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Correcting a LED response curve?

Hi,

I am designing a product using a Vactrol (led controlled resistor) to simulate a RTD. The led intensity will be control by the buffered output of a D/A chip. It is working, but to be useful, we have to operate the led in the worst region of it's curve, resulting in very coarse response. Since I do not have enough resolution on this D/A to correct that from software I am searching for a method to render the Vactrol response a bit more linear. Any one can suggest a simple way to create a amplifier using a simple opamp or transistor to reproduce the reverse curve of the led and correct the Vactrol response. I tough that maybe using a led in the feedback path of an opamp may result in a non linear amplifier response and that can be use to drive the Vactrol. Any suggestions?

Bye

Jacques

Reply to
Jacques St-Pierre
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Does the LED output have to be DC? If not the throw away the DAC and use PWM which will give a very linear predictable output.

Reply to
Icky Thwacket

Can you place a resistor in parallel or series to move the region of operation of the LED?

Reply to
Greg Neill

Find another approach to the problem. Best regards, Spehro Pefhany

--
"it's the network..."                          "The Journey is the reward"
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Reply to
Spehro Pefhany

Are you driving the LED with a voltage or a current?

Maybe a simple voltage to current source circuit will alleviate your problems?

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
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Reply to
Jim Thompson

I just looked up "Vactrol" and realized that resistance is inversely related to LED current.

What are you trying to accomplish? Schematic?

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
         America: Land of the Free, Because of the Brave
Reply to
Jim Thompson

That's what I was thinking as well. Just keep the switching frequency above the RTD's transient response and that should work nicely.

--
Paul Hovnanian	paul@hovnanian.com
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Reply to
Paul Hovnanian P.E. 

--
Blow off the Vactrol and substitute:

http://www.fairchildsemi.com/ds/H1%2FH11F3M.pdf
Reply to
John Fields

Note Figure 1, It has the same curvature problem as a Vactrol.

The OP is trying to get a linear resistance versus control current curve.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
         America: Land of the Free, Because of the Brave
Reply to
Jim Thompson

If I was doing this, my first step would be to add a series and parallel resistor to the Vactrol so that the network was just capable of producing the small range of resistance needed over the full usable range of LED current. Then I would add a voltage to current converter between the DAC and the LED.

--
Regards,

John Popelish
Reply to
John Popelish

"Jacques St-Pierre"

** Vactrol is a TRADE NAME - you wanker.

(led controlled resistor) to simulate a RTD.

** Don't use pretentious acronyms without explanation.
** Current drive should help.

Or better still, use PWM - the average light output will then be a linear function of duty cycle.

...... Phil

Reply to
Phil Allison

would this be better?

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martin

Reply to
Martin Griffith

And yet on the front page it says ">= 99.9% linearity".

At any rate, I think RTDs are around 100 ohms, so the device does not seem well suited here.

--

John Devereux
Reply to
John Devereux

Perkin Elmer VTL5C3 1.5 ohms to 30k ohms is $3.79.

Bob

Reply to
sycochkn

Sorry if I offend anyone using the Vactrol name for the device. Since I did not know of any other similar device, I tough using Vactrol should be clear enough, but yes it's a "LED Controlled Resistor" as mention by Phil.

In fact I am using VTL5C4.

Thanks for your comments, I will experiment on them and post the results.

For now I intend to test this:

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It may be what I needed.

Bye Jacques

Reply to
Jacques St-Pierre

But figs 5 and 6 are pretty scary.

John

Reply to
John Larkin

Maybe his keyboard doesn't have those little "tm" or letter-c-in-a-circle gadgets.

You think "RTD" is pretentious? How do you feel about "CPU", or "FPGA" or even "IC"?

John

Reply to
John Larkin

Is this a general purpose RTD simulator or can you take advantage of a particular application in some way. For example, if the RTD is used to develop a DC voltage then an isolated differential current in shunt with a precision resistor would be more straightforward. If it's inside a synchronous detector loop then the vactrol will cause problems and you have to do something else. There are things called 'digital pots" which can be configured as 'digital rheostats' which maybe suitable, it is very easy to isolate the logic commands to the digital rheostat, and there are various ways to configure several of these for high resolution, in excess of 16-b.

Reply to
Fred Bloggs

am

Any

Vactrol

I guess any (real-world) curve is 99.9% "linear" if the range is small enough :)

--

John Devereux
Reply to
John Devereux

This was one of the solutions suggested.

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Bob

Reply to
sycochkn

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