16f877 with LM335 problem

[Aussie Barry]

Why not use the LM35?
This sensor is calibrated directly in Celcius, has a linear 10mV/degree C scale factor, is rated for -55 to +150 C range and does not require any external calibration or trimming to provide typical accuracies of =/-0.25C at room temperature and +/-0.75C over the full -55 to +150C range.

Cheers
Barry
VK2XBP

[flotulopex]

Thanks for the idea Barry.

It's just, I have a LM335 in the drawer....

I can give it a go with the LM35 - need to order one but here, in Switzerland, the LM35 is ten times more expensive than the LM335

Anyway, I would still like to understand how the LM335 could be used with an opamp.

[Ioannis]

Attached is one idea. First op-amp works as an adder (or subtracter). It removes the DC offset voltage from the + input. So output is around 0-0,58 volts for -30 to 30 deg.

Then the second op-amp works as an DC amplifier to boost the 0,58 to 5 volts. So you have to make two adjustments, trimmer R9 at 2,4315 volts and R10 to get 5 Volts for 30 deg.

Also very critical is to select op-amps that are able to work at 5Volts and be rail to rail. I thing LMC662 is such a chip.

Ioannis

[flotulopex]

Ioannis,

Thank you very much for the schema and your comment.

I need to check what the opamps in my drawer can do and if they don't, I'll order some to test this.

It makes it quite a "large" circuit in comparison of the usage of a single LM35.

What would be the advantage of this circuit vs a LM35 please?

[Ioannis]

If it seems too complicated, you can remove R5, R6, R8 making exact adjustment more critical.

The circuit will give you more resolution for the range you asked, -30 to +30 for a 0 to 5 volts output or about 0,083 volt/deg celsious.

Also it gives you an output for negative temperatures.

For the LM35 you will get 10mV/C. This means you will get from 0-0,3 volts for 0-30 C. If you want to measure from -30, then you must raise the ground of the LM35 above say 0,3 or better 0,6 volts. Maybe with a diode. Your output then will not be referenced to ground but from the pins of the LM35 (out and LM35 gnd). Your span for -30 to +30 will be 0,6 volts. If you need to drive an ADC then it would be better to amplify this small signal too.

HTH,
Ioannis

[Dave]

I think you are better off with the LM335 as it's output is 10mv/deg K. The LM35 is 10mv/deg.c. There is going to be a problem with the accuracy below 0 deg C. when using the LM35. If you would like rather than analog you could use a TMP-03/04 from Analog Devices which is a pulse width output. I have used these before and work quite well. Just a thought...

[flotulopex]

I think you are better off with the LM335 as it's output is 10mv/deg K. The LM35 is 10mv/deg C.

Hum...can you explain a little more please?

I thought Kelvin were used to express a temperature difference and Celsius/Fahrenheit would represent a value. I must be wrong.