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Capacitor symbol question.....

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mech3d

New Member
What is the difference between C8 and C9? I know C9 is a polarized cap but what about C8? Sometimes I see a cap symbol as 2 straight parallel lines. Is that also the same thing as C8?
 

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BrownOut

Banned
C8 is drawn as a polarized cap, just doesn't have the "+" on it. However, I'm not sure C8 should be polarized for this application.
 
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mech3d

New Member
Thanks for the quick reply. This is part of a schematic for an FM transmitter. There are a bunch of caps drawn like C8 in fact all but 4 out of 22.

I just took a look at the parts list, the caps with the plus sign is specified as Tant caps w/polarity and all the other cap are as monolithic caps.
 

Boncuk

New Member
C8 is a nonpolarized cap.

There is no such thing as a 1nF (0.001µF) electroytic cap.

There was a thread containing ANSI and IEE symbols for electronic components.

I don't remember the headline of it. May be another member remembers what it is.

Boncuk
 
Usually the side with the straight line denotes the positive leg (when there is a straight line and a curved line), but in this case, its nonpolarized
 

mech3d

New Member
Thanks again for such a quick reply.

It seems then that I need to be careful only with the 4 Tant caps with regards to polarity.
 

Dean Huster

Well-Known Member
There are quite a few different schematic symbols for capacitors, some geographically mandated (Euro cap symbols differ or have differed from U.S. symbols), some according to year, others according to company drawing standards. In 1944, caps were drawn as two straight lines. The curve of one plate appeared during the late 1950s, early 1960s. The curved side usually meant negative lead for electrolytics or "outside foil" for axial-lead paper or plastic dielectric caps.

The smallest electrolytic (polarized) capacitor that I recall seeing is a little 0.47µF (and there may have been a 0.1µF) tantalum that I have in my stock. If you compare the same value electrolytic against a non-polarized version, you'll find a LOT less series inductance and ESR on the electrolytic, which could be critical in some applications.
 

Mikebits

Well-Known Member
There are quite a few different schematic symbols for capacitors, some geographically mandated (Euro cap symbols differ or have differed from U.S. symbols), some according to year, others according to company drawing standards. In 1944, caps were drawn as two straight lines. The curve of one plate appeared during the late 1950s, early 1960s. The curved side usually meant negative lead for electrolytics or "outside foil" for axial-lead paper or plastic dielectric caps.

The smallest electrolytic (polarized) capacitor that I recall seeing is a little 0.47µF (and there may have been a 0.1µF) tantalum that I have in my stock. If you compare the same value electrolytic against a non-polarized version, you'll find a LOT less series inductance and ESR on the electrolytic, which could be critical in some applications.

Nice history lesson. Good info :)
 

mech3d

New Member
I had another schematic of a mixer with the two difference symbols for the caps and I used polarized electrolytic in all places where I positoned the cap with the positive at the straight line & the negative at the curved and the mixer worked great.

Was I just lucky?
 

audioguru

Well-Known Member
Most Helpful Member
Many audio circuits need non-polarized capacitors.

I have seen many Japanese audio circuits that use non-polarized electrolytic capacitors.
 

MikeMl

Well-Known Member
Most Helpful Member
Can you make a non-polarized capacitor by connecting two higher capacitance polarized capacitors as in ---|(---)|--- ?
 
Just my two pen'worth.

I thought that 'polarised' capacitors were formed by a reversable chemical reaction on plates that are actually 'touching' each other except for the tiny insulating layer formed by the chemical reaction due to the applied voltage and the electrolyte. So this is where the 'electrolytic' name comes from.

Since the plates are so close the capacity can be made very high per unit volume.

So, as long as you keep the polarity correct the layer seperating the plate will not 'unmake' itself and cause a short. When used as a reservoir the polarity is very obviously always one way round all the time.

