Can someone please explain to me the function of a bipolar capacitor?

Discussion in 'General Electronics Chat' started by boatsman, Jul 24, 2015.

1. boatsman Thread Starter Senior Member

Jan 17, 2008
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Can someone explain to me the function of a bipolar capacitor? I have a model railway circuit which has a bipolar capacitor in parallel to the output to the rails, i.e. between the positive to the 12vdc motor and the ground. Can I use an ordinary capacitor? The component is listed as 10mfd 25v bipolar (Digikey P1125-ND)

2. MikeML AAC Fanatic!

Oct 2, 2009
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Is the voltage to the tracks (or motor; I can't tell from your description where the capacitor is connected) ever reversed? If so, then you need a "bipolar" capacitor. Bipolar implies that the voltage across the capacitor terminals can be +- or -+.

How does the locomotive run in reverse?

3. #12 Expert

Nov 30, 2010
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First you need to know what the function of a polarized capacitor is. It's a capacitor that can only survive one polarity of voltage. Now it seems obvious that a bipolar capacitor can survive voltage in both directions. So, how DOES a train run backwards? Reverse the voltage on the tracks?

Jul 18, 2013
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A little into the reason, electrolytics are compact compared to most other forms of dielectric materials, but they 'suffer' from being unipolar, to make the equivalent size value in other materials results in an often impractically large size.
In order to use them in bipolar applications, they can be coupled back to back, polarity wise, or manuf this way in one case for convenience.
Max.

5. boatsman Thread Starter Senior Member

Jan 17, 2008
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391
Thanks MikeML for your reply. Yes the voltage is reversable . The output to the track is from a pair of Darlingtons acting as a push/pull controlled by a potentiometer so one direction of the potentiometer activates the pnp Darlington and the other direction activates the npn. Is it possible to just use an ordinary non-electrolytic capacitor for this function? I have a schematic that I downloaded but I don't know how to post it in this forum.

Apr 5, 2008
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Hello,

You can upload files using the "Upload a File" button under the edit box.

Bertus

7. #12 Expert

Nov 30, 2010
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It's possible, but I just did a search and all I got was mislabeled parts and \$32 for a bunch of multi-section caps that were mislabeled.

Try Mouser, digikey, jameco, allied for strange, expensive parts.

8. alfacliff Well-Known Member

Dec 13, 2013
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if you check your rail to rail voltage, it reverfses when you reverse the engine. a polarized cap wouldnt like that. we had here a guy that put a bunch of electrolytics across a plating rectifier, when the shop reversed polarity to deplate the hangers, BOOM> an 18,000 amp plating rectifyer can do that.

Jul 18, 2013
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A 10μf in any other type than electrolytic will be fairly large, the alternative if nothing found is to use two 10μf electrolytic in series back to back polarity.
Max.

10. crutschow Expert

Mar 14, 2008
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Two normal electrolytic (polarized) 20μF, 25V capacitors in series back-to-back (negative-to-negative) will work as a substitute.

Edit: Max beat me to it.
Two 10μF in series are likely adequate, since the reverse biased capacitor generally conducts like a diode, but using 20μF adds some margin.

11. boatsman Thread Starter Senior Member

Jan 17, 2008
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Jul 18, 2013
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Looks like a no longer good number for Digikey.
You can either use the two back to back or look for one in a non polarized package if not, just a bit larger if the physical dimensions are not an issue.
Plenty to chose from at Digikey.
Max.

13. ian field Distinguished Member

Oct 27, 2012
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What I would refer to as a non-polarised electrolytic capacitor often has "BP" printed on it - I would assume that is what you mean.

Its basically an electrolytic with 2 anodes. They tend to be bigger for a given voltage and capacitance, and some of the other parameters are also not optimum.

Metalised foil capacitors that big are comparatively rare - but not impossible to find.

Multilayer ceramic chip capacitors with that capacitance are less rare, but they're tiny and usually SMD - I'd worry about the component overheating if it had to handle a lot of hash from motor brushes.

Another possibility is an inverse series pair of polarised electrolytics, each needs a parallel diode to prevent it ever being charged in reverse - with the diodes, series connection halving the capacitance doesn't apply. I'd put at least 0.1uF non-electrolytic in parallel with the series pair.

Jul 18, 2013
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15. djsfantasi AAC Fanatic!

Apr 11, 2010
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boatsman, what scale are you working in? How much space do you have for this capacitor? The one Max linked to is about 1" x 1.25"x 0.6" in size. That is a tight squeeze in an HO locomotive... if the capacitor is supposed to be mounted close to the motor.

Jan 17, 2008
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17. boatsman Thread Starter Senior Member

Jan 17, 2008
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I have several new electrolytic capacitors and quite a few removed from old boards. All of them are several years old. What is the shelf life of electrolytic capacitors? What would happen if I were to use old capacitors? Would they cause a short or just not conduct? Is there any simple way of testing them. I only have a multimeter and no access to an oscilloscope.

18. boatsman Thread Starter Senior Member

Jan 17, 2008
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I'm working with N scale and I am not touching the locomotives. The circuit shown is for the external controller feeding power to the rails.

19. MikeML AAC Fanatic!

Oct 2, 2009
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The whole idea of using the bipolar capacitor is to put it inside the locomotive. That way it stores the energy required to keep the motor running across dirty sections of track... Putting it across the track does nothing...

djsfantasi likes this.
20. boatsman Thread Starter Senior Member

Jan 17, 2008
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So why is it indicated in the circuit above that I uploaded?