Re: Electrolytic Capacitor Substitute

[Dan Fraser]

A non polarized capacitor of the same capacitance and voltage rating can
replace a polarized capacitor in almost every application with NO
modifications.

In fact the diode trick won't work because current has to flow both in
and out of a capacitor.

However, why bother as polarized caps are far cheaper than unpolarized.

--
Dan Fraser

From Costa Mesa in sunny California
949-631-7535 Cell 714-420-7535

Check out my electronic schematics site at:
http://www.schematicsforfree.com
If you are into cars check out www.roadsters.com

 

[Rob Judd]

Actually, most NP versions are nearly twice the size physically as
their polarized counterpart, given the same voltage spec. That would
mean that many applications would not fit the bigger cap if the
original was tightly packed in the initial design.

Two. Back to back. Quick and cheap. NP caps cost less than two,
however, in normal buying circumstances. They are harder to find in
single quantities though, which would make your statement probably
correct. They really ARE two caps back to back inside. That is what
an NP cap is.

Look in the audio section under "crossover caps". That's generally where
they're kept.

Rob

 

[DarkMatter]

Look in the audio section under "crossover caps". That's generally where
they're kept.

There ya go!

 

[Robert Monsen]

Don't electrolytic caps require a bias voltage to keep them healthy? Seems
like wiring two back to back would cause problems with this.

Regards,
Bob Monsen

 

[DarkMatter]

Don't electrolytic caps require a bias voltage to keep them healthy? Seems
like wiring two back to back would cause problems with this.

Non-polarized electrolytics ARE physically two caps of the given
value in a back to back configuration, in a single package.

 

[Spehro Pefhany]

[original comment deleted]

Non-polarized electrolytics ARE physically two caps of the given
value in a back to back configuration, in a single package.

I've also heard something like this (notably in Grob's "Basic Electronics"),
but I still don't believe it is strictly true.

If a non polarized electrolytic capacitor consists of two back-to-back
(series connected) simple electrolytic capacitors, then a reverse charge
will be put across one of the capacitors during any charge cycle. I was
under the distinct impression that subjecting any electrolytic to more than
2 V of reverse bias would result in a dangerous situation, possibly leading
to venting of electrolyte and explosion. Connecting the capacitors in
parallel would likewise result in a reverse bias.

The common gets charged to a negative or positive voltage.

From what I can determine by searching the web (references below), I believe
that nonpolarized electrolytic capacitors are actually electrolytic
capacitors in which the dielectric oxide layer has been formed on both the
anode and cathode during the manufacturing process. In a polarized
electrolytic capacitor, only the anode has this dielectric oxide layer
(actually, the cathode has it also, except its too thin to withstand more
than about 2V. This, I gather, is for protection in the case of an
accidental reverse bias.)

Absolutely correct. They look identical to any other electrolytic
inside. It's all in the processing of the foil to create the oxide
layer before they are assembled. Cut one apart and see (I have).


Best regards,
Spehro Pefhany

 

[Stig Carlsson]

Some details: A normal polarized electrolytic capacitor has two foils,
anodic and cathodic, separated by paper impregnated with electrolyte. The
anodic foil has an oxide layer which is the dielectric of the capacitor. The
thickness of the oxide layer is propotional to the voltage applied when the
foil is manufactured. There is also an oxide layer on the cathodic foil, but
it is very thin, about 1 Volt.

The oxide layer can only support voltage in one direction.

If the voltage is applied correctly it vill be across the anodic foil and
all is well. If applied backwords the anodic foil will be shorted out, a
large current will pass and the cathode foil will start to grow its own,
thicker oxid layer. This will lead to gas and heat being produced, and the
capacitor will explode if the availible current is big enough.

In a nonpolarized capacitor there are two anodic foils so it does not matter
which of the foils gets the plus or minus connection, one one the foils will
always be connected correctly.

Regards
Stig Carlsson

 

[Jim Meyer]

From what I can determine by searching the web (references below), I believe
that nonpolarized electrolytic capacitors are actually electrolytic
capacitors in which the dielectric oxide layer has been formed on both the
anode and cathode during the manufacturing process. In a polarized
electrolytic capacitor, only the anode has this dielectric oxide layer
(actually, the cathode has it also, except its too thin to withstand more
than about 2V. This, I gather, is for protection in the case of an
accidental reverse bias.)

Thus, I believe its probably not a good idea to put electrolytic capacitors
back to back, as suggested previously.

Regards
Bob Monsen

This comes up about once every two or three years.

It *IS* OK to put two polarized caps back-to-back to make an AC
cap out of them. The reason is that there is a tiny bit more leakage
in the reverse biased cap. That leakage charges the "properly" biased
cap in the right direction to keep it happy when the current reverses.
Since the circuit is symetrical, each cap's leakage protects the
other.

Do a test with real capacitors (or even a SPICE simulation). You
will find that after only a couple of cycles that each cap will have a
DC voltage across it that is of a magnitude and polarity such that it
will be completely protected from reverse voltages.

You can guild the lilly with a couple of diodes. One diode
across each cap in a direction that will prevent the cap's voltage
from going more than a diode's drop reverse biased. The diodes are
not necessary but may give you that belt and suspenders feeling of
confidence.

Jim

 

[Robert Monsen]

I just tried to do your experiment, since it seemed absurd to me (how could
one side charge without passing charge the wrong way through to the other
side?) 14v of AC (off of a step down transformer) across two back to back
(positive side out) 100uF 25v polarized capacitors.

