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 Forum index » DIY Hardware and Software » MusicFromOuterSpace.com designs by Ray Wilson
Anybody know where to get 2.2M pots?
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fabreeze



Joined: Mar 13, 2006
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Location: Olympia, WA

PostPosted: Sun Aug 27, 2006 12:41 pm    Post subject: 2.2M pots Reply with quote  Mark this post and the followings unread

Anybody know where to get 2.2M pots?
I need them for a sequencer/drum machine project.
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Blue Hell
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Joined: Apr 03, 2004
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PostPosted: Sun Aug 27, 2006 12:46 pm    Post subject: Reply with quote  Mark this post and the followings unread

I split this off from the origunal (noob) thread as it seemed to be an ubrelated subject to me.
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bigtex



Joined: Mar 30, 2006
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Location: Cupertino, California

PostPosted: Sun Aug 27, 2006 2:10 pm    Post subject: Reply with quote  Mark this post and the followings unread

I think the first question is: why? That seems like kind of an odd magic number.

You might be able to find one, but almost all potentiometers that I know of are in round number values (100k, 1M, 5M, 10M) so good luck finding a 2.2M pot.

It may be possible to sort of change the value of an existing pot by putting a resistor in parallel with the complete resistance of the pot, but that might alter how it works. I know I learned something about how to do this once, but I've since forgotten.

A quick google search turned up this one result:
http://www.allaboutcircuits.com/vol_6/chpt_3/9.html

and I'm sure there's plenty more information out there. So you could start with a 5M or 10M pot, and put some resistors in parallel with portions of it and see what you get on a multimeter. Wother a shot.
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bigtex



Joined: Mar 30, 2006
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PostPosted: Sun Aug 27, 2006 2:17 pm    Post subject: Reply with quote  Mark this post and the followings unread

Okay, here. Put a resistance in parallel with a larger value pot and you'll get your 2.2M ohm potentiometer:

1 / ( ( 1 / R1 ) + ( 1 / R2 ) ) = R

1 / ( ( 1 / 10M ) + ( 1 / R2 ) ) = 2.2M

( 1 / 10M ) + ( 1 / R2 ) = 1 / 2.2M

1 / R2 = ( 1 / 2.2M ) - ( 1 / 10M )

R2 = 1 / ( 1 / 2.2M ) - ( 1 / 10M )

R2 = 2.82 M ohm

So you'd need a 2.82M resistor, which you could either find or make by putting other resistors in series.
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Blue Hell
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PostPosted: Sun Aug 27, 2006 2:51 pm    Post subject: Reply with quote  Mark this post and the followings unread

Piher still seems to make large valued carbon pots (couldn't find 'm for Alps or Bourns) : http://www.piher-nacesa.com/pdf/24-T21v03.pdf

BTW: shunting a potmeter with a resistor will only work when it is used as a rheostat (variable resistor), but it will change the characteristic from linear to something else.

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bigtex



Joined: Mar 30, 2006
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Location: Cupertino, California

PostPosted: Sun Aug 27, 2006 3:06 pm    Post subject: Reply with quote  Mark this post and the followings unread

Blue Hell wrote:
BTW: shunting a potmeter with a resistor will only work when it is used as a rheostat (variable resistor), but it will change the characteristic from linear to something else.


I knew there was something like that, but I couldn't remember the details. Thanks for pointing it out.
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fonik



Joined: Jun 07, 2006
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PostPosted: Mon Aug 28, 2006 6:00 am    Post subject: Reply with quote  Mark this post and the followings unread

mister laurie biddulph has this great article about tailoring pots on his site (elby designs):

http://www.elby-designs.com/documents/tailoringpotentionometers.pdf

cheers,
matthias
http://www.modular.fonik.de
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fabreeze



Joined: Mar 13, 2006
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Location: Olympia, WA

PostPosted: Mon Sep 25, 2006 11:45 am    Post subject: yes Reply with quote  Mark this post and the followings unread

Thanks fonik,
that's exactly what I needed.
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goodrevdoc



Joined: Sep 11, 2006
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PostPosted: Mon Sep 25, 2006 12:37 pm    Post subject: Reply with quote  Mark this post and the followings unread

Mouser sells 2M pots in various varieties, the simplest and cheapest probably being the Alpha 24mm Part# 31VA602
This will certainly suffice for a 2M2 in most cases. Posting a schem of what exactly you're trying to accomplish may give some insight if this part is needed at all. I.E.- changing a cap or resistor value and using a 1M pot is certainly easier than stacking resistors on a large value or tracking down one weird value.
-justin
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