Temperature measures application very extensive, produce craft to need temperature control not only, some electrons product still needs to undertake metrical to the temperature of their oneself, if the computer wants the temperature of monitoring CPU, motor controller should know the temperature of power drive IC is waited a moment, a few kinds of commonly used temperature sensor introduce below. 　

 

Temperature is the parameter that the test often needs in applying actually, make semiconductor production from iron and steel, a lot of craft should rely on temperature to come true, temperature sensor is the bridge between applied system and real world. The article has brief introduction to different temperature sensor, and the interface between the introduction and circuit system. 　

　

Thermistor

With the sensor sort that will measure temperature a lot of, thermistor is among them one of. A lot of thermal resistor have negative temperature coefficient (NTC) , that is to say its resistance when temperature drops will be elevatory. In sensor of all passive type temperature, the sensitivity of thermal resistor (namely temperature every change for a time when the change of resistor) highest, but the resistor of thermal resistor / temperature curve dispute is linear. 　　

 

 

Express the 1 NTC thermistor performance data that is a model.

 

These data have measuring getting to Vishay-Dale thermal resistor, but the overall case that it also represented NTC thermal resistor. Among them resistance gives out with form of a rate (R/R25) , of the block value that this rate represents to current temperature falls and the block value when 25 ℃ than, normally the thermistor of same set has similar character and identical resistance / temperature curve. It is in order to express 1 medium thermal resistor series exemple, the block when 25 ℃ is worth the resistor that is 10KΩ , when 0 ℃ resistor is 28.1KΩ , the resistor when 60 ℃ is 4.086KΩ ; As similar as this, the resistor when 25 ℃ is 14.050KΩ in the resistor when 0 ℃ for the thermal resistor of 5KΩ .

 

 

Graph the 1 temperature curve that is thermal resistor, can see resistance / of linear of temperature curve dispute.

 

Although the thermal resistor data here is increment with 10 ℃ , but some thermal resistor are OK with 5 ℃ even 1 ℃ is increment. If want to know the block below two some temperature is worth, can estimate with this curve, OK also and direct calculation gives resistance, computational formula is as follows:

 

 

 

T shows the family name is very here temperature, a, B, C, D is invariable, according to the character of thermal resistor each have different, these parameter are offerred by the manufacturer of thermal resistor. 　　

 

Thermal resistor has an error range commonly, use the consistency between formulary sample. Differ according to use data, error value is in normally 1% to 10% between. When some thermal resistor design application can crossing-over, use at cannot having the situation of spot adjustment, the exemple is consistent platform instrument, user or spot engineer can change only thermal resistor and cannot undertake calibration, this kind of thermal resistor wants than common precision a lot of taller, also want expensive much. 　

　

Graph 2 it is the typical circuit that uses thermal resistor to measure temperature. Resistor R1 pulls the voltage of thermal resistor rise referenced tension, general the referenced voltage of it and ADC is consistent, so if the referenced voltage of ADC is 5V, vref also will be 5V. Thermal resistor and resistor series connection produce cent pressing, change of its block value makes the tension that node handles also produces change, the precision of this circuit depends on the precision of the error of thermal resistor and resistor and referenced voltage.

 

 

 

Heat up a problem oneself

Because thermal resistor is a resistor, the meeting when electric current has shed it produces certain quantity of heat, because this circuit designs personnel to should ensure,help litre of resistance enough big, heat up in order to prevent thermal resistor oneself excessive, otherwise what the system measures is the heat that thermal resistor gives out, is not the temperature of surroundings. 　　

 

The energy that thermal resistor wastes expresses to the influence of temperature with dissipation constant, it shows thermal resistor temperature rises taller than environmental temperature the fine long hair that 1 ℃ place needs wattage. Dissipation constant because of thermal resistor enclose, pin norms, cover material and other element are different and different. 　　

 

Systematic place allows from quantity of heat and size of current limliting resistor by measure precision to decide, measure what precision is ±5 ℃ to measure a system to be ±1 ℃ to survey a system than precision but susceptive thermal resistor wants from heat big. 　

　

The block cost that should notice to help litre of resistance must have consideration, with demarcate whole measure temperature range inside heat up power comsumption oneself. After giving a resistance, because thermal resistor block is worth change, dissipation power also differs somewhat below different temperature. 　　

 

Need to so that get proper temperature resolution,undertake demarcate to the input of thermal resistor sometimes, graph 3 it is 10 ～ limits of 40 ℃ temperature expands ADC is whole the circuit of interval of input of 0 ～ 5V. 　　

