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aniruddha

Polar Capacitor as a Diode?

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Hi everyone,
I have a few conceptual probs. pls hlp.

1- What is the constructional difference between polar and non-polar capacitors?

2- What is the operational(functional) difference between polar and non-polar capacitors?

3- If a polar capacitor is connected with opposite polarities will it act like a diode?

4- Also are there any different theory/formulae/laws governing polar capacitors or they are same as those for non-polar capacitors?

5- Also pls give details on polystrene (excuse the spelling mistake) and electrolytic capacitors.

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But what if a low frequency (eg.  0.5 hz) a.c. vtg is present across it with a negative d.c. shift (less than the amplitude of a.c.) ?
say it gets charged during the positive part and discharged during negative but as duration of -ve will be more after getting completely discharged then what?
will it act as open ckt? or closed switch similar to diode?
i tried simulating but there it charged it in both directions which is obviously not possible.

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If a circuit has bias voltages that causes a capacitor to have reversed polarity during part of the waveform then re-bias the circuit to avoid it, use a non-polarized capacitor or connect two polarized capacitors back-to-back like this:

post-1706-142791432719_thumb.png

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Hello aniruddha

i don't know what you are upto? ::)

i defintely have answer for your thrid Question:

No, it wouldn't act like a Diode, but you'd hear a bang, then your room will be filled with fine mist when that Polarised electrolytic Cap is put in wrongly(polarity)

this is my personal Experience!

why do you want to use Costly Caps as diodes when that job has been taken up by cheap diodes

nura

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aniruddha,
If two, same-value, aluminum electrolytic capacitors are connected in series, back-to-back with the positive terminals or the negative terminals connected, the resulting single capacitor is a non-polar capacitor equal in capacitance to either of the original pair. The two capacitors rectify the applied voltage and act as if they had been bypassed by diodes. When voltage is applied, the correct-polarity capacitor gets the full voltage. On a capacitance meter with no bias voltage the two capacitors measure half capacitance as you would expect from capacitors in series. In non-polar aluminum electrolytic capacitors and motor-start aluminum electrolytic capacitors a second anode foil substitutes for the cathode foil to achieve a non-polar capacitor in a single case.

Here is an article from University of Guelph on making your own high voltage capacitors:
http://www.uoguelph.ca/~antoon/circ/hv/hvcap/hvcap.html
It has the formulas that you asked about.

Here is a fun site that experiments with two sheets of insulated aluminum foil to make a capacitor that you can test in a digital clock circuit:
http://www.doctronics.co.uk/owncap.htm

Care to go a step further? Here is an article on building your own Air Variable Capacitor:
http://www.eham.net/articles/5217

Have fun experimenting!

MP

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Thanks a lot guys it really helped

  thks audioguru , i did search google but getting an accurate answer on google when a question is too specific is a bit difficult.

  thks nura, sharing your experience surely seems to have saved me from getting into trouble at college for trying this out  ;)

  thks Zeppelin, the uogoelph site is really good and has lots of other useful topics too.  :)

  thks MP , i m looking forward to try those tempting experiments. They really appear to be exciting , the practical details mentioned in it are truly very very helpful for an amateur like me. ;D

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Hi aniruddha, what are you up to man, when dealing with polarized caps always keep the polarity in mind and the maximum voltage rating also. ( if connected wrongly or the voltage exceeds the rating, the capacitor can burst and giving off quite a nasty smell). Electrolytic caps have a tendency to dry out with time raising their ESR (Equivalent Series Resistance). An old Electrolytic may not show much drop in its capacitance but the ESR goes up quite fast. To check the ESR you need an ESR meter , a Capacitance meter will not work in this case. If you are simulating your circuits on a PC using softwares like Electronics work bench, Multisim, or circuit maker, never rely totally on the softwares. As per my personal experience these softwares work well for digital circuits but not for the Analog circuits. So stick to the old breadboards to get the best results. ;) 

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