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'[PIC]: current generator with a PIC'
2001\12\27@163058 by ashly Dearden

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I'm thinking of doing some temperature sensing using the approach that
Dallas and Maxim has with the current source and measuring the voltage
across a diode-connected transistor.  They don't go into alot of detail on
what they are doing...other than around 100uA of current. I assume this must
be a stable current source and just measure the voltage that is generated?
I'd like to do this in just a PIC, since its there doing some other stuff as
well.  The alternate is to use the MAX1805 and interface the I2C to the PIC.
But one chip is better than two...

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2001\12\27@174046 by Dave Dilatush

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ashly Dearden wrote...

>I'm thinking of doing some temperature sensing using the approach that
>Dallas and Maxim has with the current source and measuring the voltage
>across a diode-connected transistor.  They don't go into alot of detail on
>what they are doing...other than around 100uA of current. I assume this must
>be a stable current source and just measure the voltage that is generated?

Chips like the MAX1805 are a bit more sophisticated than that; they're
actually pretty clever.

Simply taking a diode (or diode-connected transistor) and putting a
constant current through it will give you a temperature-dependent
voltage that you could measure; and its temperature coefficient will be
pretty predictable, at -2.1 mV per degree C; but different diodes and
transistors will give you very different voltage drops at any given
temperature, making it necessary to calibrate each individual sensor and
making interchanging of sensors very difficult.

What MAX1805-type devices do is modulate the current through the diode
or transistor, alternating between two levels of current that have a
fixed ratio to one another- 2:1, 3:1, 10:1, etc.  The chip measures the
diode voltage drop at each level and subtracts the two voltages,
measuring the difference.

It turns out that this difference in voltage at the two current levels
is a function of temperature and current ratio only, and is very nearly
independent of diode/transistor characteristics and the actual current
levels used.  The voltage difference is proportional to absolute
temperature.  Transistors/diodes can be interchanged without more than a
couple of degrees C error.

>I'd like to do this in just a PIC, since its there doing some other stuff as
>well.  The alternate is to use the MAX1805 and interface the I2C to the PIC.
>But one chip is better than two...

If you try to replicate the functionality of the MAX1805 with discrete
stuff, you'll very likely end up with way more hardware than if you just
used the MAX1805 itself.

Sometimes Maxim just can't be beat...

Dave

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2001\12\27@191350 by Brandon Stewart

picon face
unless you are just out to experiment, I would just use a national LM34 or
an analog devices tmp04F79





{Original Message removed}

2001\12\28@021439 by Dwayne Reid

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At 10:38 PM 12/27/01 +0000, Dave Dilatush wrote:
>ashly Dearden wrote...
>
> >I'm thinking of doing some temperature sensing using the approach that
> >Dallas and Maxim has with the current source and measuring the voltage
> >across a diode-connected transistor.  They don't go into alot of detail on
> >what they are doing...other than around 100uA of current. I assume this must
> >be a stable current source and just measure the voltage that is generated?

L Stephen Woodward did a design idea several years back in Electronic
Design or EDN (I don't remember which) but he credits Jim Williams of
Linear Technology for the idea.

Its actually pretty simple: modulate the forward current by 10:1 (100 uA, 1
mA).  AC couple the resulting voltage change to an amplifier and use a
synchronous detector to rectify the output.  Feed to an A/D convertor.

I did a simple version a few years back where a PIC pin supplied the
modulation current.  Calibration was simple: just adjust the gain (in
software) so that the display agreed with the actual temperature.  Because
it was AC coupled, there is only a gain adjustment required: AC signal
amplitude is a straight line from absolute zero (0K) to whatever
temperature is being measured.  Simple and accurate!  Long term accuracy is
dependent on supply voltage but I use LP2950 regulators which seem to have
adequate long term accuracy.

I did find that diode connected transistors (B & C tied together) worked
much better as sensors than just simple diodes - I could swap in a 2n4401
or 2n4403 as the sensor without affecting calibration.

I've got source code (12ce673) and a schemo somewhere - it was part of a
project that got put on the back burner.

dwayne



Dwayne Reid   <dwaynerspamKILLspamplanet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax

Celebrating 17 years of Engineering Innovation (1984 - 2001)

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2001\12\28@025719 by Peter L. Peres

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There is a National sensor that works like that (more or less), the LM35 I
think.

Your problem is to detect the -2.2 mV/K signal from the sensor which will
have to be calibrated individually. Connected directly to a 10 bit A/D at
5V supply you can only read about 1 deg. C. Maybe this is enough for you,
else there are schemes to make the sense transistor an amplifier too. They
are complicated to get working other than in ones (i.e. calibration
problems).

Peter

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2001\12\28@040603 by Roman Black

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face
ashly Dearden wrote:
>
> I'm thinking of doing some temperature sensing using the approach that
> Dallas and Maxim has with the current source and measuring the voltage
> across a diode-connected transistor.  They don't go into alot of detail on
> what they are doing...other than around 100uA of current. I assume this must
> be a stable current source and just measure the voltage that is generated?
> I'd like to do this in just a PIC, since its there doing some other stuff as
> well.  The alternate is to use the MAX1805 and interface the I2C to the PIC.
> But one chip is better than two...


