electro-music.com   Dedicated to experimental electro-acoustic
and electronic music
 
    Front Page  |  Radio
 |  Media  |  Forum  |  Wiki  |  Links
Forum with support of Syndicator RSS
 FAQFAQ   CalendarCalendar   SearchSearch   MemberlistMemberlist   UsergroupsUsergroups   LinksLinks
 RegisterRegister   ProfileProfile   Log in to check your private messagesLog in to check your private messages   Log inLog in  Chat RoomChat Room 
 Forum index » DIY Hardware and Software » Developers' Corner
Back-to-Back Nonpolarized Caps
Post new topic   Reply to topic Moderators: DrJustice
Page 1 of 1 [6 Posts]
View unread posts
View new posts in the last week
Mark the topic unread :: View previous topic :: View next topic
Author Message
Inventor
Stream Operator


Joined: Oct 13, 2007
Posts: 6221
Location: near Austin, Tx, USA
Audio files: 267

PostPosted: Sun Sep 13, 2009 7:46 am    Post subject: Back-to-Back Nonpolarized Caps
Subject description: How do they work?
Reply with quote  Mark this post and the followings unread

We all know about that trick of putting two electrolytic caps in series, with their negative terminals tied together. This makes a non-polarized cap of half the value. The quesion is: why does it work?

I've asked around in prior workplace(s) about this and nobody seems to know why it works, and why half the value. What's your guess?

Les

_________________
"Let's make noise for peace." - Kijjaz
Back to top
View user's profile Send private message Send e-mail
blue hell
Site Admin


Joined: Apr 03, 2004
Posts: 24079
Location: The Netherlands, Enschede
Audio files: 278
G2 patch files: 320

PostPosted: Sun Sep 13, 2009 8:27 am    Post subject: Reply with quote  Mark this post and the followings unread

Half the value is expected for two caps in series, but why it becomes unpolarized was never clear to me either.
_________________
Jan
also .. could someone please turn down the thermostat a bit.
Posted Image, might have been reduced in size. Click Image to view fullscreen.
Back to top
View user's profile Send private message Visit poster's website
DrJustice



Joined: Sep 13, 2004
Posts: 2114
Location: Morokulien
Audio files: 4

PostPosted: Sun Sep 13, 2009 7:09 pm    Post subject: Reply with quote  Mark this post and the followings unread

The only real attempt at explaining this that I could find is in this discussion, 8th posting down :

"It *IS* OK to put two polarized caps back-to-back to make an AC
cap out of them. The reason is that there is a tiny bit more leakage
in the reverse biased cap. That leakage charges the "properly" biased
cap in the right direction to keep it happy when the current reverses.
Since the circuit is symetrical, each cap's leakage protects the
other.

Do a test with real capacitors (or even a SPICE simulation). You
will find that after only a couple of cycles that each cap will have a
DC voltage across it that is of a magnitude and polarity such that it
will be completely protected from reverse voltages."

Better read the following messages too.

BTW, both common + or - will work.

BTW(2), if it's for audio decoupling, designers commonly use a single electrolytic cap and get away with it even in expensive eqiupment. This has never seemed quite right to me...

DJ
--
Back to top
View user's profile Send private message Visit poster's website
Uncle Krunkus
Moderator


Joined: Jul 11, 2005
Posts: 4761
Location: Sydney, Australia
Audio files: 52
G2 patch files: 1

PostPosted: Sun Sep 13, 2009 10:24 pm    Post subject: Reply with quote  Mark this post and the followings unread

Nice bit of research DJ Wink
_________________
What makes a space ours, is what we put there, and what we do there.
Back to top
View user's profile Send private message Visit poster's website
daverj



Joined: Jul 07, 2009
Posts: 28
Location: USA

PostPosted: Mon Sep 14, 2009 10:58 am    Post subject: Reply with quote  Mark this post and the followings unread

My understanding was that this is reliable with tantalum caps only, but that with aluminum caps you need to put a bias resistor on the point where the positive nodes meet.

At least that's what I was told, many many moons ago.
Back to top
View user's profile Send private message
DrJustice



Joined: Sep 13, 2004
Posts: 2114
Location: Morokulien
Audio files: 4

PostPosted: Mon Sep 14, 2009 12:15 pm    Post subject: Reply with quote  Mark this post and the followings unread

Well, some say that tantalum caps should never be used in this application. However, e.g. Kemet mentions this a perfectly fine application in one of their applications notes. They also mention that they sell prepackaged back-to-back pairs of tantalum caps for convenience. The same goes for aluminium electrolyte caps. That's good enough for me... but I'd rather spend on some non polar, non electrolytic caps (e.g. polypropylene) for audio AC coupling to max out the quality.

DJ
--
Back to top
View user's profile Send private message Visit poster's website
Display posts from previous:   
Post new topic   Reply to topic Moderators: DrJustice
Page 1 of 1 [6 Posts]
View unread posts
View new posts in the last week
Mark the topic unread :: View previous topic :: View next topic
 Forum index » DIY Hardware and Software » Developers' Corner
Jump to:  

You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum
You cannot attach files in this forum
You can download files in this forum


Forum with support of Syndicator RSS
Powered by phpBB © 2001, 2005 phpBB Group
Copyright © 2003 through 2009 by electro-music.com - Conditions Of Use