Nonpolarized cap from a polarised one?

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.

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

--
"it\'s the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

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

oil

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 =^.^=

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

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.

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

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

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

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

"Christopher"

** 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

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.

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

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

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

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.

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

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)

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.