If you are interested in minimum parts count
a thermistor is cheap and smaller than the
transistor and current source solution. Both
systems will require compensation in software.
Or use Jinx' nifty PIC thermometer which uses
the watchdog timer referenced against the
main clock crystal. How accurate do you need?
I wish people would state EXACTLY what their
needs are when asking for help. :o)
-Roman

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2001\12\29@014057 by ards, Justin P

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face
I have been reluctant in the past to precisely state the requirements as I
believed it would consume too much of peoples time reading, so I tried to
summarise.

Have learnt since that it is efficient to be precise and detailed initially

{Original Message removed}

2001\12\29@050829 by Peter L. Peres

picon face
Dwayne, I missed the fact that you do tie B and C together, disregard this
in my previous post. Can you supply a helpfull pointer wrt. theory ? Is
this based on the fact that both probe currents are several orders of
magnitude above the saturation current of the junction(s) ?

tia,

Peter

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2001\12\29@050847 by Peter L. Peres

picon face
Dwayne, the reference you mentioned cannot be traced down for some reason,
so could you please elaborate a little about what you used in the
practical circuit ?

Like, what is the expected AC pk-pk level (approximately) on the sensor at
room temperature ?

And do you tie C and B together on the transistors used for sensors ?

Peter

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2001\12\30@185431 by Dwayne Reid

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At 10:15 AM 12/29/01 +0200, Peter L. Peres wrote:
>Dwayne, the reference you mentioned cannot be traced down for some reason,
>so could you please elaborate a little about what you used in the
>practical circuit ?
>
>Like, what is the expected AC pk-pk level (approximately) on the sensor at
>room temperature ?
>
>And do you tie C and B together on the transistors used for sensors ?
>
>Peter

I had a look for the files on my laptop - I have PIC code but don't have
the schematic here - its either on the server back at my shop or may even
still exist only in paper form in my note book (also back at my
shop).  Unfortunately, that is all about 1200 KM away from me right now -
I'm at my parents home for the holidays.

Based upon memory and what I see in the code, I used a 1 meg resistor from
+5V biasing the sensor and switched a 100K resistor in parallel with the 1
meg resistor at about a 40 Hz rate (the backplane rate for my LCD
display).  I think that the amplifier gain was about 70 - a LM324 is
adequate because of the AC coupling.  The synchronous detector is 1 section
of a 4053 analog switch: the output of the amplifier is a 10 uF cap tied to
the armature of the analog switch, when only the 1M resistor is tied to the
sensor, the output cap is switched to GND.  When the 100K resistor is in
parallel with the 1M resistor, the output cap is switched to another 10 uF
cap tied to the input of the a/d input.  Amplifier gain is set so that the
a/d does not over-range while measuring the highest temperature desired but
is otherwise not critical.

I used the 8 bit a/d convertor of a 12ce673 - but with a dither source and
oversampling.  I wind up with a noisy 14 bit result that is then heavily
filtered for an effective accuracy of 10 or 11 bits.  Native 8 bit accuracy
is not enough because of the 273C offset between 0K & 0C.

As far as the math is concerned, the function is a straight line that
passes through absolute zero.  The slope is dependant on gain and sensor
temperature.  All math and calibration is done in degrees Kelvin and
converted to degrees C for display.

The detail I do not remember for sure is amplifier gain - but its not all
that important!  Build the circuit, measure the output of the
detector.  Adjust amp gain so that the a/d does not over-range at the
highest desired temperature.

My implementation (hardware wise) is almost identical to L Stephen
Woodward's design: all 3 sections of a 4053, a LM358 dual op-amp (VCC tied
to unreg supply of +17Vdc), something like 8 resistors and 4 capacitors
gave me all the processing needed for 2 temperature sensors made of 2n4401
transistors.  I wrote an initial pass of the software to prove the
concept.  Then I shelved it!

I'll dig it up when I return and probably post it on Piclist.com.

dwayne

PS: I think the name of Mr Woodward's article was: Low Cost Precision
Thermometry.  Try doing a web search on that string.



Dwayne Reid   <.....dwaynerKILLspamspam.....planet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax

Celebrating 17 years of Engineering Innovation (1984 - 2001)

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2001\12\30@190918 by Dave Dilatush

picon face
Dwayne Reid wrote...

>PS: I think the name of Mr Woodward's article was: Low Cost Precision
>Thermometry.  Try doing a web search on that string.

I didn't get anything off google on that string, but Linear Technology
application note AN-45 has what I think is Jim Williams' original
circuit of the "delta-Vbe" thermometer, at:

http://www.linear-tech.com/pdf/an45.pdf

on page 7.  It has a pretty good explanation of the circuit's principle
of operation, and should be easily adaptable to run off a 5V supply for
input to a PIC's A/D converter.

