Search This Site:  Advanced Search?   Search Tips

MODULE V - FUNDAMENTALS OF ELECTRONICS VOLTAGE DIVIDERS  (Page 2) Temperature sensors The resistance of most common types of thermistor decreases as the temperature rises. They are called negative temperature coefficient, or ntc, thermistors. Note the -t� next to the circuit symbol. A typical ntc thermistor is made using semiconductor metal oxide materials. (Semiconductors have resistance properties midway between those of conductors and insulators.) As the temperature rises, more charge carriers become available and the resistance falls.

Although less often used, it is possible to manufacture positive temperature coefficient, or ptc, thermistors. These are made of different materials and show an increase in resistance with temperature. How could you make a sensor circuit for use in a fire alarm? You want a circuit which will deliver a HIGH voltage when hot conditions are detected. You need a voltage divider with the ntc thermistor in the Rtop position: How could you make a sensor circuit to detect temperatures less than 4�C to warn motorists that there may be ice on the road? You want a circuit which will give a HIGH voltage in cold conditions. You need a voltage divider with the thermistor in place of Rbottom : This last application raises an important question: How do you know what value of Vout you are going to get at 4�C? To answer this question, you need to estimate the resistance of the thermistor at 4�C. Lots of different types of thermistor are manufactured and each has its own characteristic pattern of resistance change with temperature. The diagram below shows the thermistor characteristic curve for one particular thermistor: On the y-axis, resistance is plotted on a logarithmic scale. This is a way of compressing the graph so that it is easier to see how the resistance changes. Between 100 and 1000 , each horizontal division corresponds to 100 . On the other hand, between 1000 and 10000 , each division corresponds to 1000 . Above 10000 , each division respresents 10000 . As you can see, this thermistor has a resistance which varies from around 70  at 0�C to about 1 at 100�C. Suppliers catalogues usually give the resistance at 25�C, which was 20 in this case. Usually, catalogues also specify a 'Beta' or 'B-value'. When these two numbers are specified, it is possible to calculate an approximate value for the resistance of the thermistor at any particular temperature from the equation: Where: RT is the resistance at temperature T in Kelvin (= �C +273) RT0 is the resistance at a reference temperature T0 in Kelvin. When the reference temperature is 25�C, T0 = 25+273. e is the natural logarithm base, raised to the power in this equation. B is the B-value specified for this thermistor. You don't need to think about applying this equation at the moment, but it is useful to know that the information provided in catalogues is sufficient to allow you to predict thermistor performance. Using a spreadsheet such as Excel, it is possible to to generate characteristic curves for any thermistor by calculating resistance values for a range of temperatures. With RT0 = 20 and B =4200, resistance changes from 0 to 10�C are as follows: From the graph, the resistance at 4�C can be estimated as just a little less than 60 . By calculation using the equation, the exact value is 58.2 . KEY POINT: The biggest change in Vout from a voltage divider is obtained when Rtop and Rbottom are EQUAL in value. What this means is that selecting a value for Rtop close to 58.2 will make the voltage divider for the ice alert most sensitive at 4�C. The nearest E12/E24 value is 56 . This matters because large changes in Vout make it easier to design the other subsystems in the ice alert, so that temperatures below 4�C will be reliably detected. Sensor devices vary considerably in resistance and you can apply this rule to make sure that the voltage dividers you build will always be as sensitive as possible at the critical point. Thermistors turn up in more places than you might imagine. They are extensively used in cars, for example in: electronic fuel injection, in which air-inlet, air/fuel mixture and cooling water temperatures are monitored to help determine the fuel concentration for optimum injection. air conditioning and seat temperature controls. warning indicators such as oil and fluid temperatures, oil level and turbo-charger switch off. fan motor control, based on cooling water temperature frost sensors, for outside temperature measurement acoustic systems Thermistors are used to measure surface and deepwater sea temperatures in helping to monitor El Ni�o ocean currents. Less obviously, thermistors are used to measure air flow, for instance in monitoring breathing in premature babies. Voltage Dividers - Final Page >>>

Electron Theory and Atoms Resistance Resistors Ohms Law Current and Voltage Voltage Dividers Capacitance Inductance Reactance Resonance and Impedance Diodes Transistors "Q" Radio Terminology A-L Radio Terminology M-Z Atttenuators Chokes Decibel & Toroids Digital Basics Power Supplies Conclusion  Subscribe For A Free Ham Radio E-Tutorials  Back To Top

Tell A Friend! Type In Your Name: Type In Your E-mail: Your Friend's E-mail: Your Comments: Get copy: