Correcting a LED response curve?

[Jacques St-Pierre]

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

 

[Jan Panteltje]

Hi,

I am designing a product using a Vactrol (led controlled resistor) to
simulate a RTD.

What is a RTD?

The led intensity will be control by the buffered output of
a D/A chip.

A LED had light proportional to the *current* through it.
So you should drive it with a 'voltage (from your DA) controlled current source'.

opamp
----------- + a k
from DA out ---------- LED ---
-- - |
| |
------------------------------|
R sense

 

[Icky Thwacket]

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

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.

 

[Jan Panteltje]

Hi,

I am designing a product using a Vactrol (led controlled resistor) to
simulate a RTD.

What is a RTD?

The led intensity will be control by the buffered output of
a D/A chip.

A LED had light proportional to the *current* through it.
So you should drive it with a 'voltage (from your DA) controlled current source'.

opamp
----------- + a k
from DA out ---------- LED ---
-- - |
| |
------------------------------|
R sense
|
///
Ground got cut of, here it is.

 

[Greg Neill]

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?

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

 

[Spehro Pefhany]

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?

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

 

[Jim Thompson]

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

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

 

[Jacques St-Pierre]

What is a RTD?

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.

A LED had light proportional to the *current* through it.
So you should drive it with a 'voltage (from your DA) controlled current
source'.

opamp
----------- + a k
from DA out ---------- LED ---
-- - |
| |
------------------------------|
R sense
|
///
Ground got cut of, here it is.

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

Bye
Jacques

 

[Jacques St-Pierre]

Can you place a resistor in parallel or series

to move the region of operation of the LED?

I tried, but did not get much results from it.

Bye
Jacques

 

[Jim Thompson]

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

 

[Paul Hovnanian P.E.]

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.

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

 

[Jan Panteltje]

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

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.

 

[DJ Delorie]

Make sure your op amp supply is high enough to account for the LED forward 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.

 

[Jacques St-Pierre]

How about if the op-amp drove a transistor?

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

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

 

[Jacques St-Pierre]

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

What are you trying to accomplish? Schematic?

...Jim Thompson

I wish to use the Vactrol to replace an RTD on a existing system. I wish to
simulate the RTD so the system will think it still read temperature, while
it will be a value set by my equipment. I need something completely
isolated, it's why the Vactrol appear to be a acceptable way to go.

I can use it has is, but I have really not much resolution in the value of
resistor require. The circuit is simple, the D/A output drive a transistor
as buffer that drive the Vactrol LED via a simple resistor. I can set the
D/A output from 0 to 5 Vdc, the resistor as been select for 40ma on the LED
at 5Vdc. The Vactrol LED need 40ma for minimum resistor output.

So my hope is to find a way to build a simple circuit that can will reverse
the curve of the LED and end up with something more linear. This should give
me a better resolution in the range I need.

Bye
Jacques

 

[John Devereux]

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.

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)

 

[Jim Thompson]

I wish to use the Vactrol to replace an RTD on a existing system. I wish to
simulate the RTD so the system will think it still read temperature, while
it will be a value set by my equipment. I need something completely
isolated, it's why the Vactrol appear to be a acceptable way to go.

I can use it has is, but I have really not much resolution in the value of
resistor require. The circuit is simple, the D/A output drive a transistor
as buffer that drive the Vactrol LED via a simple resistor. I can set the
D/A output from 0 to 5 Vdc, the resistor as been select for 40ma on the LED
at 5Vdc. The Vactrol LED need 40ma for minimum resistor output.

So my hope is to find a way to build a simple circuit that can will reverse
the curve of the LED and end up with something more linear. This should give
me a better resolution in the range I need.

Bye
Jacques

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

 

[John Devereux]

I wish to use the Vactrol to replace an RTD on a existing system. I wish to
simulate the RTD so the system will think it still read temperature, while
it will be a value set by my equipment. I need something completely
isolated, it's why the Vactrol appear to be a acceptable way to go.

I can use it has is, but I have really not much resolution in the value of
resistor require. The circuit is simple, the D/A output drive a transistor
as buffer that drive the Vactrol LED via a simple resistor. I can set the
D/A output from 0 to 5 Vdc, the resistor as been select for 40ma on the LED
at 5Vdc. The Vactrol LED need 40ma for minimum resistor output.

So my hope is to find a way to build a simple circuit that can will reverse
the curve of the LED and end up with something more linear. This should give
me a better resolution in the range I need.

Do you need to generate a complete curve, or just a couple of points?
You could use relays to directly switch in a few resistors.

Or make a relay-driven R2R ladder. A relay DAC!

 

[Jim Thompson]

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

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

...Jim Thompson

 

[Jim Thompson]

Do you need to generate a complete curve, or just a couple of points?
You could use relays to directly switch in a few resistors.

Or make a relay-driven R2R ladder. A relay DAC!

John, That's quite a nice idea to make an isolated adjustable
resistor! I've done similar things non-isolated using MOS switches,
for an AGC.

...Jim Thompson