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 Forum index » DIY Hardware and Software
Variable Controlled Resistors
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shawn



Joined: Dec 13, 2005
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PostPosted: Thu Jan 18, 2007 11:34 am    Post subject: Variable Controlled Resistors Reply with quote  Mark this post and the followings unread

I've been wondering about variable controlled resistors for some time. I have a few circuits that I'd like to add to my portable modular synthesizer but I'm unsure how to go about this particular pitfall. What various ways can you implement such a thing? I've only seen people mention using an LM13700, VCR7N or a JFET such as the 2N3819 but haven't seen any applications that do so. Do they only work if one end of the 'resistor' is tied to ground? I've been trying to figure out a way, without using vactrols mind you, of adjusting resistance through vc over a wide range. Are there limitations to the range available?
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bugbrand



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PostPosted: Thu Jan 18, 2007 12:16 pm    Post subject: Reply with quote  Mark this post and the followings unread

I've done a bit with LM13700s - took the datasheet examples as a starting point and tweaked from there. It's not all worked out quite right though - I'm really not so sure on tweaking to the correct resistance (calculation style)...

There's a bit of an example in the VC BugCrusher circuit here

..i'd also be interested to hear of other ideas and methods..

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shawn



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PostPosted: Thu Jan 18, 2007 3:16 pm    Post subject: Reply with quote  Mark this post and the followings unread

Ah very cool indeed. What range did you manage to obtain with that setup? Looks interesting!
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State Machine
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PostPosted: Thu Jan 18, 2007 5:03 pm    Post subject: Reply with quote  Mark this post and the followings unread

This got me thinking guys, I wonder how an analog switch, like the MAX326, would behave if the switch "control" terminal were biased below the specified input level under "normal" operating conditions in such a way that the user could obtain intermediate resistive values other than the Rds on specified in the data sheet. I know many also have the CD4066 quad bilateral switches on hand, you might want to try and play around with that device.

Nobody says you have to operate the the IC's like the data sheet says and it might be interesting to operate the switches in the "subthreshold" region. Very Happy

Technically, what you are looking for is a "floating node voltage variable resistor"


Bill
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shawn



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PostPosted: Thu Jan 18, 2007 6:07 pm    Post subject: Reply with quote  Mark this post and the followings unread

Ahhh so floating means it's not attached to ground?
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State Machine
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PostPosted: Fri Jan 19, 2007 12:24 pm    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
Ahhh so floating means it's not attached to ground?


Yes, correct. It's a variable resistance that can be put anywhere in circuit where you want to control the resistance, under voltage control, between two points (nodes) in a circuit.

Bill
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mediatechnology



Joined: May 10, 2006
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PostPosted: Tue Jan 23, 2007 11:34 am    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
Technically, what you are looking for is a "floating node voltage variable resistor"


Has anyone mentioned the H11F1? It's an optically-isolated LED/ FET. The FET side is completely floating. It has an added advantage of not having any control feedthrough by virtue of opto coupling.

Like most FETS it's only an OK VCR. But one of the bullet-points on the datasheet mentions filter control. I've used them in a very fast noise gate and the complete lack of feed through was very useful. And like a FET they don't like high levels IIRC <50 mV for decent distortion.

http://www.fairchildsemi.com/ds/H1/H11F1.pdf

Last edited by mediatechnology on Tue Jan 23, 2007 4:44 pm; edited 1 time in total
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State Machine
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PostPosted: Tue Jan 23, 2007 4:30 pm    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
Has anyone mentioned the H11F1? It's an optically-isolatedLED/ FET. The FET side is completely floating. It has an added advantage of not having any control feedthrough by virtue of opto coupling.


Yes, useful, but you have to really watch those signal levels or the device will not behave very well. Seems like the device has a very linear response in the 100 to 10K ohm region or three decade worth or resistance.

Nice. Thanks for posting the data sheet for us .... Very Happy

Bill
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mediatechnology



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PostPosted: Tue Jan 23, 2007 4:43 pm    Post subject: Reply with quote  Mark this post and the followings unread

Correct. As a voltage controlled attenuator in shunt configuration they distort heavily at high signal levels. As a switch, in current mode or as series element they might not be so bad. I guess it depends on how he wants to use it. But it's cool the drain and source float.
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