16f877 with LM335 problem

[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.

[Ioannis]

Deg Kelvin references differently. 0 deg Kelvin is the absolute zero. The point that every thermal motion stops. To convert it to Celsius, just add 273,15. So the 0 Celsius is 273,15 Kelvin and 0 Kelvin -273,15 celsius.

Ioannis

[flotulopex]

Thanks Ioannis.

This (schema) is where I have come up to now - using the simple simplest, maybe too simple (...) The opamp acts as a voltage doubler (seems enough for what I need).

BTW, I'm using a 16F690 with 10 bits ADC.

At 24°C, the LM35 outputs 1,199V so the opamp's Vout shows 2,396V.

Now, what is the LM35 going to output at 0°C or even less, around -20°C?

Since 1,199V represent 24°C, can I say that at 0°C, I will have 0,959Volts (LM35 Vout)?

Talking "temperature", what is the reference when adjusting temperature sensors at home?

Is it this?

Noooooooooooooo......

NB: You're absolutely right about Kelvins. In the field, talking with cooling systems engineers, to make sure we don't get messed up while setting up machines, we (wrongly) use Kelvins to express a temperature difference or threshold and degrees to set a temperature value