Nonpolarized cap from a polarised one?

[ark1]

Hi,

I've heard two polarized capacitors in series (+ - - +) will form a
nonpolarized capacitor.
Is that ALWAYS true? Specifically, I need a 2uF/250V AC capacitor to be
used as a load on 230V mains. Would it be OK if make it from two
electrolytic caps rated for 400V DC?

Thanks.

 

[Ancient_Hacker]

Hi,

I've heard two polarized capacitors in series (+ - - +) will form a
nonpolarized capacitor.
Is that ALWAYS true? Specifically, I need a 2uF/250V AC capacitor to be
used as a load on 230V mains. Would it be OK if make it from two
electrolytic caps rated for 400V DC?

Thanks.

Yes and no.

You could hook up two 4uF electrolytics back to back and you'll get
something.

But you're depending on the polarized nature of the capacitors to keep
the voltage polarities all kosher. A bit dubious in the long run.

In addition many non-polar applications require a capacitor with low
dissipation factor, which the electrolytics won't be.

For example, a motor-run capacitor has to be low loss, so electrolytics
are likely to overheat within a few minutes.

 

[Spehro Pefhany]

Hi,

I've heard two polarized capacitors in series (+ - - +) will form a
nonpolarized capacitor.
Is that ALWAYS true? Specifically, I need a 2uF/250V AC capacitor to be
used as a load on 230V mains. Would it be OK if make it from two
electrolytic caps rated for 400V DC?

Thanks.

Probably not. You'll likely exceed the ripple current rating of the
caps and blow them up. Use a film capacitor rated for across-the-line
operation. It won't be really cheap or really small.

Best regards,
Spehro Pefhany

 

[Tam/WB2TT]

Hi,

I've heard two polarized capacitors in series (+ - - +) will form a
nonpolarized capacitor.
Is that ALWAYS true? Specifically, I need a 2uF/250V AC capacitor to be
used as a load on 230V mains. Would it be OK if make it from two
electrolytic caps rated for 400V DC?

Thanks.

It will work for a small load, like an antenna rotator. Put a 1n4XXX diode
across each capacitor, so that it is never reverse biased. Since only one
capacitor is in line at any given time, you do not have to double the
capacitor value. I would first look for a non polarized capacitor. 2 uF oil
filled 600V caps used to be common in the surplus market.

Tam

 

[colin]

It will work for a small load, like an antenna rotator. Put a 1n4XXX diode
across each capacitor, so that it is never reverse biased. Since only one
capacitor is in line at any given time, you do not have to double the
capacitor value. I would first look for a non polarized capacitor. 2 uF oil
filled 600V caps used to be common in the surplus market.

If im not mistaken the capacitors will charge up so that the diodes no
longer conduct and therefore you still have 2 capacitors in series (both
biased corectly 99% of the time), so need twice the value.

Without the diodes the capacitors should self rectify so that they both
charge up initialy and once charged only ever being reverse biased just
enough to cancel the leakage current in the oposite direction and so will
stil work ok, however this is not so good for the capacitors so the diodes
might help.

You have to watch the ripple current and temperature rise and consequential
derating due to temp, electrolytic capcitors are often current rated for
smoothing rectified dc so expecting to have a voltage ripple of considerably
less than their rated voltage at 100hz, a full voltage ripple at 50hz might
exceed the ripple current rating.

They can explode quite spectacularly if anything goes wrong and lifetime is
an issue if worked hard, a big voltage safety margin might be a good idea.
ie use at least 500vdc.

Amongst other types already sugested you can get X-rated caps at 1uf
(designed for supresuon accross line) quite easily I dont know how cheaply,
these will undoubtedly be safer and more reliable.

If there is any inductance ie using for motor start you have to watch for
much higher voltages due to resonance.

Colin =^.^=

 

[Tam/WB2TT]

If im not mistaken the capacitors will charge up so that the diodes no
longer conduct and therefore you still have 2 capacitors in series (both
biased corectly 99% of the time), so need twice the value.

Yeah, I think you are right. I was quoting an old wives tale.

Without the diodes the capacitors should self rectify so that they both
charge up initialy and once charged only ever being reverse biased just
enough to cancel the leakage current in the oposite direction and so will
stil work ok, however this is not so good for the capacitors so the diodes
might help.

I ran LTSpice on this. Problem is that it ignores reverse bias on an
electrolytic. So, in my simulation the diodes did prevent the caps from
reverse biasing. Most of the diode current occurred during the first cycle
after turn on. You really don't want to use the caps as diodes.

You have to watch the ripple current and temperature rise and
consequential
derating due to temp, electrolytic capcitors are often current rated for
smoothing rectified dc so expecting to have a voltage ripple of
considerably
less than their rated voltage at 100hz, a full voltage ripple at 50hz
might
exceed the ripple current rating.

They can explode quite spectacularly if anything goes wrong and lifetime
is
an issue if worked hard, a big voltage safety margin might be a good idea.
ie use at least 500vdc.

We once use 20V Tantalums at 18 V. Not a good idea.

Amongst other types already sugested you can get X-rated caps at 1uf
(designed for supresuon accross line) quite easily I dont know how
cheaply,
these will undoubtedly be safer and more reliable.

If there is any inductance ie using for motor start you have to watch for
much higher voltages due to resonance.

Colin =^.^=

He only needs a 2 mfd; couldn't be very expensive to get the real thing. I
am assuming this is a hobby project.

