cheap RTD measurement

AlastairM · Joined: 28 Apr 2008 Posts: 28

Has anyone tried using the internal comparator on a 12F510 or similar for a relaxation oscillator to measure an RTD accurately or at least better than the 8-bit ADC that's available on these devices? (C1OUT connected internally to T0CKI to measure period and hence the RTD resistance)
http://ww1.microchip.com/downloads/en/appnotes/00895a.pdf Figure 2.
The app notes always show an external comparator but the internal one looks up to the job. I was hoping for a sanity check!
Thanks Al

temtronic · Posted: Sat Jun 08, 2013 6:05 pm

hmm...
from the apnote....

The relaxation oscillator offers a single comparator
solution for cost-sensitive applications. It is a simple
solution for an application that needs the fast thermal
response time of RTD, with a temperature
measurement accuracy approximately equal to ±3°C.

...

I do not consider +-3*C to be good enough for me! I'd rather use the LM34/35 for analog or the DS18B20 for digital . Yes, costs a bit more(maybe, what's the RTD probe cost ?) but FAR better accuracy!!

sanity check...home heating systems are good for about +-1*C...

hth
jay

AlastairM · Joined: 28 Apr 2008 Posts: 28

sorry, forgot to mention my temperatures are up to 300 degrees so I can't use the chip solution.

ezflyr · Joined: 25 Oct 2010 Posts: 1019 Location: Tewksbury, MA

Hi,

Use the MAX31855 IC, and do your temperature measurement with a thermocouple!

John

AlastairM · Joined: 28 Apr 2008 Posts: 28

Thanks,
I didn't spot the MAX31855 that's a great one chip solution. I'll get on with the PCB design ASAP!
Al