The excellent linearity of integrated-circuit temperature sensors makes them ideal for direct analog compensation of the thermally induced errors inherent in many circuits. For example, a negative-sloped temperature sensor could compensate for an increase in amplifier bias current resulting from a rise in temperature.
Some circuits, however, exhibit a positive temperature coefficient in one temperature range and negative coefficient in another. One example is the frequency drift of XT-cut crystals, which have a parabolic curve with the center frequency typically specified at 25°C. The dual-slope temperature sensor circuit in Figure 1 combines one negative-sloped and one positive-sloped temperature sensor to create a V-shaped output, which can be used to compensate for a dual-temperature-coefficient thermal response.
The transfer function of the LM19 (IC1) can be closely approximated at room temperature as a straight line with the equation:
where T is temperature in °C. The transfer function of the LM61 (IC2) is a straight line with the equation:
The two lines are plotted in Figure 2, which shows that the temperature of intersection (TINT) is 58.55°C. Since the LM19 and LM61 essentially do not sink current (less than 10 µA), VOUT will be equal to whichever device has the higher output voltage. This results in a V-shaped output curve with a minimum at TINT.
To use this circuit to compensate for a dual-temperature-coefficient circuit, it is desirable to have the minimum of the V-shaped curve occur at the same temperature as the maximum of the thermal response being compensated. To reduce TINT to the desired temperature (e.g., 25°C), an additional offset voltage VOS is added to the LM61 via the R1-R2 voltage divider, giving a new equation for the LM61 output:
VOS is calculated by setting equations 1 and 3 equal to each other and substituting the desired TINT for T. For a TINT of 25°C, the value of VOS is 0.728 V. The values of R1 and R2 must therefore be selected such that
I1 is determined by R1 and R2, and it should be set to significantly more than the quiescent current through the LM61 (125 µA max) in order to reduce errors caused by this quiescent current passing through R2. Choosing I1 to be 10 times greater than the quiescent current gives the condition:
Solving the two equations 4 and 5 for the two variables R1 and R2 results in the values R1 = 398 Ω and R2 = 582 Ω. The circuit in Figure 1 uses the standard resistor values of 360 Ω and 560 Ω to satisfy Equation 5 and gives a ratio of 0.636, very close to that of Equation 4.
Finally, R3 was selected so that I2 is within the operating range of the LM4041 voltage reference.
Figure 3 shows the output of the dual-slope temperature sensor circuit, measured over a temperature range of -25°C to 85°C. The average error over the full temperature range was 25.4 mV (approximately 2.5°C), and the maximum error was 46.4 mV (4.6°C).
hi there, just wondering about this temperature sensor, is it measuring the wet temperature, or the dry 1? as now i m doing my final year project in electronic, i couldnt find any idea about the dry temperature sensor. pls help me, or give me any idea... thanks~!
Jason KS -May 15, 2008
i'm doing my final project which using temeperature sensor as my main component...i need u to help my design the circuit with this condition
my project consist of 2 temperature sensor.during normal condition, the sensor will only detect atmospheric temperature but if something burning the room, the alarm will be activated. both sensor's reading are compared using microcontroller..the alarm will be activated if only the comparison reach certain value...the reading also being recorded into the system every half an hour....please help me to design this project....
firdaus -March 11, 2008
i want circuit diagram and its assembly language by using analog to digital converter
Anonymous -February 28, 2008
PANNA PULAYADIMONE NIRTHEETTU ANDI ADIKKAN PODA
CEETHAL MARY COCHIN -January 22, 2008
PODA POOORE
MINU P -January 22, 2008
please am planning on my final year project and i need info on any project topic i can work with. am studying computer engineering, anyone with an idea??? please help me out
Agbara Kelechi -June 25, 2007
Click the links you idiots.
Anonymous -March 26, 2007
i need pictures of the thermometre circuit, this would help understand xx
jenifer keeley -January 04, 2007
i hate you i want a circuit you poo
From Chris Evans
xx
jon dean -May 19, 2006
i hate you and want to kill you
Anonymous -May 19, 2006 (Article Rating: )
reason is i need a circuit diagram of a digital thermometer. not a mathermatical equation to get a rocket to the moon.
Anonymous -May 07, 2006
utter Tripe
Anonymous -May 07, 2006 (Article Rating: )
I need to now how to construct a heat sensor using a thermistor transistor 2 resistors a motor and 9v battery on a piece of copper strip board
tony yayo -May 01, 2006
does any1 *** on dis site its not of any help
50 cent -May 01, 2006
pure rubbish
50 cent -May 01, 2006
poda pulle
Anonymous -April 10, 2006
i need some images of the circuit of the temperature sensor...please i really need it to do my research as soon as possible.
radioheadbuz -October 19, 2005 (Article Rating: )
hellow, we want some graphical image(photographs) of temp sensors like LM35.
sweta -April 07, 2005
I want specification details for a heat sensor circuit output signal to go though digital signals.
Indunil -February 14, 2005
Nice work indeed.
shyam -January 12, 2004
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