* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Application of Thermistors
Survey
Document related concepts
Negative resistance wikipedia , lookup
Josephson voltage standard wikipedia , lookup
Superconductivity wikipedia , lookup
Valve RF amplifier wikipedia , lookup
Operational amplifier wikipedia , lookup
Thermal runaway wikipedia , lookup
Schmitt trigger wikipedia , lookup
Power electronics wikipedia , lookup
Electrical ballast wikipedia , lookup
Lumped element model wikipedia , lookup
Voltage regulator wikipedia , lookup
Switched-mode power supply wikipedia , lookup
Opto-isolator wikipedia , lookup
Surge protector wikipedia , lookup
Current source wikipedia , lookup
Power MOSFET wikipedia , lookup
Rectiverter wikipedia , lookup
Current mirror wikipedia , lookup
Transcript
Application of Thermistors • • • • • 3/22/2003 Physics review – electricity Basic circuits Ohm’s Law Thermistor properties / circuits Laboratory information BAE 1022 1 of 10 Current and Voltage • Current: – Flow of electrons • The quantity of electrons per unit time flowing through a conducting medium • Units of Amperes (A), abbreviated “amps“ or fundamentally coulombs per second (coulomb=6.03x1023 electrons) • Voltage: – Electromotive force (EMF) • A potential or “tension” between two points of a conducting medium that can drive the flow of electrons through the medium expressed as work per number of electrons • Analogous to pressure in a fluid that can drive flow of fluid through a pipe • Units of Volts (V) or fundamentally joules per coulomb, the energy (potential) per unit of electrons. 3/22/2003 BAE 1022 2 of 10 Power dissipation • Power – Electromotive Force times quantity of electrons moved per unit time • Work done per unit time • Voltage X Current – [Joules/Coulomb] x [Columbs/second] = [Joule/second] = Watts • Units of Watts 3/22/2003 BAE 1022 3 of 10 Current flow in circuits • Schematic with a voltage source supplying a potential to a resistive load (R) with a current (i). + Vsupply i R • An abbreviated schematic showing the source and indicating the return to source is ground referenced. Vsupply i 3/22/2003 BAE 1022 R 4 of 10 Resistors and Ohms Law • Property of a resistor – Flow of current is proportional to voltage (or vice versa). The proportionality constant is known as resistance: v Ri Vsupply • For the following circuit: Vsupply Ri i R 5V i 10 kW • Resistance has units of Ohms (W) – (fundamentally, volts per amp) • The current could be computed in the circuit above given Vsupply and R: i = 5V / 10,000W = 0.0005 V = 0.5 mV 3/22/2003 BAE 1022 5 of 10 Circuit computations Vsupply • For a more complex circuit (half bridge) – The current (i) must be the same for both resistors (no other path) i iR1 iR2 (1) – The supply voltage dropped across both resistors must be the sum of the voltage across R1 and R2 Vsupply VR1 VR2 (2) R1 i R2 – We can compute the voltage across R2, (VR2) as a function of Vsupply and the resistances by using Ohm’s law: Vsupply – From (2) V i R i R i R R i supply – From (1) i VR 2 R2 3/22/2003 1 2 1 2 VR2 R1 R2 R2 Vsupply R1 R2 VR 2 Vsupply BAE 1022 R2 R1 R2 6 of 10 Thermistors - characteristics • Read Thermometrics handout. • Thermistor is a resistor where resistance is strongly a function of temperature • Important characteristics – Mass – larger masses = slower response – Temperature coefficient • NTC – Resistance decreases with temperature – Exponential relationship between temperature and resistance T0 T RT RT0 e T0T • PTC – Resistance increases with temperature – Packaging – Temperature range 3/22/2003 BAE 1022 7 of 10 Thermistor circuits for temperature measurement • Half bridge (voltage divider) Vout R2 Vsupply RT R2 Vsupply RT T R2 • For a NTC thermistor: Variable Symbol Direction Temperature T Increase Thermistor resistance RT Decrease Voltage out Vout Increase Vout • Design issue: select Vsupply so that self-heating is insignificant – Does RT heat up due to resistive power dissipation? (P=VI) 3/22/2003 BAE 1022 8 of 10 Theoretical Performance of Voltage Divider Circuit R_2 3450 10000 Vout 1.286374 1.376124 1.468281 1.562551 1.658619 1.756156 1.854822 1.954269 2.05415 2.154121 2.253847 2.353002 2.451281 2.548394 2.644074 2.73808 2.830192 2.920219 3.007996 3.093382 3.176262 Vsupply 5 Resistance vs. Temperature for NTC Thermistors 30000 3.2 3 RT 25000 2.8 Vout 2.6 20000 2.4 2.2 15000 2 Vout (V) T0 25 RT 28868.95 26333.94 24053.43 21998.96 20145.56 18471.27 16956.77 15585.01 14340.97 13211.32 12184.3 11249.45 10397.5 9620.204 8910.211 8260.974 7666.646 7122.002 6622.364 6163.541 5741.773 Resistance (Ohms) RT0 10000 T 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 1.8 10000 1.6 1.4 5000 1.2 0 5 10 15 20 25 30 35 40 Temperature (C) 3/22/2003 BAE 1022 9 of 10 Information for the laboratory • Prototype board internal connections • Resistor color codes First Digit Second Digit Multiplier (x10) Precision 3/22/2003 0 Black 1 Brown 2 Red 3 Orange 4 Yellow 5 Green 6 Blue 7 Violet 8 Grey 9 White 5% Gold 10% Silver BAE 1022 10 of 10