electro-music.com   Dedicated to experimental electro-acoustic
and electronic music
 
    Front Page  |  Articles  |  Radio
 |  Media  |  Forum  |  Wiki  |  Links  |  Store
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
Resistance swing to CV conversion
Post new topic   Reply to topic Moderators: jksuperstar, Scott Stites, Uncle Krunkus
Page 1 of 1 [7 Posts]
View unread posts
View new posts in the last week
Mark the topic unread :: View previous topic :: View next topic
Author Message
mike page



Joined: Sep 26, 2016
Posts: 125
Location: norwich, uk

PostPosted: Mon May 27, 2019 12:53 am    Post subject: Resistance swing to CV conversion Reply with quote  Mark this post and the followings unread

Lets say I have a resistance change swinging from 1k to 100k in a fairly linear.
Whats the best way to convert this into a useful control voltage for a eurorack system? Ideally maintaining the linearity of the response.
All I can think of currently is using it as one half of a carefully balance voltage divider, or somehow use with a LM317 or similar.

Any thoughts?
Back to top
View user's profile Send private message
ixtern



Joined: Jun 25, 2018
Posts: 93
Location: Poland

PostPosted: Tue May 28, 2019 10:57 pm    Post subject: Re: Resistance swing to CV conversion Reply with quote  Mark this post and the followings unread

mike page wrote:
Lets say I have a resistance change swinging from 1k to 100k in a fairly linear.
Whats the best way to convert this into a useful control voltage for a eurorack system? Ideally maintaining the linearity of the response.
All I can think of currently is using it as one half of a carefully balance voltage divider, or somehow use with a LM317 or similar.

Any thoughts?

Depends what kind of resistance. If purely resistive, then connect it to reference voltage in a voltage divider circuit. If other kind (e.g.semiconductor type) - current source may be needed and careful reading of datasheet (linearity, max voltage and current...).

In every case Op Amp voltage follower with high input impedance or other way speaking with low bias currents is needed to avoid linearity errors.

LM317 is not suited for precision voltages. Fine more stable voltage source - for entry level it may be 50ppm TL431 or (even better) old 30ppm uA723. Upper case is 2ppm LT1021BCN8, 3ppm REF5040, 2.5ppm REF102CU.
Back to top
View user's profile Send private message
JovianPyx



Joined: Nov 20, 2007
Posts: 1836
Location: West Red Spot, Jupiter
Audio files: 218

PostPosted: Wed May 29, 2019 6:38 am    Post subject: Reply with quote  Mark this post and the followings unread

Using an opamp, create an inverting amp.

Ground the noninverting input.

Connect the variable resistance to the inverting input.

Connect a fixed resistor larger than the highest value possible from the variable resistor from the opamp output to the opamp inverting input.

Put a fixed negative voltage on the other end of the variable resistor. The voltage should be small enough to give a full scale output (10 volts) when the variable resistance is at max ohms.

The variable resistor will provide and control the current into the opamp.

The output will change voltage as the resistor changes value. The higher the resistance, the higher the voltage. The resulting voltage should linearly follow the changes in the variable resistance.

_________________
FPGA, dsPIC and Fatman Synth Stuff

Time flies like a banana.
Fruit flies when you're having fun.
BTW, Do these genes make my ass look fat?
corruptio optimi pessima
Back to top
View user's profile Send private message Visit poster's website
mike page



Joined: Sep 26, 2016
Posts: 125
Location: norwich, uk

PostPosted: Wed May 29, 2019 10:10 am    Post subject: Reply with quote  Mark this post and the followings unread

Amazing! Thanks!!
Back to top
View user's profile Send private message
JovianPyx



Joined: Nov 20, 2007
Posts: 1836
Location: West Red Spot, Jupiter
Audio files: 218

PostPosted: Wed May 29, 2019 1:20 pm    Post subject: Reply with quote  Mark this post and the followings unread

On second thought, it won't be linear. Part of the range will be close to linear. The same is true of the voltage divider method.
_________________
FPGA, dsPIC and Fatman Synth Stuff

Time flies like a banana.
Fruit flies when you're having fun.
BTW, Do these genes make my ass look fat?
corruptio optimi pessima
Back to top
View user's profile Send private message Visit poster's website
Blue Hell
Site Admin


Joined: Apr 03, 2004
Posts: 22785
Location: The Netherlands, Enschede
Audio files: 231
G2 patch files: 319

PostPosted: Wed May 29, 2019 3:51 pm    Post subject: Reply with quote  Mark this post and the followings unread

So this would be resistance to voltage conversion, right? So ... how about then .. maybe:

Set up a current mirror, control the ingoing current from a reference voltage and that resistor, then at the other end of the mirror use a fixed resistor equal in size to the lowest value of the input resistor (with a larger value it would clip against ground or Vcc, depending on NPN or PNP config).

Then maybe use an opamp follower after that to get a lower output impedance. The follower would have to invert .. or the mirror would have to be done with PNP transistors .. I'd use NPN tho I think .. without spending much thought on it ;-)

Or alternatively ... set up something with an OTA .. some fixed small input value on its inputs, and control the gain ( Iabc) with the resistor.. basically this is the same idea .. as an OTA is mostly a bunch of current mirrors, it would need a bit less build space thoughI think.. but .. watch the Iabc to not get too large. This would be less of an issue with a discrete current mirror.

_________________
Jan
also .. please don't march .. we are on a bridge right now.
Back to top
View user's profile Send private message Visit poster's website
mike page



Joined: Sep 26, 2016
Posts: 125
Location: norwich, uk

PostPosted: Thu May 30, 2019 9:34 am    Post subject: Reply with quote  Mark this post and the followings unread

Once again thanks, that's awesome.
Everyone loves a sassy lil' transistor circuit!
Back to top
View user's profile Send private message
Display posts from previous:   
Post new topic   Reply to topic Moderators: jksuperstar, Scott Stites, Uncle Krunkus
Page 1 of 1 [7 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
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
e-m mkii

Please support our site. If you click through and buy from
our affiliate partners, we earn a small commission.


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