Temp readings and voltage input

[Acetronics2]

for 10 bits ...

use V ref = 4.096v or 2.048 v ... but not 2.5 v

for 4.096 v you get 1 count for 1 mv
for 2.048 v you get 2 counts for 1 mv input ...

so, your formula ... is really awful !!!

100°C = 1 v ...

with a 2.5 v ref : 1v /2.5 *1024 counts ... 409.6 > 409

409*2500/256 / 4 = 998.5 ...

you will find the good numbers easily !

Alain

[Acetronics2]

Errata !!!

for 10 bits ...

use V ref = 4.096v or 2.048 v ... but not 2.5 v

for 4.096 v you get 1 count for 4 mv
for 2.048 v you get 1 count for 2 mv input ...


Alain

[ruijc]

Thanks Alain

[Squibcakes]

Hey Ace,

The Carbsmart uses a 100K thermistor (supposedly a ntsa0wf104fe1b0). I have a spare and was thinking of using it with a pic to do some temp recordings to see how my temps are going.

I just need some advise on the circuit design. I was going to use a 5V regulator to supply the pic and ref for the thermistor. Also use a 100k resistor as a voltage divider as my analog voltage input to the pic. Is this enough?

Also, the thermistor has a current rating of 0.14ma @25C. Does that mean max current draw through the ntc over the entire temp range or what?

See attached circuit.

[ruijc]

Hi Squibcakes,

Just a question,

Does that thermistor has a linear variation with temperature ?

Where do you get this thermistor ? Any special supplier ?

Thanks

[Squibcakes]

Hi Squibcakes,

Just a question,

Does that thermistor has a linear variation with temperature ?

Suprisingly no it's not that linear. I plugged in the values from the data sheet into excel and created a graph of the curve. It's linear on the higher temps (say 90C +). I wired this up on the bench as per the diagram I posted above and it seems to be working. I've seen mention of the LM35 sensor which is linear and has a higher max temp range which I think would be better but finding it hard to source these...

Where do you get this thermistor ? Any special supplier ?

RS sell them pretty cheaply.
http://australia.rs-online.com/web/s...tsa0wf104fe1b0


Thanks

[ruijc]

The LM35 is very easy to find. Any local store should have it.

The problem with non linear thermistors is the math.
I dont know how to program in a way that the result is always accurate from the "real life".

The LM35 is linear and accurate and is capable of temps up to 150º. You can easly program and get true results.
For applications as the one you want ( model engines ) , you will need a sensor that can go over 135º.

The only problem i see is to get the package of the LM35 fit inside the engine fins.
You could sandpaper off a bit of the package but you will lose accuracy.

.

[Acetronics2]

Hey Ace,

The Carbsmart uses a 100K thermistor (supposedly a ntsa0wf104fe1b0). I have a spare and was thinking of using it with a pic to do some temp recordings to see how my temps are going.

I just need some advise on the circuit design. I was going to use a 5V regulator to supply the pic and ref for the thermistor. Also use a 100k resistor as a voltage divider as my analog voltage input to the pic. Is this enough?

Also, the thermistor has a current rating of 0.14ma @25C. Does that mean max current draw through the ntc over the entire temp range or what?

See attached circuit.

Hi, Squib

as always ... it relies on what you want to do !

your scheme works fine if you just want to switch a relay ( a home thermostat generally works between 17 and 23 °C !!! ) or run an alarm ... NTCs perfectly can be used on limited temp ranges where you use a linear or 2nd order approx.

now, you also can use a retrive table ( You did it with Excel, it seems ... ) and linearize between points: look at Microchip AN's !!!

The main problem is NTC's often come with 10 or 20% nominal value tolerance ... so, it's not possible to get something precise without any calibration ( my home thermostat or my car temp indicator offer you to correct the read temp by adding or substracting 0 to 5 degrees ... simplest and easiest way !!! )


For your thermistor current rating 0.14 ma ^2 x 100 000 = 2 mW ...

let's suppose it's the maximum current not to disturb the thermistor temp. by self heating !!!



Now, ... once more, the sensor will be chosen from the temp range and the use you will do with the output ...

trying to know a model engine head temp @ +/- 1°C is a nonsense ... as you won't be able to place the sensor very close to the combustion chamber ... you just measure "something indicative ... " and the same measurement, with cold ot hot weather do not show ... not negligible ... temp changes at the "critical" point !!!

Alain