Fun with LC circuits and diodes Physics 117 Lecture 3 Winter 2016 Prof Pietro Musumeci LC circuit discussion at the board Diode Old School diode Vacuum tube as diode hot cathode plate electrons can flow only one direction I V characteristic you measured The Diode Simplest model cathode anode Current flows from anode to cathode with 0 6V drop electrons flow from cathode to anode No current flows in reverse direction Understanding the Diode semiconductor Resistivity Si 6 40 102 SI units what are they conductor Resistivity Ag 1 65 10 8 insulator Resistivity quartz 7 5 1017 Question How does resistivity relate to resistance ie what are units above N doped silicon 1 m P doped silicon hole The P N junction Phosphorus doping N Boron doping P Put them together free electrons in N move to P leaving N region positively charged The P N junction P N depletion region electron carriers excess h excess e hole carriers E field established i e V Put them together free electrons in N move to P leaving P region negatively charged Field creates depletion region The P N junction P N h e hole carriers electron carriers E field established i e V Question which way does current flow best The P N junction N cathode P h e I hole carriers electron carriers anode I E field established i e V a as shown OK hole coming in on right pushes holes to the left Eventually hits an electron in the middle and electrons move to the right All along current is flowing to the left To cathode Just need to overcome E d V Typically 0 6 V b Reverse direction Take holes away from RHS take electrons away from LHS Current briefly flows until huge depletion region Simplest model generally good enough I 0 6V V I e any positive current flows as long as can overcome 0 6V drop no negative current can flow Really a little curvature Dynamic Resistance 0 6V Turning AC into DC rectification simplest but not best method Vout 110VAC 9VAC 2k Called a half wave rectifier Questions What does output look like How can you smooth it out Why do we need the final resistor Full wave rectifier 9VAC no ground defined Vout 2k NB careful where you put your scope s ground Questions Ignoring drop across diode what does output look like How many diodes working at a time With the 0 6V drop what does it look like Diode clamp Questions at the board 1 How would you use a diode and a DC power supply to limit the voltage into a circuit at 10V 2 10 V I V curve Zener Diode Scope Probe x1 x10 switch signal clip ground clip to scope Can be better than just clipping a banana cable to your circuit Higher input impedance Can look at faster signals BUT bulky Probe presents 10 M to the circuit Turn switch to 10x First where is the measurement point by the scope Again we say voltage at a point but we know what we mean probe tip probe 9M scope 1M This is the impedance INSIDE your scope Scope sees 1 10 of the voltage drop than at probe tip You set the attenuation on the scope to x10 to compensate When you have done this without the probe you have seen crazy voltages x10 More expensive scopes auto detect the probe type Real world scope has some capacitance low pass filter and or RC time constant probe tip probe 9M scope 1M 100pF unavoidable scope measurement point Probe compensation adjust to the rescue cal signal output on your scope probe tip probe 0 20pF under comp 9M scope over comp 1M 100pF N B There are many fancier scope probes can will find Active differential choke condenser droop inductor capacitor lowering of voltage from a circuit when a load is attached risetime time for waveform to rise from 10 of its lowest value to 90 of its highest value rms root mean square value For this class of a waveform For AC Power Irms Vrms For a sine wave how does amplitude relate to Vrms ripple Vrms 1 sqrt 2 amplitude 0 707 amplitude variation of voltage on a power supply due to imperfect filtering primary of a transformer input coils of a transformer input impedance RC Time constant bleeder resistor Hi pass filter 3 dB 20 dB 23 dB impedance looking into a load Usually want it to be large Charging or discharging time of a capacitor through a resistor to or from 1 e of Maximum A resistor to quickly discharge a capacitor once a device is powered off A filter that PASSES high frequencies and blocks low ones A factor of half in power A factor of one hundred in power A factor of two hundred in power phase shift Shift in time of a sine wave measured in degrees or radians as it passes through a filter bandpass filter Filter that PASSES a range of frequencies above some minimum and below some maximum frequency not 0 or infinity notch filter Filter that BLOCKS a range of frequencies not including 0 or infinity DVM Digital Voltmeter but usually is a multimeter ie DMM but nobody says that Thevenin Resistance Reactance an inductive load The resistance of a circuit when recast as an ideal voltage in series with a resistor The imaginary part of an impedance a circuit with a positive reactance