 

Operation amplifier outputs formula to be as follows: 　

Once the input demarcate of thermal resistor is finished later, can represent the corresponding case of effective resistance and temperature with chart. As a result of linear of thermal resistor dispute, so need represents with chart, the system should know correspondence how much is the value of each temperature ADC, the precision of the watch is specific be be increment with 1 ℃ or want to be decided according to specific applying for increment with 5 ℃ . 　　

 

Accumulate an error

When measuring temperature with thermal resistor, want to choose good sensor and other component in input, so that mix,the precision photograph of a need matchs. Some circumstances need precision to be the resistor of 1% , and need precision a little possibly to be the resistor of 0.1% . The for love or money applies cipher out of a piece of form of all component accumulate the error influence to measuring precision, these component include resistor, referenced voltage and thermal resistor itself.

　　

If claimed accuracy is tall and want to spend a bit money less, calibration undertakes to it after needing to had been built in systematic compose, because circuit board reachs thermal resistor to must change in the spot, do not suggest to be done so usually so. Below the case that equipment cannot make the spot change or the engineer has temperature of other method monitoring, also can let software propose the form that ADC of correspondence of a piece of temperature changes, need to measure actual temperature value with other tool at this moment, software ability founds corresponding form. Change to must wanting the site a little the system of thermal resistor, can the component that will change (sensor or whole imitate front) good with respect to calibration before leave factory, save calibration result on disk or other memory medium, of course, the software after component changes must want to be able to know to use the data after calibration.

 

 

 

As a whole, thermal resistor is measurement technique of temperature of a kind of low cost, and use very simple also, we introduce resistor temperature detector and thermocouple temperature sensor below. 　

　

Resistor temperature detector

Resistor temperature detector (the lead that RTD) is one Ghent different actually, its resistor changes along with temperature and change, normally RTD material includes copper, platinic, nickel and nickel / ferroalloy. RTD component can be a lead, also can be a film, use eletroplate or splash ejaculation methodological coating to be in pottery and porcelain kind on data base. 　

　

The resistance of RTD serves as with value of 0 ℃ block nominal value. Resistor of platinic RTD of 0 ℃ 100Ω is worth in its block when 1 ℃ it is 100.39Ω normally, 119.4Ω is when 50 ℃ , graph 4 it is RTD resistor / the resistor of temperature curve and thermal resistor / the comparison of temperature curve. The error of RTD should compare thermal resistor small, to platinum, the error is in commonly 0.01% , nickel is commonly 0.5% . Divide resistor of sum of errors lesser beyond, the interface circuit of RTD and thermal resistor is basic and identical. 　　

 

Thermocouple

Thermocouple is united in wedlock by two kinds of different metals and become, the meeting when its be heated produces little tension, voltage size depends on the stuff of two kinds of metals that comprises thermocouple, iron – constantan (J) , copper – constantan (T) with chromium – aluminous (K) thermocouple is 3 the most commonly used kinds. The tension that thermocouple produces is very little, have a few millivolt only normally. K thermocouple temperature every change change of the voltage when 1 ℃ has only about 40μV, the voltage change that because this surveys a system,wants to be able to measure a 4μV measures precision to just can achieve 0.1 ℃ . 　　

 

As a result of two kinds the metal of different type is united in wedlock together to be able to produce voltage, so thermocouple and the join that survey a system also can produce tension. Put join drop commonly it is on heat insolation piece affect in order to reduce this, make two node place falls with same temperature, reduce an error thereby. Also can measure the temperature of heat insolation piece occasionally, in order to compensate the influence of temperature (graph 5) .

 

 

 

The gain that surveys thermocouple voltage requirement is commonly 100 to 300, and thermocouple picks the multiple that extraction noise also can put Great Harmony kind. Use normally measure amplifier to magnify signal, because it is OK,eliminate thermocouple connects a line in in all modular noise. Thermocouple signal conditioner still can be bought on the market, be like the AD594/595 of company of imitate parts of an apparatus, usable will simplify hardware interface. 　

　

Solid state heats up sensor

The simplest semiconductor temperature sensor is knot of a PN, for example diode or transistor base – blast off extremely the PN knot between. If constant electric current has flowed to slant the silicon PN knot of buy, to pressure fall in temperature every change the meeting when 1 ℃ reduces 1.8mV. A lot of IC use this one character of semiconductor to measure temperature, the LM335 of half and LM74 wait the MAX1617 that includes beautiful letter, country a moment. The interface form of semiconductor sensor is diversiform, it is OK to output interface of serial SPI/ small line from voltage.

　　

Temperature sensor is a lot of more phyletic, through choosing software and hardware correctly, can find the sensor that suits oneself to apply certainly.