Happy New Year!

Searching on the forum, there are very few threads about polarized vs non-polarized caps,
and none that talks about their cost (that I could find)...so here we go...

Recently I was building a little board to play with a generalized 555 timer.
Two pots - 100K and 1meg - and a row of capacitors from .0001 uF to 220 uF -
all connected to dip switches so I can easily change the capacitance.
It allows me to put the 555 in an infinite number of "states" and watch LEDs blink off and on.

While I was soldering the caps, I noted on a 1 uF that there was now "white line" that I had to line up with the negative side of the circuit.
I also noticed an "NP", and decided to do some research.
(I'm sure I just hit the Jameco "buy" button based on the value I needed)

Now I know that there are "polarized" and "non-polalrized" (NP) electrolytic capacitors - duh.

I searched a bit more and read things about using the NPs in AC circuits, and that the NPs cost more.
More research.

Here are some data from Jameco:
Capacitance___________Cost NP____________Cost Polarized
1 uF___________________.12__________________.08
3.3 uF_________________.10__________________.15
4.7 uF_________________.06__________________.09

So I wonder about the following:

a. There seem to be fewer values of non-polarized caps.
b. For a hobbiest, there is no great difference in price between polarized and non-polarized electrolytics
c. For "high" value capacitance in an AC circuit, you have to use an NP cap.
d. If you can find an NP cap in the value you need, and it's not too expensive, use it over a polarized cap.

Are these correct?
Do you have any other thoughts on the difference between NP and P caps?
How do you use them?

 

Look at this:
http://en.wikipedia.org/wiki/Electrolytic_capacitor

 

All of your assumptions are incorrect, except a.

What is more important is the application of the capacitor and the circuit in which it is to be used.

The first criteria is the value of the capacitance.
Capacitors under 1μF are generally non-polarized.
For larger values, non-polarized caps tend to be physically large.
Polarized electrolytic caps are physically smaller.

If the application is in a DC circuit such as to smooth or filter the power supply voltage, use an electrolytic capacitor.

The control signal in a 555 timer circuit is varying DC and an electrolytic cap would work but a non-electrolytic NP cap would be preferred.

If the signal current or voltage reverses direction, you want to use a NP cap, examples, interstage signal coupling, horizontal sweep circuit, speaker cross-over.

The second criteria is the operating voltage.
The third criteria is the physical size, construction, package.
Other criteria are type of material - ceramic, mylar, polysester, aluminum, tantalum.
Finally, one should consider ESR (Electrical Series Resistance) when choosing caps > 1μF for critical filter circuit applications.

Price is not usually a deciding factor.
(I am referring to capacitances under, say, 1000μF. When you get into higher values >1000μF, higher voltages >100V, or supercaps in the >1F values, price may become a factor.)

 

You can also roll your own non-polarized electrolytic thusly...

 

Look at this:
http://en.wikipedia.org/wiki/Electrolytic_capacitor

Ah yes, I saw that. Thanks for the reminder!

All of your assumptions are incorrect, except a.

What is more important is the application of the capacitor and the circuit in which it is to be used.

The first criteria is the value of the capacitance.
Capacitors under 1μF are generally non-polarized.
For larger values, non-polarized caps tend to be physically large.
Polarized electrolytic caps are physically smaller.

If the application is in a DC circuit such as to smooth or filter the power supply voltage, use an electrolytic capacitor.

The control signal in a 555 timer circuit is varying DC and an electrolytic cap would work but a non-electrolytic NP cap would be preferred.

If the signal current or voltage reverses direction, you want to use a NP cap, examples, interstage signal coupling, horizontal sweep circuit, speaker cross-over.

The second criteria is the operating voltage.
The third criteria is the physical size, construction, package.
Other criteria are type of material - ceramic, mylar, polysester, aluminum, tantalum.
Finally, one should consider ESR (Electrical Series Resistance) when choosing caps > 1μF for critical filter circuit applications.

Price is not usually a deciding factor.
(I am referring to capacitances under, say, 1000μF. When you get into higher values >1000μF, higher voltages >100V, or supercaps in the >1F values, price may become a factor.)

I love "RULES" and "PEARLS" - thanks very much as usual.

You can also roll your own non-polarized electrolytic thusly...

An interesting option - I'll "roll my own" in future!

Thanks for all the excellent comments.

 

You can also roll your own non-polarized electrolytic thusly...

Is is necessary that the value of both caps be equal?