Temp readings and voltage input

[ruijc]

Hi all,

i'm my 16F88 temp monitor project i have a LM35 sensor with no vref.
The input voltage for the workbench was always 5v and everything is working perfectly.

After moving for the pcb i have found an issue that puzzled me.

For the PCB aplication i have it connected ( via 7805 ) to a 4.8V NiCd pack. This pack when fully loaded is 5.3V and after some use it drops to +-4.5V.

This is causing some bad temperature readings when the pack drops bellow 5v.

The code i used for temperature is like this :

temp= (in */5000)>>2

I have no vref because i can live with the 0.5ºC stepping ( 10Bit resolution ) and this code allows me to have an output like XXX,X.

My question is:

Is there a better code to be flexible with the input voltage and still have accurate readings ?

Thanks

[Acetronics2]

The answer is simple ...

Use a Voltage Ref !!! a TL431 i.e is cheap, TO92 shaped, and easy to trim ...

OR use a Temp to Digital ( DS18B20 i.e.) or temp to PWM ( SMT 160-30 i.e ) sensor ...

No miracle to wait for !!!

Alain

[sougata]

Hi,

I personally prefer voltage references from the Microchip Analogue familiy. MCP1541 is which I use most. While using TL431 cap values are important and improper design causes noise which would set things up. Find the attachment and open in Microsoft Excel. It is a TL431 calculator. Make sure macros are turned on from your Excel security settings.

[Acetronics2]

Hi, Sougata

Thanks for the XLS sheet ...

TL431 is to consider like a Zener diode ... noise involved !!!

I always use a solid aluminium 10µF cap ( Alusol C 122 ) or tantalum cap ...

With a good 4.5 digits multimeter for trimming - and quality components - ... performance is to compare to MAX or AD best voltage refs ...

Not so bad, for such an oldie !!!

Alain

[ruijc]

Thanks Acetronics,sougata,

The idea for the digital sensor has crossed my mind. Actually i would prefer but i have the problem that the DS18B20 does not reach 150C ( that i what ) and the SMT 160-30 is not available here

I guess i will have to use the Vref. and have 2,5V there.

I never used the vref on this chip. Correct me if i'm wrong here:

In the data sheet it's marked Ra2 and RA3 as Vref- and Vref+.

I may use only the Vref+ pin and set the ADCON1 REGISTER (ADDRESS 9Fh) like :

ADCON1=%10100000

And i have to change the code to:

temp= (in */2500)>>2

Is this right ?

[Acetronics2]

Ha,ha,ha ...

I think VENOM might tell you something !!!

Now, this would be much easier to use a NTC Thermistor or a KTY 81-1xx NTC sensor.

Just use a Lookup table with a linear regression between points ... and HOP, in the pocket ( as told the Kangaroo ... )

...

BTW ... choose your "gas" carefully ... lots of degrees to win here !!!

see :

http://www.carburant-modelisme.com/

and

http://www.meccamo.com/fr/indexFR.htm

for really serious blenders ...

Alain

[ruijc]

Acetronics,

you lost me there...

.