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fabreeze
Joined: Mar 13, 2006 Posts: 9 Location: Olympia, WA
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Posted: Sun Aug 27, 2006 12:41 pm Post subject:
2.2M pots |
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Anybody know where to get 2.2M pots?
I need them for a sequencer/drum machine project. |
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blue hell
Site Admin
Joined: Apr 03, 2004 Posts: 24079 Location: The Netherlands, Enschede
Audio files: 278
G2 patch files: 320
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Posted: Sun Aug 27, 2006 12:46 pm Post subject:
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I split this off from the origunal (noob) thread as it seemed to be an ubrelated subject to me. _________________ Jan
also .. could someone please turn down the thermostat a bit.
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bigtex
Joined: Mar 30, 2006 Posts: 323 Location: Cupertino, California
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Posted: Sun Aug 27, 2006 2:10 pm Post subject:
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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 Posts: 323 Location: Cupertino, California
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Posted: Sun Aug 27, 2006 2:17 pm Post subject:
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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
Site Admin
Joined: Apr 03, 2004 Posts: 24079 Location: The Netherlands, Enschede
Audio files: 278
G2 patch files: 320
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Posted: Sun Aug 27, 2006 2:51 pm Post subject:
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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. _________________ Jan
also .. could someone please turn down the thermostat a bit.
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bigtex
Joined: Mar 30, 2006 Posts: 323 Location: Cupertino, California
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Posted: Sun Aug 27, 2006 3:06 pm Post subject:
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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 Posts: 3950 Location: Germany
Audio files: 23
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fabreeze
Joined: Mar 13, 2006 Posts: 9 Location: Olympia, WA
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Posted: Mon Sep 25, 2006 11:45 am Post subject:
yes |
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Thanks fonik,
that's exactly what I needed. |
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goodrevdoc
Joined: Sep 11, 2006 Posts: 288 Location: Philadelphia, PA
Audio files: 1
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Posted: Mon Sep 25, 2006 12:37 pm Post subject:
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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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