Correcting a LED response curve?

[John Popelish]

A precious-metal RTD changes less than +0.4%/K. Accuracy of the
*sensor* is typically +/-0.3 to +/-0.03K, so you'd normally want an
instrument to be considerably more accurate and stable than that. An
LDR doesn't have a hope in h-e-double-hockeysticks of getting there
unless this is a particularly sloppy & crappy application. The usual
method is to use an active analog/digital circuit.
Best regards,

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.

 

[John Larkin]

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

But figs 5 and 6 are pretty scary.

John

 

[Jacques St-Pierre]

Something like this?

Bob

http://home.earthlink.net/~sycochkn1/rtd.pdf

I test the circuit, on the good old breadboard, here some results. I used a
10ma current source to feed the feedback LDR and I add a transistor as
follower at the output of the Op-Amp to get the 40ma require to drive those
LDRs LEDs.

VDAC

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

2.2

2.4

2.6

2.8

3

3.2

3.4

3.6

3.8

4


Rout of LDR

64

64.1

64.2

82

102

124

144

164

184

204

235

304

400

546

778

1025

1321

1758

2500

4000





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

 

[John Larkin]

"Jacques St-Pierre"



** Vactrol is a TRADE NAME - you wanker.

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

(led controlled resistor) to simulate a RTD.


** Don't use pretentious acronyms without explanation.

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

John

 

[Jim Thompson]

I test the circuit, on the good old breadboard, here some results. I used a
10ma current source to feed the feedback LDR and I add a transistor as
follower at the output of the Op-Amp to get the 40ma require to drive those
LDRs LEDs.

VDAC

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

2.2

2.4

2.6

2.8

3

3.2

3.4

3.6

3.8

4


Rout of LDR

64

64.1

64.2

82

102

124

144

164

184

204

235

304

400

546

778

1025

1321

1758

2500

4000





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

 

[Fred Bloggs]

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:

http://home.earthlink.net/~sycochkn1/rtd.pdf

It may be what I needed.

Bye
Jacques

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.

 

[John Devereux]

But figs 5 and 6 are pretty scary.

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

 

[[email protected]]

does it have to be a vactrol? how about one of the dual chip matched
analog optocouplers that are designed for this, I seem to recall a
Siemans part.
I also remember it shown in a analog devices app note.

Steve Roberts

 

[sycochkn]

I test the circuit, on the good old breadboard, here some results. I used
a
10ma current source to feed the feedback LDR and I add a transistor as
follower at the output of the Op-Amp to get the 40ma require to drive
those
LDRs LEDs.

VDAC

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

2.2

2.4

2.6

2.8

3

3.2

3.4

3.6

3.8

4


Rout of LDR

64

64.1

64.2

82

102

124

144

164

184

204

235

304

400

546

778

1025

1321

1758

2500

4000





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

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

Bob

 

[sycochkn]

I test the circuit, on the good old breadboard, here some results. I used
a
10ma current source to feed the feedback LDR and I add a transistor as
follower at the output of the Op-Amp to get the 40ma require to drive
those
LDRs LEDs.

VDAC

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

2.2

2.4

2.6

2.8

3

3.2

3.4

3.6

3.8

4


Rout of LDR

64

64.1

64.2

82

102

124

144

164

184

204

235

304

400

546

778

1025

1321

1758

2500

4000





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

This is your response curve.

http://home.earthlink.net/~sycochkn1/rtd1.pdf

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

 

[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:

http://home.earthlink.net/~sycochkn1/rtd.pdf

It may be what I needed.

Bye
Jacques

This was one of the solutions suggested.

http://home.earthlink.net/~sycochkn1/rtd2.pdf

Bob

 

[john jardine]

"Fred Bartoli" <" "> wrote in message

[...]

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

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

I imagine something along the lines of ...

BOSS: Production and rework desparately need a floating RTD simulator.
Get moving!.
OP: Jardi says that'll be $18000 plus 10 weeks for an engineered
solution.
BOSS: F*** that!. Product is piling up and now the repair people have
started screaming.
OP: I've a Vectrol idea that might help.
BOSS: I've rethought the spec'. Looks like a functional test will be OK.
Look into that Vectrol thing.
OP: We can give you floating resistance to +/- 5% within 2 weeks and
next to no cost.
BOSS: TFFT. Make it so!
OP: You remember that salary increase I mentioned? ...

 

[whit3rd]

A precious-metal RTD changes less than +0.4%/K. Accuracy of the
*sensor* is typically +/-0.3 to +/-0.03K, so you'd normally want an
instrument to be considerably more accurate and stable than that.

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.

 

[Spehro Pefhany]

That's not really as compelling as it sounds. The RTD changes
proportional to absolute temperature, so at helium temperature

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 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.

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.

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.

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


Best regards,
Spehro Pefhany

 

[Jim Thompson]

Please post the model somewhere.

In SwitcherCAD "led_ldr.asc" in Vactrol/Examples/Apps

...Jim Thompson

 

[JosephKK]

What is a RTD?

Resistance Temperature Detector, nominally 100.0 Ohms at 20 degrees C.
For more send "RTD" to your favorite search engine.

 

[JosephKK]

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

Actually a better place to put it would be in parallel with the LDR.
Say about 250 Ohms.

 

[JosephKK]

My line of thought, use a dual Vactrol...

Then a feedback loop so that control current versus "resistor" value
can be defined linearly, the second "resistor" serves as your RTD
tracking the first resistor.

I need a good Spice model for a Vactrol. I found one in a schematic
in SwitcherCAD, but haven't quite made sense of it yet. Helmut?

...Jim Thompson

Please post the model somewhere.