Correcting a LED response curve?

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.

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

Find another approach to the problem. Best regards, 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

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

What are you trying to accomplish? Schematic?

...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.

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

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.

"Jacques St-Pierre"

(led controlled resistor) to simulate a RTD.

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

...... Phil

would this be better?

martin

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.

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

Bob

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:

It may be what I needed.

Bye Jacques

But figs 5 and 6 are pretty scary.

John

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

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.

am

Any

Vactrol

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

This was one of the solutions suggested.

Bob