Pats Electronics Lecture basics Water Analogy helped me

Pat’s Electronics Lecture basics

Water Analogy (helped me…) Current flow Water source Pressure pushes water through pipe Battery Does useful “work” Laptag MILL and SWEATSHOP Flow Current return Does some kind of work (Water drain = “return”)

Water Analogy, cont’d Water Pressure (the higher the pressure the more water flows) Voltage (The higher the voltage the more current flows) Water Flow Rate Current (e. g. gallons per minute) (which is actually charge flow rate: 1 coulomb/sec = 1 ampere)

“Work” • In both pictures, potential energy is converted to “work”, where • Work = – Kinetic energy – Heat – Some other kind of potential energy • Physics note: Total Energy is conserved

What’s Happening • Electric charges can flow in conductors • Like charges repel • Unlike charges attract ++++ Battery ----- Because of the chemistry inside the battery, there is a voltage set up across the terminals

What’s Happening • Electric charges can flow in conductors • Like charges repel • Unlike charges attract +++ + + + Battery - - - - If we connect wires, they also become charged up

What’s Happening • Electric charges can flow in conductors • Like charges repel • Unlike charges attract + charges Battery + charges

What’s really happening • Electrons are flowing out of bottom of battery, around to the top • Since they are negative, the direction of the current flow (by convention) is opposite their physical movement • It is MUCH EASIER to think of positive charges flowing, even though they are slightly fictitious

Typical Elements of a circuit • Wires • Voltage Sources • Electronic Components – Resistors – Capacitors – Inductors – Modular circuits (e. g. amplifiers) – Occasionally diodes and transistors

Wires • These are good conductors, with practically unimpeded flow of current • Electrons in metal form a kind of plasma • Any flowing current creates a magnetic field (which btw can be used to measure the current) • Size is measured by “AWG”, American Wire Gauge, since the 1850 s

Interesting note on AWG • The gauge number is similar to decibel measurement for sound • 20 steps in AWG is (almost) a factor of 10 in wire diameter • For instance, #1 AWG wire is ~ 10 x the diameter of #20 AWG • We typically use #20 to #24 for circuits

Voltage • (the Electrical version of pressure) • Measured with a meter, if time variation is slow enough • Measured with a scope and typically a scope probe if fast time variation • Hazards: – HIGH VOLTAGE CAN KILL YOU – (actually it’s the current through your heart…)

Pressure is not exactly Voltage • One difference: voltage is always measured between two points (e. g. a meter has a “common” probe and a measurement probe. • The reason for this goes back to the attraction of charges, • Still a very good analogy, though

Water flow is not exactly Electrical current • Water can flow even when there is not an (obvious) return path

2 hazards we will encounter • 1: DO NOT USE A SCOPE OR METER TO MEASURE THE AC LINE VOLTAGE!!! (what is AC voltage? We will cover this) WHY? • THE METER CAN LITERALLY EXPLODE • You might kill a $10, 000 scope ► ► ►Use a “Wiggy” instead

2 d hazard: Death • High voltages in our lab can kill you. Best case scenario: you accidentally touch a high voltage terminal, and current starts to flow through your arm. If this current is much larger than your nerve impulses, you can no longer pull your arm away, because your muscles don’t receive the command. It hurts. You begin to think about how dumb you were to have one hand resting on ground while you poked around with the other one. Next, some guy who also didn’t listen grabs onto you to try to pull you away. Current flows through him, too, so he is useless. Finally someone who paid attention to this lecture finds a non-conducting hook and saves both victims. Worst case: sufficient current finds its way through your heart to stop it, too.

High Current • This can also be dangerous: – wires can heat up, and cause fires. – Circuit elements (wires) can literally explode if a lot of energy is dumped into them quickly – More subtly, interrupting a high current can give a high-voltage transient!!! Of all the hazards, this is the only one I personally had experience with that actually did kill a guy. (We will get to the reason for this. )

Resistors • Resistors impede the flow of electrical current • Like a pin-hole for water flow Water source High pressure Lower pressure Constriction in pipe resists the water flow · need more pressure to get the same flow · pressure after the constriction is lower Similarly, there is a voltage drop across a resistor when current flows through it.

Resistors • Symbol • Measured in ohms: A resistance of 1 ohm will let 1 Amp of current flow for a voltage drop of 1 Volt (across the resistor).