I should have been wearing safety goggles. I was just about to hook up the
oscilloscope to check out this mysterious DC bias, when one of the 100uf
capacitors exploded right in front of me. I've never seen that before! Its
amazing how much stuff sprayed out all over the place. They were both brand
new. The electrolyte got all over my new tek probes and superstrip!

Regards,
Bob Monsen

 

[Robert Monsen]

Actually, once I started cleaning up the mess from the explosion, I realized
that BOTH had exploded. The reason I didn't notice is that one only sprayed
electrolyte. The CAN exploded on the other one. This is NOT a good way to
hook up capacitors, and will serve as a lesson to me to be careful with
polarized electrolytics during circuit failure modes. It took less than 10
seconds to blow it up.

Regards,
Bob Monsen

 

[DarkMatter]

Be careful with electronics, and take anything you read on Usenet
with a grain silo of salt.

That would be one high sodium intake!

 

[Dave Platt]

They will do this in DC circuits if installed backwards,

or if excessive valtage is applied.
I have used 2 to replace 1 NP cap in DC circuits but
have never attempted it for a pure AC circuit.
Did you measure the AC waveform P-P or RMS?
P-P would be the only way and allow a large spread
between peak and cap voltage rating, like 2X.
Your test may not have allowed for excessive voltage
or it may do the same again.

I'd guess that it wasn't a matter of overvoltage - it was a matter of
ripple current, and the resulting heat dissipation.

Running a large AC current through a 'lytic cap is generally a bad
idea. Aluminum electrolytics have a relatively high dissipation
factor... their equivalent series resistance is fairly high and
they'll heat up rapidly if high currents are run through them.

I'd guess that the OP didn't puncture the dielectric (as an
overvoltage would have done) - he boiled the electrolyte into steam.

 

[Jeff]

I just tried the same "test" with the same value caps
and a 5 ohm resistor it has been sitting at 11V AC RMS
for a lot longer than 10 seconds. (varriac)
Very little current draw and nothing getting hot or
exploding, RMS voltage across each cap ~5.5VAC
Now for the surprise, DC reading across each cap
6-7VDC polarized + and- with each capacitor.
Jeff

 

[Robert Monsen]

One of the earlier posters suggested that there is some kind of leakage
effect that pumps the junction between the capacitors up to a high dc level,
high enough to prevent the AC voltage from causing a reverse bias.

Given a diode over one of the capacitors pointing towards the junction, I
can see that it would get pumped up to the highest voltage, minus the .6
drop on the diode. If the diode was pointed away from the junction, then the
opposite would be true, ie, the junction would drop to the lowest voltage +
the .6 PN drop.

If there is asymetrical leakage of current, then this would also happen
without the diode, one way or another. If the asymetric leakage is toward
the junction, then the junction would pump up to the highest voltage. If its
away from the junction, then the junction would drop to the lowest voltage.

Given the above mentioned posting, my guess was that it will leak current
from - to + more readily than + to -. Thus, I put on goggles, and tried the
experiment again, this time joining the + leads rather than the - leads.
Also, I used caps with a higher voltage (50 VDC) rating. The maximum voltage
pp at any one time is about 40v, so I guessed that it should be better able
to handle the stress.

The first experiment was with a diode from the - to + terminal of one of the
caps. The DC voltage between the caps grew to 16vdc, as expected.

Next, I discharged the caps, and tried it again without the diode. The caps
did not explode, but the dc voltage seemed to land at a random place
initially, and then slowly climb. Thus, its probably true that the leakage
is greater from - to + than from + to -, at least with these new caps.
However, it seems to take a long time for these 100uf caps to charge up to a
value that will protect them from being charged with incorrect polarity
(without the diode, anyway). I let the caps charge for 30 seconds, and it
looked like they were charging at about 2v per minute. Thus, the 'bad'
polarity charging would occur in my test setup for about 7 minutes, albeit
with smaller and smaller negative voltages all the time.

I did not repeat the experiment with the caps connected in the other
direction, although by symmetry it should simply charge the negative
junction to a large negative value over time. However, I'm still pulling cap
stuffing out of my hair from the last experiment, so I though I'd leave
things as they stand... Any brave souls who want to give it a try might
email me the results at rcmonsen @ comcast.net

I'm guessing that the explosion from before occured because the leakage
current wasn't fast enough to prevent a big negative overvoltage of the
caps, or perhaps one of the caps had a big leak in the wrong direction.

I'd say that in order to replace nonpolarized caps with polarized caps, I'd
use a diode to quickly charge the junction between them up (or down). Point
the diode from the - to the + terminal over one of the caps. Not using the
diode risks an explosion, or at the least degradation of the caps over time
due to reverse voltages for a time while the system charges up.

The other point is that since the caps are in series, the total value is
C1*C1/(C1+C2), so not only do you need two caps, you need twice the value
that the original non-polarized cap had (in addition to the diode). Also,
both of these caps need to have a dc rating at least twice the peak to peak
AC voltage expected.

Regards

 

[Fred Abse]

one of the 100uf
capacitors exploded right in front of me. I've never seen that before!

You must be one of the newer fellas

When I was (much) younger, I and a few of the guys used to amuse
ourselves with a piece of pipe closed at one end, a few electrolytic
capacitors, and the AC supply. We called it the "Cap Gun"