A design to do what you want is a big challange to one of your experience.
You will probably spend considerable money and a great deal of time and
still not achieve your goal
There are a multitude of RTD digital panel meters that do what you want
available in the UK
Just buy one - it will work and probably be cheaper than you can do it
yourself
Search internet for RTD temperature panel meter UK
Dan
--
Dan Hollands
1120 S Creek Dr
Webster NY 14580
585-872-2606
[email protected]
www.QuickScoreRace.com
I had a read of the MAXIM application notes, which use the MAX197 IC
and a couple of op amps for amplification. This seems to be perfect,
but the only problem is that it costs £20 for a single 28-pin DIN
MAX197, which makes it a bit unviable for me. It is a one-off personal
project, but others on my brewing forum may well want to build units
themselves.
I think I understand what's required at a high level, but my
electronics isn't up to it at the low-level. I need a range of 0C to
100C and 0.5C accuracy/resolution for my application.
High level things I think I need (high-level block diagram)
4-wire PT100 RTD
"Something" to generate the excitation current - I think this needs to
be extremely accurate and invariable.
Something to amplify the small voltage across the RTD in to the range
of an ADC. Looks like NS ML4140A-2.500 is used for this in the links
above?
An ADC to convert the analogue signal in to a format for my PIC to use.
Are there not PIC's which have on-board ADC's that I can use for this
purpose or is this not a good idea?
I think I know how to convert from the digital voltage in to a
temperature - divide the known voltage range up in to chunks based on
the bit resolution of the ADC. Say - 0-5v ADC and 10-bit, I would then
5/((2^10)-1) to get the voltage increment per bit. Then map the
'voltage' reading back to resistance (known excitation current), and
use a look-up table to 'linearise' and then interpolate to give a
reasonable approximation of temperature.
One thing I'm not sure about is how to calibrate this process.
Another is that IIRC 0C is 100R and 100C is 138.5R (for Platinum PT100)
which @ 1mA gives a voltage reading range of 100mV-138.5mV. I'm sure I
can scale this up so that the 100C resistance maps somewhere near 5
volts, but the 0C will not be near 0v. so some of the 10-bit signal is
wasted. Could I not get better accuracy (perhaps it's not needed with
10-bits) by translating the 0C voltage/resistance somewhere near 0v and
then scale/amplify so that 100C is near 5v so I'm using a larger
portion of the bit range?
Regards,
Mark