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 Forum index » DIY Hardware and Software
Reset timing cap of AR to pull output to zero
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fonik



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PostPosted: Wed Aug 04, 2010 1:21 am    Post subject: Reset timing cap of AR to pull output to zero
Subject description: why does it not work in real life?
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i built an AR generator on breadboard. my idea was to short the timing cap to force the circuit to go down to zero volt in the release phase at some point.
the idea was to use a comparator to create a short pulse that controlls a NPN in parallel to the timing cap (circuit below).

it works in vitro (screenshot of scope in the simulation software), but not in vito (picture of the analoge scope). in real life the 100n cap seems to try to reset when upper limit is reached?

any ideas?


AR_scope2.gif
 Description:
Simulation Software
 Filesize:  21.4 KB
 Viewed:  1888 Time(s)

AR_scope2.gif



AR_scope.JPG
 Description:
real life scope with circuit on breadboard
 Filesize:  46.31 KB
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AR_scope.JPG



AR_01.gif
 Description:
AR gen circuit
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 Viewed:  121 Time(s)
This image has been reduced to fit the page. Click on it to enlarge.

AR_01.gif



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fonik



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PostPosted: Thu Aug 05, 2010 6:45 am    Post subject: Reply with quote  Mark this post and the followings unread

anyone? Laughing

i thought i had triplechecked the breadboard, so what could cause this behaviour? ideas? should i try a comparator instead a quarter of the TL074?

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Blue Hell
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PostPosted: Thu Aug 05, 2010 7:06 am    Post subject: Reply with quote  Mark this post and the followings unread

Not sure if its a problem ... but when your comparing-opamp goes negative the switching transistor is reverse biased and that may do some odd things if it goes into zener mode .. which it probably will.
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fonik



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PostPosted: Thu Aug 05, 2010 7:12 am    Post subject: Reply with quote  Mark this post and the followings unread

Blue Hell wrote:
Not sure if its a problem ... but when your comparing-opamp goes negative the switching transistor is reverse biased and that may do some odd things if it goes into zener mode .. which it probably will.

thanks for the reply, jan!

by 'zener mode' you are refering to the transistor?
the comparator going negative... this would explain the difference from software to breadboard, i think. i should try a single rail powered comparator then...

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PostPosted: Thu Aug 05, 2010 7:16 am    Post subject: Reply with quote  Mark this post and the followings unread

fonik wrote:
by 'zener mode' you are refering to the transistor?


Yes, the BE diode of that transistor - like in a noise generator, there the zener effect is used to generate the noise.

You could try a diode after the opamp. Why did you use AC coupling for the discharge circuit BTW?

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fonik



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PostPosted: Thu Aug 05, 2010 7:31 am    Post subject: Reply with quote  Mark this post and the followings unread

Blue Hell wrote:
fonik wrote:
by 'zener mode' you are refering to the transistor?


Yes, the BE diode of that transistor - like in a noise generator, there the zener effect is used to generate the noise.

You could try a diode after the opamp. Why did you use AC coupling for the discharge circuit BTW?

AC coupling??? Laughing i just wanted to create a short pulse from the comparators output. i did not recognize that it is ac coupling actually.
do you think this is a problem? from my thinking i need a short pulse to allow the timing cap to charge again after reset. without this gate to pulse conversion i would constantly bypass the timing cap.

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Blue Hell
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PostPosted: Thu Aug 05, 2010 9:40 am    Post subject: Reply with quote  Mark this post and the followings unread

fonik wrote:
from my thinking i need a short pulse to allow the timing cap to charge again after reset. without this gate to pulse conversion i would constantly bypass the timing cap.


Embarassed that makes sense, yup.

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BananaPlug



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PostPosted: Fri Aug 06, 2010 7:15 am    Post subject: Reply with quote  Mark this post and the followings unread

Rethinking the original problem. Your timing cap discharges through a diode (for separate attack/decay resistors) so I assume that's where the slight residual charge you want to dump comes from. You have two op amps. How about using them one after the other (as inverting op amps) for the output buffer. Now you have a summing point at the first one and can sum in a small negative voltage to null out the effect of the residual charge. You could put a diode in the feedback path of the final op amp to guarantee uni-polar output.
Just a thought.
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