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I've found myself a number of time trying to build a circuit from a schematics that does not provide any polarity indication for caps (probably because the designer used non-polarized caps in the first place), and all I have in my junk box is a bunch of polarized electrolytics with the same value. This typically happens for 1μF's.

The only way I've found so far to figure out how to place the polcap on my circuit is to simulate it with LTSpice and verify what the voltage at the input of the caps looks like.

I was wondering if there was perchance a simple way/trick to do this ?

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  • \$\begingroup\$ The trick is to get a bunch of 1 uF ceramic caps. \$\endgroup\$ Feb 28 '15 at 13:43
  • \$\begingroup\$ Electrolytic capacitors typically have a very wide range, which might cause problems. \$\endgroup\$ Feb 28 '15 at 14:00
  • \$\begingroup\$ You can usually choose polarity just by simple inspection of the circuit. \$\endgroup\$ Feb 28 '15 at 16:36
  • \$\begingroup\$ +Dwayne Reid: true, there are a number of times where the answer is obvious, e.g. one side of the cap is grounded. But there are times where the answer isn't necessarily obvious. I'm a beginner, often trying to build circuits just to understand how they work, and conducting a full pen and paper circuit analysis prior to actually building it is unfortunately still out of my reach. On the other hand, when I build to understand it, I'd rather it didn't blow up in my face :) That's why I was hoping there was some sort of simple trick. But from Olin's comment, I guess I'm out of luck :). \$\endgroup\$ Feb 28 '15 at 16:40
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Capacitors having the same capacity are not all the same. If a designer uses a non polarized cap, e.g. polyester or ceramic, usually there is a reason for it. You cannot usually replace a 1μF polyester cap with a 1μF electrolytic one without consequences. There are many more factors involved in non-trivial circuits that mandate a different technology for caps. For example, electrolytic caps can have rather large ESR (equivalent series resistance), in other words they act as a cap with a resistor in series, and this could alter (for example) the bandwidth of an amplifier or the switching time of some driver circuit. Electrolytic caps are more prone to failure due to thermal stress: they don't like to live near heat sources and they may fail miserably if, let's say, they are put near another component that gets too hot (some electrolytic caps are rated for 80°C max, and some power circuits may get hotter than this in some places).

EDIT

To be clearer about that. Substituting a cap type with another should be done knowing at least some basics about the different technologies and their advantages and drawbacks. As a more evident example of this, consider that in some schematics you'll find two caps in parallel across a power rail and ground, one could be (say) 100μF electrolytic and the other 100nF polyester. A 3 order of magnitude difference! Knowing only that two caps in parallel are equivalent to a single one having a capacity that is the sum of their capacity would lead to a puzzling question: "why the 100nF one is needed, since its capacity is swamped by the other one?". The answer is: the bigger one is there to store energy, helping leveling out voltage dips due to peaks in current demand from powered circuits, the polyester one is there because the ESR of the electrolytic prevents it to be a real short circuit for high frequencies, therefore high frequency components that may reach the rail are shorted to ground much better by the 100nF.

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  • \$\begingroup\$ Sorry to disagree, but you are correct only part of the time. Many times, someone will use ceramic capacitors because they are becoming so ubiquitous but where a polarized cap would have worked just fine. In other words, it depends on the circuit. Because no circuit was shown, it is not reasonable to assume that one should not substitute capacitors. \$\endgroup\$ Feb 28 '15 at 16:08
  • \$\begingroup\$ @DwayneReid Please, reread my post. I didn't say that one shouldn't ever substitute caps or that non-polarized are always better than polarized ones (i.e. high-capacity ceramics can exhibit microphonic effects or have big tempco). I just told the OP, since he/she seemed totally unaware that different types of caps have different characteristics, beside capacity value. Then I stressed some disadvantages of electrolytic caps because the OP seemed to have the inclination of use them indiscriminately. Sorry if I wasn't sufficiently clear about it in my post. \$\endgroup\$ Feb 28 '15 at 17:28

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