Ohm’s Law

Computing resistance • Resistance Where ρ (rho) is the “resistivity” of the material L is the length A is the area A = length L

Some Resistivities

Resistor Marking • Color Code • First 2 bands = digits • 3 d band = power of 10 • 4 th band = tolerance: gold 5%, silver 10%, none 20% 0 1 2 3 4 • E. g. brown black red is = 1 0 00 = (a one followed by a zero followed by 2 zeros) Other Notes: 3 d band = gold: divide by 10 3 d band = silver: divide by 100 5 6 7 8 9

Remember • • • Black = 0 (no color) White = 9 (all colors) Grey is close to white, so make it 8 Brown = ? Might as well be 1 The rest correspond to the spectrum – ROYGBV (You may have heard of this guy: Roy G. Biv) Red = 2…etc.

From http: //www. token. com. tw/resistor/image/color-code. jpg

Simple Circuit Diagrams 1 • 1 Voltage Source (e. g. battery) • 1 resistor Given a 9 V battery, and a 1000 ohm resistor, what current will flow?

Simple Circuit Diagrams 2 • Resistors in series:

Simple Circuit Diagrams 3 • Resistors in parallel:

Convenient formulas: • Series resistors: • Parallel resistors: Note: it may help to think about the construction of a resistor

Another circuit

…think about what happens in this arrangement: Water source High pressure Lower pressure

What about this one? Hint: symmetry helps

Other useful components • • Inductors Capacitors Diodes Integrated Circuits (e. g. RF amplifier) MOSFETs Occasionally transistors Rarely vacuum tubes

Electrical Power • Power is rate of dissipation of energy • Also rate of getting work done • Energy is conserved, so if we are not storing any energy: Power in = Power out + heat dissipated as losses

AC Voltage, Current • AC stands for alternating current • Nevertheless people still talk about “AC current” coming out of the wall. • The voltage alternates: if you had a really fast meter, you would see the polarity reversing 60 times a second* * Or just use an oscilloscope, BUT DON”T HOOK IT UP DIRECTLY

Water analogy: • 2 buckets on a see-saw Water source/sink

Why AC? • See “War of Currents” on wikipedia – Edison wanted DC – Tesla wanted AC • No good way to transform DC to a different voltage (at least in 1900) – Transmission requires high current – Must generate near point of load • AC can be transformed up to high voltage, low current, for transmission, then back to safer levels (110 V) near point of load

AC Outlet: 110 V (rms) Low side, or neutral Ground High side, or line In an AC line cord, standard colors are: Green for ground, White for neutral, and Black for line NOTE: in most AC wiring, BLACK is the hot, or high voltage, side

AC Voltage Measurement Level is quoted as – Peak-to-peak (least ambiguous) – Peak – RMS = root mean square, which is the average value of the square of the voltage. This is what a typical handheld voltmeter reads on the AC setting. • 110 V is the RMS value, peak is around 160 V, or

Transformer • 2 sets of windings, with their magnetic fields coupled. • Use iron to channel the field from one set to another • Step up or down the voltage according to the turns ratio “primary” winding “secondary” winding

Transformers, cont’s where Also Note: Power is conserved:

Capacitors • Symbols: • Let AC through, but not DC; another way of saying this is that they tend to keep the voltage across them constant • Have an impedance (not a resistance because they don’t dissipate any power)

Capacitor construction 2 conductors separated by a physical space d A C, in Farads, is a measure of how much charge can be stored for a given voltage

Water Model • Water balloons in a sealed oil-filled enclosure:

Capacitors, cont’d • Often the gap is filled with a “dielectric” material to increase the capacitance; using an insulator also allows the gap to shrink, d 0, but voltage stays the same without breakdown. • All dielectrics have a safe operating voltage, which is given as the voltage rating • Sometimes the dielectric can only be charged in one direction: the capacitor is polarized, or electrolytic – advantage is higher capacitance • Ugly fact that we will not worry about: most dielectrics change their value as they are biased to higher voltages!

Inductors • Symbol • Let DC through, but not AC; another way of saying this is that it tends to keep the current flowing through it at a constant level • Have an impedance (not a resistance because they don’t dissipate any power)

Inductor Construction • Any coil of wire • Sometimes iron is added to increase the magnetic stored energy, which increases the inductance

Inductance Why N 2 ? • Current flowing through the windings produces a magnetic field; more turns produces more field, proportional to the number of turns in a given length. • Each turn then picks up voltage from the changing magnetic field; with the turns “in series” the voltage adds, so the total is proportional to the total number of turns.

