PHYS225 Lecture 3. Electronic Circuits

Transcription

1 PHYS225 Lecture 3 Electronic Circuits

2 Course Web Page Up now Contains All lecture notes (PDF) All assignments Useful links

3 Last lecture Devices not like a resistor Zener diode Tunnel diode Capacitor Signals Sinusoid Frequency, phase, and amplitude Fourier transform Can be used to characterize complex signals

4 Sinusoidal Time variable signal Characterized by Frequency Phase Amplitude

5 Sinusoidal Many sinusoids of top of each other Many frequencies, phases, amplitudes added Fourier transform to sort out

6 Fourier transforms

7 Fourier transforms

8 Other kinds of signals

9 These have Fourier transforms too

10 Lots of combinations

11 Pulses

12 Machines available to generate these signals Function generator Pulse generator Signal generators Generally characterized by frequency, shape of pulse, etc.

13 Circuits with capacitors Capacitors Q = CV I = C dv/dt Current is proportional to rate of change of potential Change in potential proportional to current Power stored as energy in internal electric field Can get it back again later Parallel capacitance add C = C1 + C2 + C3 + Serial capacitors add like parallel resistors 1/C = 1/C1 + 1/C2 + 1/C3 + Many different kinds of capacitors Each has unique and useful properties

14 Capacitors Capacitor Battery Unit = Farad Pico Farad - pf = F Micro Farad - uf = 10-6 F

15 Capacitor types Ceramic disk Monolithic ceramic Dipped siver-mica Mylar Mylar Ceramic disk Monolithic ceramic Dipped silvered-mica Mylar or polyester Aluminum electrolytic (+/-) Tantalum (+/-) Solid tantalum, polarized Radial aluminum electrolytic Axial aluminum electrolytic

16 Capacitors Capacitance is determined by 3 factors Plate surface area Plate spacing Insulating material (dielectric)

17 Capacitor ratings Physical size of capacitors is related to voltage handling ability WVDC working voltage DC Temperature coefficient may also be important can be + or or nearly zero Temperature coefficient depends upon dielectric material

18 Circuits with capacitors Potential across capacitor changes when a current flows through it

19 Circuits with capacitors C dv/dt = I = -V/R V = A e -t/rc Capacitors will charge up over time after application of an initial voltage Approaches the applied potential Will also discharge over time if the applied potential is reduced

20 Capacitor Charging

21 Capacitor Discharge

22 RC time constant

23 RC time constant Product of RC in a simple circuit For R in ohms and C in farads, RC is in seconds 1 µf across 1KΩ = 1 ms Characteristic time of response for the circuit Sets frequency response of circuit How quickly circuit responds How much of which frequencies get through the circuit

24 Some applications Time-delay circuit: Can induce a delay in a signal

25 Another application I = C d/dt(v in V) = V/R V = RC d/dt(v in V) For small changes in dv/dt V RC dv in /dt Circuit differentiates the incoming signal For square wave input, output is a series of pulses

26

27 Unintentional capacitive coupling

28 Circuits with capacitors Integrators V << V in Ramp generators If provide constant current, Voltage continues to increase All sometimes useful

29 Inductors V = L di/dt; L is inductance A simple coil of wire! Putting a voltage across an inductor causes the current to ramp Power stored in as energy in the magnetic field 1 V across 1 henry produces 1 amp Rare to use, but useful in some circumstances RF chokes Transformers Two closely coupled inductors

30 Inductors Values specified in henries (H), millihenries (mh) and microhenries (μh) A coil of wire that may be wound on a core of air or other non-magnetic material, or on a magnetic core such as iron powder or ferrite. Two coils magnetically coupled form a transformer.

31 Inductor types Molded inductor & air-wound inductor Adjustable air-wound inductor Ferrite core toroidal transformer Air wound inductor Iron powder toroidal inductor 31

32 Inductor ratings Wire gauge and physical size of the coil determine the current handling capacity. Core material will have a temperature dependence. Air is best, followed by iron powder, then ferrites.

33 Transformers Two closely coupled coils AC voltage applied across one will appear across the other at a different voltage Change depends on ratio of the number of turns in the coil Power is conserved So if voltage goes up, current will go down Generally very efficient

34 Transformer

35 Transformers Useful to change line power to something else At the heart of everything used to power computers, cell phones, etc. Isolate circuit from actual connection to the power line