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 Forum index » DIY Hardware and Software » Thomas Henry designs
question about expo converter
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Grumble



Joined: Nov 23, 2015
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PostPosted: Fri Feb 02, 2018 8:03 am    Post subject: question about expo converter Reply with quote  Mark this post and the followings unread

Posted Image, might have been reduced in size. Click Image to view fullscreen.
I have seen this diagram several times now, but it says only tempco 2k for R1, but is it a PTC or an NTC?
I hope NTC because they are sold more frequently and cheaper Rolling Eyes

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Blue Hell
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Joined: Apr 03, 2004
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PostPosted: Fri Feb 02, 2018 10:21 am    Post subject: Reply with quote  Mark this post and the followings unread

The short answer .. +3300 ppm/C - so a PTC.

The long answer :

The voltage to current conversion is performed by the transistor in two stages. First stage is the voltage to current of the BE junction, 2nd state base current to collector current according to Hfe.

The first stage is temperature sensitive according to : https://en.wikipedia.org/wiki/Shockley_diode_equation and this is what you want to compensate for. With a linear tempco this is never perfect, but a first order approximation is what is usually used. The 3300 ppm comes from assuming some specific room temperature - I think 25C, but did not calculate it.

Some more info:

https://www.davidhaillant.com/about-tempcos-in-exponential-converters/

https://www.allaboutcircuits.com/projects/diy-synth-series-vco/

http://schmitzbits.de/expo_tutorial/index.html (which also illustrates an alternate way to compensate with an NTC).

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Grumble



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PostPosted: Fri Feb 02, 2018 12:12 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thanks Jan,
Now I have to start looking to buy some ptc’s, since I already ordered a handfull of ntc’s Confused

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gasboss775



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PostPosted: Fri Feb 02, 2018 12:36 pm    Post subject: Reply with quote  Mark this post and the followings unread

This might be flawed but since you've got the ntc 2K thermistors might be worth a try. Put the 2K ntc thermistor in the place of the 100K input resistor and use a 40ohm resistor in the feedback path of the amplifier.

Also you'll probably need to put an opamp buffer at the input as the input resistance would be a bit low otherwise.
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Grumble



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PostPosted: Fri Feb 02, 2018 2:28 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thanks, but...
I’m populating a pcb of the Electro-Music Karplus Strong ( thanks AlanP) so rearranging components is not an option. For future perfboard builds this is an option, so thank you!

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gasboss775



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PostPosted: Sat Feb 03, 2018 11:28 am    Post subject: Reply with quote  Mark this post and the followings unread

Grumble wrote:
Thanks, but...
I’m populating a pcb of the Electro-Music Karplus Strong ( thanks AlanP) so rearranging components is not an option. For future perfboard builds this is an option, so thank you!


You're welcome!

I should also add that this would probably only work if the temperature coeffient was the same but (-) instead of (+), in other words - 3300 ppm / °C
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