8.1 Application Information

To insure good sensing accuracy, several precautions must be taken. Like any temperature-sensing device, self-heating can reduce accuracy. The LM135 should be operated at the lowest current suitable for the application. Sufficient current, of course, must be available to drive both the sensor and the calibration pot at the maximum operating temperature as well as any external loads.

If the sensor is used in an ambient where the thermal resistance is constant, self-heating errors can be calibrated out. This is possible if the device is run with a temperature-stable current. Heating will then be proportional to zener voltage and therefore temperature. This makes the self-heating error proportional to absolute temperature the same as scale factor errors.

8.2 Typical Application

Figure 13. Basic Temperature Sensor

8.2.3 Application Curve

Figure 14. Reverse Characteristics

8.3 System Examples

Figure 15. Wide Operating Supply

Wire length for 1°C error due to wire drop

Figure 17. Average Temperature Sensing

Figure 19. Simple Temperature Controller

Adjust R2 for 2.554V across LM336.

Adjust R1 for correct output.

Figure 21. Ground Referred Fahrenheit Thermometer

To calibrate adjust R2 for 2.554V across LM336.

Adjust R1 for correct output.

Figure 23. Fahrenheit Thermometer

Figure 16. Minimum Temperature Sensing

Figure 18. Isolated Temperature Sensor

Figure 20. Simple Temperature Control

Adjust for 2.7315V at output of LM308

Figure 22. Centigrade Thermometer

8.3.1 Thermocouple Cold Junction Compensation

Compensation for Grounded Thermocouple

Select R3 for proper thermocouple type

Figure 24. Thermocouple Cold Junction Compensation

THERMO-COUPLE	R3 (±1%)	SEEBECK COEFFICIENT
J	377 Ω	52.3 μV/°C
T	308 Ω	42.8 μV/°C
K	293 Ω	40.8 μV/°C
S	45.8 Ω	6.4 μV/°C
Adjustments: Compensates for both sensor and resistor tolerances Short LM329B Adjust R1 for Seebeck Coefficient times ambient temperature (in degrees K) across R3. Short LM335 and adjust R2 for voltage across R3 corresponding to thermocouple type. J 14.32 mV K 11.17 mV T 11.79 mV S 1.768 mV

THERMO-COUPLE	R3	R4	SEEBECK COEFFICIENT
J	1.05K	385Ω	52.3 μV/°C
T	856Ω	315Ω	42.8 μV/°C
K	816Ω	300Ω	40.8 μV/°C
S	128Ω	46.3Ω	6.4 μV/°C
Adjustments: Adjust R1 for the voltage across R3 equal to the Seebeck Coefficient times ambient temperature in degrees Kelvin. Adjust R2 for voltage across R4 corresponding to thermocouple.
J	14.32 mV
T	11.79 mV
K	11.17 mV
S	1.768 mV

Select R3 and R4 for thermocouple type

Figure 25. Single Power Supply Cold Junction Compensation

Figure 27. Differential Temperature Sensor

Adjust D1 to 50 mV greater VZ than D2.

Charge terminates on 5°C temperature rise.

Couple D2 to battery.

Figure 29. Fast Charger For Nickel-Cadmium Batteries

Figure 31. Ground Referred Centigrade Thermometer

Terminate thermocouple reference junction in close proximity to LM335.

Adjustments:

1. Apply signal in place of thermocouple and adjust R3 for a gain of 245.7.

2. Short non-inverting input of LM308A and output of LM329B to ground.

3. Adjust R1 so that VOUT = 2.982V @ 25°C.

4. Remove short across LM329B and adjust R2 so that VOUT = 246 mV @ 25°C.

5. Remove short across thermocouple.

Figure 26. Centigrade Calibrated Thermocouple Thermometer

Figure 28. Differential Temperature Sensor

Adjust for zero with sensor at 0°C and 10T pot set at 0°C

Adjust for zero output with 10T pot set at 100°C and sensor at 100°C

Output reads difference between temperature and dial setting of 10T pot

Figure 30. Variable Offset Thermometer

*Self heating is used to detect air flow

Figure 32. Air Flow Detector