Tam

 

[Christopher]

I've heard two polarized capacitors in series (+ - - +) will form a
nonpolarized capacitor.
Is that ALWAYS true? Specifically, I need a 2uF/250V AC capacitor to be
used as a load on 230V mains. Would it be OK if make it from two
electrolytic caps rated for 400V DC?

I need a 2uF/250V AC

Two of these below in parallel might satisfy your requirement.

Radio Shack

1.0µF 250V 10% Metal-film Capacitor $1.59

Model: 272-1055 | Catalog #: 272-1055

http://www.radioshack.com/search/index.jsp?kwCatId=&kw=capacitor film&origkw=capacitor film


The Radio Shack stores tell me that all of the components are going to
be dropped soon. Then I will retire from being a weekend hacker.
Remember the 1980's when they had all those chips hanging on the
walls. I will miss the "easy access" and the passing of an era.

Darn, I am still getting over loosing Olson Electronics.

Good Luck,

* * *
Christopher

Temecula CA.USA
http://www.oldtemecula.com

 

[Phil Allison]

"Christopher"

Two of these below in parallel might satisfy your requirement.

Radio Shack

1.0µF 250V 10% Metal-film Capacitor $1.59

Model: 272-1055 | Catalog #: 272-1055

http://www.radioshack.com/search/index.jsp?kwCatId=&kw=capacitor film&origkw=capacitor film

** That 250V DC rated cap would probably *blow up* within seconds if
connected across a 230 volt AC mains supply.

Special construction capacitors called "Class X" types are **essential**
for this job.




........ Phil

 

[Ancient_Hacker]

Two of these below in parallel might satisfy your requirement.

Radio Shack

1.0µF 250V 10% Metal-film Capacitor $1.59

Not the best choice. Metal film is only microns-thick-- maybe not so
good at handling large currents.

Try going to Home Depot-- they have ceiling-fan run capacitors in a
variety of sizes. Still not awfully rugged, but somewhat better than
the RS ones.

I too recall Olson Electronics! A tabloid size catalog printed on
cheap newsprint. The very best junk from Japan and Taiwan, before
those places had anything of quality.

 

[Luhan]

I too recall Olson Electronics! A tabloid size catalog printed on
cheap newsprint. The very best junk from Japan and Taiwan, before
those places had anything of quality.

Ever been to their store in Chicago? Conveniently, right accross the
street from a major electronics store (Allied Radio?) that was later
bought out by Tandy - thus Radio Shack.

Luhan

 

[Tam/WB2TT]

I too recall Olson Electronics! A tabloid size catalog printed on
cheap newsprint. The very best junk from Japan and Taiwan, before
those places had anything of quality
.

I recall when I was in college, I ordered a turntable from Olson. Claimed
"Old World Craftsmanship", made in England. It was the worst piece of
plastic junk I ever saw. I think Jameco is still in business.

Tam

 

[[email protected]]

If im not mistaken the capacitors will charge up so that the diodes no
longer conduct and therefore you still have 2 capacitors in series (both
biased corectly 99% of the time), so need twice the value.

Without the diodes the capacitors should self rectify so that they both
charge up initialy and once charged only ever being reverse biased just
enough to cancel the leakage current in the oposite direction and so will
stil work ok, however this is not so good for the capacitors so the diodes
might help.

You have to watch the ripple current and temperature rise and consequential
derating due to temp, electrolytic capcitors are often current rated for
smoothing rectified dc so expecting to have a voltage ripple of considerably
less than their rated voltage at 100hz, a full voltage ripple at 50hz might
exceed the ripple current rating.

They can explode quite spectacularly if anything goes wrong and lifetime is
an issue if worked hard, a big voltage safety margin might be a good idea.
ie use at least 500vdc.

Amongst other types already sugested you can get X-rated caps at 1uf
(designed for supresuon accross line) quite easily I dont know how cheaply,
these will undoubtedly be safer and more reliable.

If there is any inductance ie using for motor start you have to watch for
much higher voltages due to resonance.

Colin =^.^=

"...you can get X-rated caps.."
Can I get R-rated or PG-13 rated caps instead?

 

[Michael A. Terrell]

Olsen closed most or all of their stores in the early '80s. I went to
the store in Cincinnati, Ohio a few days before it closed.

Ever been to their store in Chicago? Conveniently, right across the
street from a major electronics store (Allied Radio?) that was later
bought out by Tandy - thus Radio Shack.

Luhan

Yes, Tandy bought Allied, then the feds forced them to sell it off.


--
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

 

[Ross Herbert]

Hi,

I've heard two polarized capacitors in series (+ - - +) will form a
nonpolarized capacitor.
Is that ALWAYS true? Specifically, I need a 2uF/250V AC capacitor to be
used as a load on 230V mains. Would it be OK if make it from two
electrolytic caps rated for 400V DC?

Thanks.

When dealing with components for direct application to 230 - 240Vac
you don't muck around trying to fabricate capacitors from polarised
electro's. Too much can go wrong, often with catastrophic results.

Get capacitors rated for the application. I would suggest an
Arcotronics C24 box series (watch url wrap)
http://www.arcotronics.com/cms/index.php?option=content&task=view&id=52&Itemid=48

A 4uF unit measures 42.5L x 37H x 28W and is rated for
10,000hrs@400Vac and it will be much more compact than any cobbled
together unit using electro's.