?

I think the capacitance jumps to 10 uF any time a diode is forward
biased, and falls to 5 uF any time both diodes are reverse biased. If
a sine wave voltage is applied across the pair of capacitors, the
diodes take turns being forward biased the first cycle, and then stay
reverse biased most of the rest of the time, with just a brief swing
into forward bias each cycle, to replace any charge lost to leakage.
 
P

Phil Hobbs

Jan 1, 1970
0
John said:
I think the capacitance jumps to 10 uF any time a diode is forward
biased, and falls to 5 uF any time both diodes are reverse biased. If a
sine wave voltage is applied across the pair of capacitors, the diodes
take turns being forward biased the first cycle, and then stay reverse
biased most of the rest of the time, with just a brief swing into
forward bias each cycle, to replace any charge lost to leakage.

Yes, this scheme is nonlinear at small swings. On the other hand, we
generally use wet electrolytics as coupling capacitors only when their
reactances are small enough to be negligible, because they're crappy
otherwise--nonlinear and time dependent.

I usually prefer to use a big resistor to one of the power supplies from
the middle of the capacitor string, to keep them both at a few volts of
bias in the right polarity. That keeps their characteristics much more
stable and makes them more linear, whereas the diode trick makes them
less linear.

Usually one or other side of the cap string is low enough in impedance
that this doesn't cause any loss problems.

Cheers,

Phil Hobbs
 
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