If you apply AC then the situation would become less clear. If the AC is superimposed on a bias voltage (ie coupling between stages) then (provided you have connected it the right way round) the capacitor will 'see' a correct polarity all the time and not deteriorate (even though the applied voltage is varying)

My guess at the reverse series connection question is that 1. the potential distribution would depend on the quite possibly different esr that the caps would have when forward and reverse connected and 2. the one that was reversed (could be both depending on what you are doing with it) would eventually fail, the time taken depending one the amount of voltage it was getting.

Putting an electrolytic on a pure AC supply (the mains for example) would probably 'confuse' the chemicals internally while they form and reform. I suspect that the insulating layer would eventually turn to slush and short out resulting in a loud bang and a smell of rotting cabbages.

An intersesting point from a vintage Radio forum I'm on is that they will often 'reform' electrolytics on radios that have been in the attic for years before applying full power - the point here is that the insulating layer may deteriorate over time but can be restored by applying a suitable potential for a while.

This is all guesswork based on some prinicples - but it sounds right to me!!

James
 

BrownOut

Banned
I was once told by an older, more knowledgable engineer that a non-polarized cap can be made by connecting two electrolytics back to back ( minus lead to minus lead ), however, the elctrolytics must have first been "made" by connecting them to a DC voltage that is within 25% of their rated voltage. In light of the above post, I think that might not be the best idea.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
I was once told by an older, more knowledgable engineer that a non-polarized cap can be made by connecting two electrolytics back to back ( minus lead to minus lead ), however, the elctrolytics must have first been "made" by connecting them to a DC voltage that is within 25% of their rated voltage. In light of the above post, I think that might not be the best idea.

I think he was been overly pessimistic, possibly due to using electrolytics pre-WWII? - there's no problems connecting them back to back.
 
I can't quite see how connecting electrolytic capacitors back to back can be acceptable or OK.

Surely one of the capacitors must be incorrectly polarised and so would be in exactly the same position as if it were a single capacitor incorrectly biassed. The only way this wouldn't be the case would be if the reversed capacitor were a dead short or somehow generated a reverse voltage, I suspect that neither is the case.

The only way that I can see it working is if the incorrectly biassed one somehow looks like a very low impedance for some chemical reason and only not gassing to distruction because the other capacitor is limiting the current flow through it.

So the reason why the capacitor doesn't fail is not because it is 'OK', but because you are 'just lucky' that some other effect has occurred. Personally I'd rather keep within the manufacturers recommended operating conditions.

James
 

Nigel Goodwin

Super Moderator
Most Helpful Member
I can't quite see how connecting electrolytic capacitors back to back can be acceptable or OK.

Surely one of the capacitors must be incorrectly polarised and so would be in exactly the same position as if it were a single capacitor incorrectly biassed. The only way this wouldn't be the case would be if the reversed capacitor were a dead short or somehow generated a reverse voltage, I suspect that neither is the case.

The only way that I can see it working is if the incorrectly biassed one somehow looks like a very low impedance for some chemical reason and only not gassing to distruction because the other capacitor is limiting the current flow through it.

So the reason why the capacitor doesn't fail is not because it is 'OK', but because you are 'just lucky' that some other effect has occurred. Personally I'd rather keep within the manufacturers recommended operating conditions.


It's been done this way commercially for decades, and it's always been said that commercial 'non-polarised' electrolytics are simply two polarised capactitors back to back in a single casing (although I've no actual knowledge of that). If they could make actual non-polarised electrlytics, why would they bother making polarised ones?.
 

Dean Huster

Well-Known Member
Well, that means that if I need to replace my 240v induction motor's starting capacitor, I can just use two electrolytics back to back. Anyone have the gonads to try that one and stay alongside the motor to see how well it works?

Dean
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Well, that means that if I need to replace my 240v induction motor's starting capacitor, I can just use two electrolytics back to back. Anyone have the gonads to try that one and stay alongside the motor to see how well it works?

As a motor start capacitor isn't a small non-polarised electrolytic, I wouldn't like to try it :p
 
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