Author Topic: Converting polar Electrolytic cap to NP - who is correct?  (Read 6938 times)

bluesdevil

Converting polar Electrolytic cap to NP - who is correct?
« on: January 26, 2007, 07:25:00 PM »
I did a search on converting polarized electrolytic capacitors to non-polar, but get conflicting info.

The DIY FAQ states:
"You can simulate a non-polarized by using 2 electrolytic caps. Connect them together negative to negative and use the positive leads as the component leads; which is probably why someone notated it as +-||--||-+ Each of the capacitors should be double the value that you need for the circuit because of the series capacitance formula:Ctotal= 1 / (1/C1)+(1/C2)"

R.G. sez in a post:
"You can make a quick and dirty NP cap by tying together the negatives of two equal-sized polar caps.

In the series-NP connection, the capacitance value is funny. Normally caps in series are a smaller capacitance than either cap by itself. If you had two 3.3uF polyester caps, then the expected value for two of them in series is 1.65uF. However, electrolytic caps actually conduct in the reverse direction, so two 3.3uF polarized aluminum electrolytic caps act like they each have a diode in parallel with them that conducts when the voltage is backwards for that one cap. So two 3.3uF caps hooked up as series non polar (i.e. negative to negative) look like a single 3.3uF NP cap.

... except for tiny region near zero volts where they withstand a tiny reverse voltage, so they look like 1.65uF there..."

So will it cut the value in half putting 'em in series (negs connected) or remain the same value of each cap?
"I like the box caps because when I'm done populating the board it looks like a little city....and I'm the Mayor!" - armdnrdy

R.G.

Re: Converting polar Electrolytic cap to NP - who is correct?
« Reply #1 on: January 26, 2007, 07:40:35 PM »
I am correct.

An aluminum electro cap conducts in the reverse direction, something like a diode, but with bad diode characteristics. The resistance of the side that is properly polarized is much much more than the reverse biased half, so the charge inside apportions itself across where it's blocked - the properly biased side. So only the properly biased side is acting as a cap, and there's only one cap's worth of capacitance there.

There is a small region around 0V where both are acting like caps but it's best avoided.

Even better, use my rule of thumb: never, ever rely on an aluminum or tantalum electrolytic cap to have a specific value. Use them only where you can make a case for "OK, if I just get a big enough capacitor..." and never where the exact capacitance matters. This is because even if the capacitance is correct today, it will change as the cap ages. And the tolerances on electros is generally bad. As bad as -20% +80%.

If you designed your circuit well, the electro caps are bigger than is absolutely needed, so make each one of the polarized caps be the value of the NP cap you want, and the circuit will work.

If you relied on the value of an NP cap for specific values, like a rolloff frequency - well, may Mother Nature have mercy on you.
R.G.

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aron

Re: Converting polar Electrolytic cap to NP - who is correct?
« Reply #2 on: January 26, 2007, 07:46:58 PM »
Thank you. I have updated the FAQ

bluesdevil

Re: Converting polar Electrolytic cap to NP - who is correct?
« Reply #3 on: January 26, 2007, 08:00:54 PM »
Wow R.G., a helluva big thanks for that quick response!!!  Please don't be insulted or offended, but have you ever actually measured the results with a meter or is your statement purely theory? I just realized after posting I have a "CAP" mode on my tester and I'm getting the half reading with two 4.7uf tantalums..... or maybe my multi-meter  tests at the "near zero" voltage you mentioned?
      Like you said the tolerances are rather loose and it'll probably be fine either way, just trying to get to the bottom of it all.
"I like the box caps because when I'm done populating the board it looks like a little city....and I'm the Mayor!" - armdnrdy

R.G.

Re: Converting polar Electrolytic cap to NP - who is correct?
« Reply #4 on: January 26, 2007, 08:09:17 PM »
Tantalum will withstand a reverse voltage of a few volts before the leakage comes up on the reverse biased side. So with tantalum the "funny area in the middle" is much bigger, maybe as much as 4-5V depending on the cap. Also, most DMM testers test with a small voltage across the cap so they give better answers when the caps are tested in-circuit. You're probably getting both effects.

In your conditions, I would expect just the results you're getting.
R.G.

Quick IQ Test: If anyone in a governmental position suspected that YOU had top-secret information on YOUR computer, how many minutes would you remain outside a jail cell?

bluesdevil

Re: Converting polar Electrolytic cap to NP - who is correct?
« Reply #5 on: January 26, 2007, 08:30:40 PM »
Thanks again. I'm getting the same results with electrolytics, but could just because I'm testing the caps out of circuit, as you mentioned. What I'll do is go bigger in value and then test again when it's powered up in the circuit.
       I'm trying to get a non-polar 2.2uf  that will be used to connect a 4 diode bridge arrangement(?) to vbias. It's for the Moosapotamus A/DA Flanger clone.
« Last Edit: January 26, 2007, 08:38:33 PM by bluesdevil »
"I like the box caps because when I'm done populating the board it looks like a little city....and I'm the Mayor!" - armdnrdy

chejkal

Re: Converting polar Electrolytic cap to NP - who is correct?
« Reply #6 on: March 09, 2007, 01:28:13 AM »
For those cases I need an NP electro, I usually use a pair, reverse, with diodes across each to pass the reversed voltage.
usually a 1n4007 ( well you can use a lower number as long as the voltage isn't exceeded, but the 1n4007 is CHEAP and
that way I only carry one item in my already overflowing junque box  ;).

chuck

Minion

Re: Converting polar Electrolytic cap to NP - who is correct?
« Reply #7 on: March 09, 2007, 01:40:22 PM »
If all you need is 2.2uF NP then why not simple use a 2.2uF Mylar or metal Film or simular Cap?? or even two 1uF caps in Paralell??



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Mark Hammer

Re: Converting polar Electrolytic cap to NP - who is correct?
« Reply #8 on: March 09, 2007, 04:59:47 PM »
If all you need is 2.2uF NP then why not simple use a 2.2uF Mylar or metal Film or simular Cap?? or even two 1uF caps in Parallel??

In theory, you are quite right, but there is a point at which a pair of back-to-back polarized caps will be both much smaller and much cheaper than their non-polarized functional equivalent.  For instance, I can recall a Boss schematic where the LFO has a pair of 33uf caps back to back.  The odds of ANY nonpolarized component/s taking up anything less than 3-4 times the equivalent space is small and the odds of it being anything less than 3-4 times the price of two electrolytics is similarly small.

In the other hand, very often you CAN get a couple .27uf  NPs to stick in parallel (or a .27 and .33) for only a wee bit more than the space and cost of a series pair of 1uf polarized.  It all depends on which value in particular.  There are certain minimum usable sizes when it comes to through-hole parts.  Some components aren't much bigger than that, and some need to be a lot bigger.