Example circuit • Initially the switch is open, so no current is flowing • Close the switch: the inductor tends to keep the same current flowing, which is zero. • Eventually the inductor looks like a wire, so the current is given by Ohm’s law: I = V ÷ R

Water analog: heavy paddle-wheel 1. Once valve is opened, paddle-wheel begins to spin 2. Paddle-wheel has heavy flywheel attached – so it is hard to spin up, but once it is spinning it tends to keep going Valve Flow 3. Eventually the paddle-wheel gets up to speed, and the flow is limited by the resistance in the line

Another circuit: the dangers of high current • Initially the switch is open, so some current flows, such that I = V ÷ R • Close the switch: current starts to increase • Suppose the current builds up to 100 x its initial value, then the switch is opened: what happens? • Inductor tries to keep the same current flowing, so initially V = 100 x the battery voltage

Generating high pressure due to current flow Suppose valve is initially closed Paddle-wheel is spinning slowly Flow Valve Then we open the valve for some amount of time, letting the flow build up (paddle-wheel spins faster)

Generating high pressure due to current flow Then valve is closed again… Paddle-wheel spins up Flow through this leg stops Valve Flow transfers to this leg The pressure ahead of the resistance goes up, since the paddle-wheel keeps spinning; eventually slows down to “steady state”

Diodes • Symbol: Pos Neg • Function: only let current flow one direction • Convert AC to DC – useful for power supplies, detecting radio signals, …

Water Analog of a diode • A flap inside a pipe flow: no flow:

SOLDERING • Solder works by forming a solution of the metals being joined in the liquid solder. • So the solder needs to be hot enough to flow, BUT • Too much heat traveling up the leads will destroy semiconductors! • The work pieces rather than the soldering iron must melt the solder • When done, the two conductors being soldered should look “wetted”

Solder wire Has “flux” inside. Flux is a wax-like goo that has a few percent acid, for cleaning the oxide layer from wires being soldered. For plumbing, the same thing happens except the flux is usually applied separately. And you can’t use lead solder anymore. And usually a torch is used instead of an iron.

Soldering Hints • Liquid solder conducts heat better than a dry tip, so it helps to put a dab of solder onto the tip before soldering. The associated flux can also help clean up the tip. • It helps to “tin” the leads being soldered individually before actually trying to solder them together. • The smoke comes from burning flux, not lead, but still probably not healthy to breathe it in. • Don’t hold solder in mouth.

Soldering Irons - experience • Temperature regulated ones are crucial • Tips are special – if you decide that you want a sharper tip, you can sand the tip down to a point, but it will dissolve a little bit each time you use it and disappear before too long.

Solder joint cross section From http: //www. emeraldinsight. com/fig/2170250306001. png

Making a Circuit Board 1. Generate a layout, using some kind of PCB software. There are programs that are free but that I know very little about (we use a bad but expensive tool, which is not even sold anymore): – Eagle, from http: //www. cadsoftusa. com/ – Kicad, from http: //www. lis. inpg. fr/realise_au_lis/kicad/ Top Bottom For our process, we generate a “positive” image: colored parts (which print as black) will be copper, white parts no copper.

Circuit Board, cont’d 2. Use laser printer to print layout (also called artwork) on a transparency 3. Align top and bottom, and tape them together. 4. Slip a pre-sensitized board between them. Top transparency Bottom transparency Circuit board, has copper on both sides, covered with “photo-resist”.

Circuit Board, etching 5. Expose in UV box for 5 minutes. The UV goes through the clear parts of the transparency, and does something to the photoresist. 6. Soak board in developer – this washes off the exposed photoresist. (Dilute the developer solution 1 part developer to 10 parts water. ) 7. Rinse developer off using water 8. Etch in Ferric Chloride solution. The photoresist that is still on the copper prevents the copper from being etched, at least for a while. Etching usually completes in 15 -45 minutes, depending on how old the solution is. You never know, so you need to keep an eye on the progress.

Circuit board fab cautions and notes: • The ferric chloride solution will irritate your skin after a few minutes, so a little is OK but generally you should rinse it off. • It will also eat holes in your clothes, if it gets on them and dries there. ( mysterious little holes next time you wear them) • There is an aquarium heater and a bubbler in the ferric chloride tank, to help speed things up – remember to turn it off. • Don’t pour ferric chloride down the copper drain pipes.