Help with LM335 circuit

The 0.5 degree accuracy relates to the absolute accuracy not the relative accuracy or precision of the sensor. You should be able to get the precision you need by amplifying the signal with an op amp as Eric suggested.

With regards to the circuit diagram, I'm a little confused with U2. U1 and 3 appear to be Pins 1-3 and 5-7 of the Op-Amp but U2 looks like a Pot. Is it a 10K pot?

R2 and R4 are pots. Does the values in black imply the resitance setting for these pots?

Cheers

David

higginsdj said:

With regards to the circuit diagram, I'm a little confused with U2. U1 and 3 appear to be Pins 1-3 and 5-7 of the Op-Amp but U2 looks like a Pot. Is it a 10K pot?

R2 and R4 are pots. Does the values in black imply the resitance setting for these pots?

Cheers

David

Click to expand...

Hi David,
You are correct R2 & R4 are pots and the 'black' value is the approx setting of the pot, you may need to slightly adjust them to get the absolute ranges you want.

Look at the LM335 d/sheet for the function of the 10K pot U2.

I guess you know that the K8055 has gain adjust resistors across the adc OPA's and that the adc is set for 8 bit operation.

Thanks Eric

Ahh well now you have said you only need +/- 0.5'C that is pretty easy! You did say in post #1; "very temperature sensitive geomagnetic sensors"

Are you planning to put magnetic shielding around the LM335?

Hi RB,

We are 'winging it'. There are about 80 of us world wide with various models of magnetometers. Some people bury their sensors in the ground, some put them in insulated boxes in a basement and the others - well they are rather silent on what they have done. The buried in the ground versions are the most 'stable' but given these instruments sensitivities to other things (like a person walking by them) we are 'guessing'. It is all still very experimental.

I am choosing +/- 0.5c because I cannot see that I could maintain a stable temperature better than that without going to extraordinary lengths. I should point out that we are interested in the relative changes in the magnetic field, not absolute values (we are trying to predict Aurora activity) so shielding the LM335 or associated circuit is likely not necessary since it will form part of the magnetic sensors environment and as long as the environment doesn't change - all is good.

The other point is that I want to log temperature with the changing field values so I can see, over time, the relationship between temperature and field value changes and make adjustments in software. The other advantage I have is that I have 2 professional magnetometers located nearby to compare by results with (one 6km SW and another 7km SE).

Sounds like a fun experiment!

You might want to test putting the LM335 and it's wiring etc in the vicinity of the mag sensor to see if it affects the sensor reading.

If you run the LM335 from a constant current (I think they need about 0.5mA as a minimum current) then ideally any mag field it generates will have a reasonable fixed field strength. Then if you secure all the wiring with glue everything should be constant so *ideally* it might not affect your relative measurements.

Looks like this project may be more problematic than first envisioned. It appears that I need to thermally insulate the shielded cables from the processor to the sensors as the temperature variations delivered by the cables are enough to destablise the geomagnetic sensors.

Cheers

David