Re: Correcting a LED response curve?

Yes, As I mentioned in...

From: Jim Thompson Newsgroups: sci.electronics.design Subject: Re: Correcting a LED response curve? Message-ID:

I just made a Vactrol model, but probably won't have time to demonstrate the loop method until morning... granddaughter chiming that she's hungry ;-)

...Jim Thompson

Vactrols are notoriously unstable, and the Fairchild thing looks pretty bad, too, not to mention its tiny linear signal range. An RTD usually changes about 4000 PPM per degree C, so you need a precise resistance to simulate to any decent accuracy. I really doubt that even servoing "matched" Vactrols or Fairchild gadgets would be good enough.

We have a design for a VME module that's an 8-channel RTD simulator; one of these days we'll get around to announcing it as a product. It's brute-force, with each channel fully isolated. The resistance is simulated using a switched super-precision resistor (each channel has

There are commercial units that just use a heap of resistors and relays, which works well except for switching transients when you change value.

We toyed with PWMing a real resistor, which is mighty appealing, but has too many side problems.

John

Vacs, or specifically the ldr's inside, have ghastly tempcos, and huge history effects, and bizarre, complex time constants.

John

John Larkin a écrit :

And "SDing" it?

Another way would be to use a "HF" pilot to sense the LDR resistance, that will be filtered out for the actual output resistance.

Yep. In practice I seriously doubt getting reasonable stability.

...Jim Thompson

A precious-metal RTD changes less than +0.4%/K. Accuracy of the

If the LDR can swing from hundreds of ohms to thousands of ohms, and you make full use of that range to swing between, say 100 and 110 ohms, by adding the appropriate parallel and series resistors, the overall stability will be a lot better than that of just the LDR, itself. Choosing the best two resistors for linearity is a least squares fit type math problem. I do this sort of thing with Mathcad, quite often.

That's not really as compelling as it sounds. The RTD changes proportional to absolute temperature, so at helium temperature it's about 25% per degree K. Low-temperature operation is most accurate if one uses AC excitation and four-wire sensing (which is easiest in an ungrounded circuit), because that keeps heat generation low.

I'm not aware of any important 'instability' in photoconductors, but there IS a long turn-off time constant, over a millisecond. The prudent way to use a photoconductor inside a feedback loop will require considerable settling time.

Uhn, no, not for Pt RTDs, they are pretty much useless below about 15K since the sensitivity drops rapidly in that range. Liquid He is ~4.2K. You really can't safely extrapolate behaviour at relatively high temperatures down to that realm.

It's even more accurate if you first toss out the Pt RTD and use a proper cryogenic sensor.

I don't recall the OP mentioning anything about sub-5K measurements, but perhaps he's holding out on us for effect.

Two RTDs would do it (in a programmable oven, of course).

Best regards, Spehro Pefhany