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AC "bipolar" capacitor

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orangelearner

New Member
Hi there. At my school we have a bunch of electronics supplies, and I basically want to make this:

Electric motors and generators

I'm thinking about using four 400-loop solenoids to achieve the inductor part, but the problem is the capacitor part. My physics teacher only has polarized capacitors, and I heard that if you put one of those in an AC circuit, it will explode. I tried to make some AC capacitors by myself, but my aluminum foil one only achieved 10 nF, and the Leyden jar didn't seem to work at all.

I also heard that you can make a "bipolar" capacitor by sticking two polar ones back to back, but I also heard that it involves a special diode part as well. Is this true? Before I actually hook up two $15+ capacitors together in AC, I'd like to make sure that they won't blow up.
 

dougy83

Well-Known Member
That's a pretty nice page. As for your capacitors, you can put a diode across each. The cathodes connect to the positive capacitor terminal, the anode to the negative terminal for each capacitor. The diode stops the capacitor from being reverse-charged more than about 0.7V, which is not so bad for it.
 
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tcmtech

Banned
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You could look around for an old furnace blower fan motor or an old air conditioner. They have AC capacitors on them.
 

Artificer

New Member
What you are looking for is called a "Non Polarized Electrolytic".

They are most frequently seen in the start (and secondary "run") windings of single phase motors. Look to an electrical supply house that markets to HVAC techs. Common values run from 35uF to 750uF. Price will vary upward proportional to operating voltage. Physical size will be quite large compared to "normal" caps. Normally in a metal can......

Made specifically for phase shift applications against large inductive loads. Also quite useful across "residential" mains for power factor correction.

Beware of very high local currents in an L-C tank circuit

Your header doesn't say where you are. If in the States, try WWGrainger or Johnstone Supply, both chain stores. They don't sell to the public but you might lay the student bit on them and get them to go along. I'm sure there are similar houses most any place.
 

orangelearner

New Member
That's a pretty nice page. As for your capacitors, you can put a diode across each. The cathodes connect to the positive capacitor terminal, the anode to the negative terminal for each capacitor. The diode stops the capacitor from being reverse-charged more than about 0.7V, which is not so bad for it.

AHH, now I see...that's really cool! It's like completing the circuit only one way...man....I knew there was a missing piece. So this is what you meant right? (image below)

9323-bipolar.png
 

RCinFLA

Well-Known Member
That will work but beware that electrolytic caps do not like ripple voltage for long period of time. These types of back to back electrolytics are used only for starting period in real use motor. An oil filled capacitor is used for a run capacitor in real use motors.
Like these; 370 VAC Single Oval Capacitors

If all you are looking to doing is proof of concept then you can use the back to back electrolytics for short run time.
 

tcmtech

Banned
Most Helpful Member
If you stop by one of your local heating and air conditioning repair places or local contractors shop and tell them what your doing there is a good chance you may get yourself a collection of assorted sizes of actual AC capacitors for free!

Most have a bunch of take offs and old units lying around some place and if you talk to the right person you probably can get a few just for explaining what your doing with them.

I know if you came here I would give you a box of assorted used capacitors just to see that they are being used for someone to learn something from. :)
 
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