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Old 31st July 2010, 05:07 PM   #21
gootee is offline gootee  United States
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Quote:
Originally Posted by wapo54001 View Post
Here is a rather accurate graph of the Silonex LDR response.

Current is divided into 460 equal steps along one axis, and the Silonex resistance is charted in relation to the current delivered. Resistance range shown is 40 ohms to 85K ohms, approximate. Pretty spectacular corner.

Second chart is LOG scale
Not sure I understand your plots. Maybe I'm just looking at them wrong. They appear to show resistance increasing with current. See jackinnj's plot in post # 19. You should have high resistance at low current and low resistance at high current. If the current increases to the right along the horizontal axis, then the resistance on the vertical axis should slope from upper left to lower right. The plots you posted look more like the voltage versus current of just the LED portion, which would be exponential.

Here is a nice paper about them, although it happens to refer to the Perkin Elmer / Vactec VACTROL types, which are similar to the Siliconix models in how they operate:

http://denethor.wlu.ca/pc300/optoiso...troduction.pdf

Last edited by gootee; 31st July 2010 at 05:22 PM.
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Old 31st July 2010, 05:44 PM   #22
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I think wapo is showing the response of the shunt LDR (in series/shunt attenuator setting)...the series LDR will follow jackinnj attached plot which is the more natural way of showing its response as an LDR starts high and lowers as power to it is controlled.
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Old 31st July 2010, 06:14 PM   #23
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Here is a paper from Perkin Elmer with Audio Application Notes for Analog Optical Isolators, such as the vactrols and the Silonex devices:

http://www.perkinelmer.co.uk/CMSReso...sAudioApps.pdf (144k)

And here is the Perkin Elmer Vactrols homepage:

Analog Optoisolators and Optocouplers|PerkinElmer
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Old 31st July 2010, 06:38 PM   #24
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Originally Posted by Blues View Post
I think wapo is showing the response of the shunt LDR (in series/shunt attenuator setting)...the series LDR will follow jackinnj attached plot which is the more natural way of showing its response as an LDR starts high and lowers as power to it is controlled.
I was not paying any attention to which direction the values go, only charting the relationship between my control input vs LDR resistance. The y axis represents a change in voltage drop in equal steps across a resistor in series with the LDR. In this first iteration I could do 450 equal steps, but that's not good enough -- many steps at the low end of the LDR range and the LDR resistance doesn't change much, and then not enough toward the top where the LDR resistance changes rapidly with minute changes in current.

With one or two additional passive components I think I can increase the current control step count to above 1000 which will give me finer resolution of the LDR resistance range. Also, I may rearrange the order of components in the limit resistor-LDR-control resistance string We'll see.
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Old 31st July 2010, 06:40 PM   #25
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Originally Posted by gootee View Post
Not sure I understand your plots. Maybe I'm just looking at them wrong. They appear to show resistance increasing with current.
No -- LDR resistance increasing as series resistance to the LED is increased, which in effect reduces current through the LED. My goal is precise control of the current through the LED. I can already do the precise control, but in not enough steps to cover with precision the dynamic range of 40 ohms to 350K ohms (which I want in order to do a T attenuator with1/2 dB steps starting at 1/2 dB attenuation.

Last edited by wapo54001; 31st July 2010 at 06:44 PM.
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Old 31st July 2010, 07:27 PM   #26
gootee is offline gootee  United States
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You could look at some different LDR models.
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Old 1st August 2010, 02:07 PM   #27
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You could look at some different LDR models.
Sorry, what is meant by "models?"
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Old 2nd August 2010, 01:57 AM   #28
gootee is offline gootee  United States
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Originally Posted by wapo54001 View Post
Sorry, what is meant by "models?"
"Different models" means either different part numbers from the same manufacturer or different manufacturer and part numbers.

Each part numer has different characteristics. Some other type/model/part number might have a slope that is much easier for you to control.

For example, look at the first of the two PDF files that I gave links for, to see the ranges of characteristics that are available in one manufacturer's LDR types.

Last edited by gootee; 2nd August 2010 at 02:00 AM.
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Old 2nd August 2010, 04:05 AM   #29
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As long as you have the "control law" you can use the equations to determine how any implementation will work.
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Old 2nd August 2010, 11:07 AM   #30
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"Different models" means either different part numbers from the same manufacturer or different manufacturer and part numbers.
Oh, I thought you were referring to different theoretical implementations or something like that . . .
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