Dave

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2001\12\30@192203 by Dwayne Reid

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At 12:06 AM 12/31/01 +0000, Dave Dilatush wrote:
>Dwayne Reid wrote...
>
> >PS: I think the name of Mr Woodward's article was: Low Cost Precision
> >Thermometry.  Try doing a web search on that string.
>
>I didn't get anything off google on that string, but Linear Technology
>application note AN-45 has what I think is Jim Williams' original
>circuit of the "delta-Vbe" thermometer, at:
>
>http://www.linear-tech.com/pdf/an45.pdf
>
>on page 7.  It has a pretty good explanation of the circuit's principle
>of operation, and should be easily adaptable to run off a 5V supply for
>input to a PIC's A/D converter.

I'm on-line for a few minutes and found the following info via google:

An excellent implementation of the idea is "Low-Cost Precision Thermometry
by W. Stephen Woodward which won the Best of Issue of Electronic Design for
August 21, 1995 and was reprinted on pg. 32 in the October 24, 1996 "Best
Ideas for Design" supplement to Electronic Design

So: check Electronic Design archives for those dates

dwayne



Dwayne Reid   <EraseMEdwaynerspam_OUTspamTakeThisOuTplanet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax

Celebrating 17 years of Engineering Innovation (1984 - 2001)

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2001\12\31@122612 by Peter L. Peres

picon face
Happy New Year everyone.

Dwayne, thank you for the detailed description, I am looking forward for
more details later.

From what you describe there seems to be a lot of hardware involved ...
I'll probably try it the nasty way:

A/D less PIC, sensor transistor mounted as common emitter amplifier and a
cap between C and gnd, and a R to a PIC IO pin. And a good quality voltage
regulator (a Seiko probably). Maybe I'll replace the PIC side resistor
with a JFET, and bridge it with a large R, this has worked well in the
past (with Up < 2V), to improve A/D linearity.

How much precision did you achieve ?

thanks,

Peter

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2001\12\31@163331 by Dwayne Reid

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At 07:18 PM 12/31/01 +0200, Peter L. Peres wrote:
>Happy New Year everyone.
>
>Dwayne, thank you for the detailed description, I am looking forward for
>more details later.
>
> From what you describe there seems to be a lot of hardware involved ...
>I'll probably try it the nasty way:
>
>A/D less PIC, sensor transistor mounted as common emitter amplifier and a
>cap between C and gnd, and a R to a PIC IO pin. And a good quality voltage
>regulator (a Seiko probably). Maybe I'll replace the PIC side resistor
>with a JFET, and bridge it with a large R, this has worked well in the
>past (with Up < 2V), to improve A/D linearity.

This sounds interesting - I'm looking forward to hearing your results!  But
I'm curious: will the calibration change if you use a different transistor
as the sensor?  I was looking for something that would not require
re-calibration if the sensor was changed.

>How much precision did you achieve ?

I only displayed whole degrees C - I wasn't trying for anything
better.  The spot checks I did were within 1 digit of my lab K-type
thermometers (both recently calibrated) - that was as good as I ever hoped
to achieve.

The project was originally going to use thermistors but I wanted to try
something different.  I got it working but then the whole thing got put on
the shelf - the client had other (higher priority) projects.  But I'll be
resurrecting it soon.

dwayne



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Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax

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'[PIC]: current generator with a PIC'
2002\01\02@104605 by Roman Black
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Dwayne Reid wrote:

> This sounds interesting - I'm looking forward to hearing your results!  But
> I'm curious: will the calibration change if you use a different transistor
> as the sensor?  I was looking for something that would not require
> re-calibration if the sensor was changed.

> The spot checks I did were within 1 digit of my lab K-type
> thermometers (both recently calibrated) - that was as good as I ever hoped
> to achieve.
>
> The project was originally going to use thermistors but I wanted to try
> something different.  I got it working but then the whole thing got put on
> the shelf - the client had other (higher priority) projects.  But I'll be
> resurrecting it soon.


Is it possble the transistor junction
characteristics will age over time causing
errors with age? How large is the junction
current?
-Roman

PS. this should probably be [EE:] now!

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2002\01\02@201902 by Dwayne Reid

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At 02:40 AM 1/3/02 +1100, Roman Black wrote:
>Dwayne Reid wrote:
>
> > The spot checks I did were within 1 digit of my lab K-type
> > thermometers (both recently calibrated) - that was as good as I ever hoped
> > to achieve.
> >
>
>Is it possble the transistor junction
>characteristics will age over time causing
>errors with age? How large is the junction
>current?

I checked - current is switching between about 5 uA and 55 uA.  Not much at
all.

I don't expect any aging effects.

dwayne


Dwayne Reid   <RemoveMEdwaynerTakeThisOuTspamplanet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax

Celebrating 17 years of Engineering Innovation (1984 - 2001)

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