LM335 precision?

On a sunny day (Sun, 26 Apr 2015 01:10:32 -0700 (PDT)) it happened Sonnich Jensen wrote in :

Yes, 25.0 + 100.0 * (2.96 - 2.982) = 22.8

It is not sitting one some resistor or transistor?

Datasheet for LM335 says: Uncalibrated temperature typical 2, maximum 6 degrees C.

Needs a 10 turn :-)

An interesting thing about temperature sensors, whether semiconductor or a thermistor, etc., is that they do pretty well if you calibrate out the initial offset. Keep in mind that 20 degrees C is 293 Kelvin, so a say 6-degree offset is actually only a 3% error. Which isn't bad for a jellybean 55-cent part (ST's price).

On a sunny day (26 Apr 2015 06:05:02 -0700) it happened Winfield Hill wrote in :

Sometimes a simple Si diode works just as well. if you have an ADC you can calibrate in software and store the calibration value in EEPROM. I did a temperature alarm in a big industrial cabinet with a Si diode in the top in the cooling airflow (was 10kW BTW) and used a ten turn pot and a comparator. Had to give alarm at 37 C, so used body temp (hold the diode) to set the level.

There is also temp_pic of course:

Have you actually read the datasheet (not just the blurb on the front page)?

the cheap version, and for the expensive 'A' version it's guaranteed

version) over temperature.

below what is normally considered acceptable for minimum indoor temperature by H&S folks. (20-23.5 Winter/23-26 Summer).

Chances are very good that if you bought the expensive A version that it's your measurements that are wrong. Even the cheap version should be within a few degrees C.

--Spehro Pefhany

Best regards, Spehro Pefhany

I'm partial to thinfilm platinum RTDs, leaded or surface mount. They are very accurate uncalibrated, and you can run them at low current or pulse/mux them to keep self-heating down. Or put big copper pours on the PCB, thermal wings, for the surfmount ones.

Here's one epoxied into a soda straw for outdoor use. No danger of RF pickup causing temperature errors.

Nice. We use them as axillary sensors on instrument PCBs to do some kinds of compensation.

Pt RTDs (wide temperature range) and super cheap precision thermistors (for limited temperature range) are the best accuracy bang-for-buck. Thermocouples are great, but carry a bit more overhead. Semiconductor sensors can't compete yet in a lot of markets (and never will in many).

You would have thought they would have figured out how to make an

HVAC market by now, but I guess it's not a priority for them, doesnt' fit with their standard test procedures/equipment or something.

--sp

Best regards, Spehro Pefhany

The support electronics for an RTD would be wa-a-ay over-budget in an HVAC application. ...Jim Thompson

Yeah, if they can sell a transistor or a mosfet for 2 cents, why not a temperature sensor (or controller!) ?

Why? All it takes is a reference resistor and some sort of ratiometric digitizer. But HVAC is narrow temperature range, inaccurate requirement, so a 1-cent thermistor is good enough.

Trimming cost? ...Jim Thompson

Most of the HVAC uses base-metal RTDs or thermistors. Some commercial stuff uses platinum RTDs. The support electronics is super cheap.

Resistors are trimmed, and cost a fraction of a cent.

They probably don't have trimming equipment that controls (or

the equipment they have.

The RTD guys have it worked out, because if they tried to foist the typical semiconductor sensor levels of accuracy on us they'd probably be out of business. I think TI has just recently introduced some that are about good enough for room thermostats without trimming.

--Spehro Pefhany

May i ask you yet again to have better contrast of your drawings? Grey on grey is not exactly conducive to readability or reproducibly. Thanks.

• Time is money. If it takes over one second to have the sensor settle

Huh? A transistor for two cents IS a temperature sensor. If you want 1 degree C accuracy, though, it needs 0.3% current source and voltage resolution...

So many unwarranted assumptions in a single sentence...

Best regards,

-- Spehro Pefhany

Sure. Just add a good ADC, and then calibrate it.