Re: Correcting a LED response curve?

Resistance Temperature Detector.

That work, but the range I need is too coarse to be useful, because of the non linear response of the LED.

I will try that and let you know what result of it.

Bye Jacques

On a sunny day (Thu, 13 Mar 2008 19:29:12 GMT) it happened "Jacques St-Pierre" wrote in :

OK, for 5 V max from the AD, and 20mA max LED current, then R sense is 5 / .02 = 250 Ohm. Make sure your op amp supply is high enough to account for the LED forward drop too.

drop too.

How about if the op-amp drove a transistor?

a k +5v --- LED ---. opamp | ----------- + |--' from DA out ---------------| -- - |->. | | ------------------------------| R sense | ///

That way, you only need to be high enough to account for the drop across the resistor, and current load on the op-amp is lower too.

I test that (with a transistor). I end up with a linear current curve on the LED, but still the output resistance of the Vactrol is not linear at all, really not. Probably because the LED light output is not linear.

Bye Jacques

More likely because the resistance of this "vactrol" thing does not vary linearly with light! LEDs are in fact quite linear.

You could try an arrangement with *two* Vactrol devices, provided their transfer functions are sufficiently similar. Close a loop around one to get linear resistance control, and feed the LED of the other with the same current. Hopefully the output resistance of the "other" will then be more linear, too.

The circuit is a bit too complicated for me to draw out in ASCII art right now - perhaps someone else will comment. It may be a waste of time anyway if you can't get matched "Vactrol" devices.

I have just looked them up - you can get dual ones apparently!

(Although I think Spehro is right - you need another approach)

It's injected carriers, so the resistance has to be roughly 1/x... mucho current, minimum resistance.

...Jim Thompson

Something like this?

Bob

The Vactrol uses a LED with an LDR. LDR is non-linear,

I tjust occured to me how I would do this, with only parts from the old junk box, I'd use one of my RC servos and connect a pot to it :-)

But as others pointed out, you can perhaps use 2 vactrols to compensate for the LDR curve. LDR is: slow, large tolerances, non-linear. I'd use the servo :-)

snipped-for-privacy@yahoo.com a écrit :

Or use a "HF" pilot so sense the LDR resistance, that will be filtered out for the actual output resistance.

Yes, looks good. Simpler than I expected since I didn't think of putting the leds in series for some reason!

Works for the dual vactrol too.

Nice.

Depending on the OP's application it may or may not be close enough.

Bob

Exactly my thoughts except I was going to use a dual Vactrol, and some resistance in series with the LED

I don't know about stability, though. The log-log plot on the data sheet hides the wild curvature, thus extreme loop gain variation.

...Jim Thompson

Works nicely. Used it for a Twin T notch null.

Or use two LDRs and some analog switches. Alternately use one while calibrating the other.

Are ldr's analog-voltage linear, in other words ohmic? RTD simulation would need extreme linearity.

John

Since it's injected carriers I'd guess it to be nonlinear, like a MOSFET, for other than small voltage drops.

...Jim Thompson

I test the circuit, on the good old breadboard, here some results. I used a

VDAC

Rout of LDR

Not linear yet, but a lot more usable in the 100 to 1000 ohms. Since it's the range I will use, it may be sufficient. I will do more test.

Bye Jacques

Jacques, What resolution (smallest increment of resistance) are you looking for?

...Jim Thompson

The VTL5C3 is the only vactrol that will preform below the 100 ohms the other ones have to high an on resistance.

Bob

This is your response curve.

How about some mercury wetted reed relays and precision resistors. The response time is faster than the VACTROL. and you dont need the DAC.

Bob