Division of Geological and Planetary Sciences



Lecture 1: Electricity and electronicsReading: Diefenderfer, Principles of Electronic Instrumentation, Ch 1, 3.IntroductionMass spectrometers are electronic instruments. At their heart, they measure the flow of ionized (charged) particles, which is inherently an electric current. It is thus essential to have a basic understanding of many principles of electronics to understand them, and we’ll spend a fair amount of time at the beginning trying to work our way through some principles.Basic physics of electricityCharge (e or q)Both protons and electrons carry equal charge, opposite sign; defined as 1.6 x 10-19 coulombs (C). Electrical current or voltage can be created by the movement of either, but for the most part this means electrons.Current (I)The flow of charged particles (electrons) is called a current, defined as I = dq/dt.Units are amperes (amps, A); 1 amp = 1 coulomb/second.Introduce analogy of water flowing through pipes. Current is equivalent to the flowrate of water at any given point. (charge per time instead of mass or volume per time).Voltage (V)The potential energy acquire in separating electrons from protons. Requires doing work, and you get that energy back when you allow the charges to recombine. Convenient to think of this as “excess charge”, or the force pushing electrons down the wire.Units are volts (V), named after physicist Alessandro Volta (first battery). Defined as 1V = 1 joule/coulomb.Equivalent to pressure in our water analogy. The farther you push water up a hill, the more work it takes, the more pressure it exerts coming back down. Note that voltage (pressure) is independent of total charge (or pressure).Sign convention. By convention, electricity flows from positive to negative. Before electrons were known, they guessed and got it wrong. Electrons actually flow from neg to positive, but doesn’t really matter, just follow the convention.GroundBecause voltage is a potential energy difference, it requires two points to measure. The common reference point is usually taken as the electrical potential of the Earth (which varies spatially over large scales, but fairly constant in one place). In electrical msmsts, ‘ground’ is usually defined as zero and we measure voltages relative to that. By analogy, we measure water pressure relative to 1atm of pressure (but note that pressure cannot be negative, whereas voltage can be).Key point: when measuring voltages, always remember what your ref point is. In a grounded circuit, this is easy, but common to have ‘floating circuits’ in which the whole thing is at some elevated potential relative to ground.This can create huge problems if the reference for voltages is different in varying places or times, which can be caused if circuits are not carefully connected to each other and to earth ground. For this reason, proper grounding of all circuits and parts is key in instrumentation, and something that we should look at carefully in our IRMS. Grounding of power circuits can be hard (because the Earth is largely an insulator), and so they drive large copper rods into the Earth.Neutral vs ground. It is possible to construct a circuit with just one wire (hot), allowing current to flow into the ground. In fact, older ‘knob and tube’ wiring in houses did just this. Not very safe though, because ground is not a good conductor. Best practice is to include a ‘neutral wire’ which is at ground potential but that carries all current back to the power supply. Thus a common 110V circuit has 3 wires: hot, neutral, and ground. Ground wire is really just a safety valve, should not carry any current. Always check that neutral/ground are at the same potential, and be worried if they are not. Can often get away with measuring voltages relative to ground (eg, the IRMS frame) but much safer to actually find the neutral wire as ref point.Resistance (R)Unit is ohms (?). All materials have some inherent resistance to flow of electrons, caused by atomic scale disorder. Materials with very low resistance (typically metals, R~10-6 ?-cm) are called conductors. Materials with high resistance (R>1011 ?-cm) called resistors. Semiconductors are in between (R ~106 ?-cm). Note that a component made specifically to provide a fixed resistance is also called a resistor; two uses of word often confused (material property versus electronic component).In an ideal resistor, current flow is directly proportional to voltage, leads to Ohm’s Law: R=V/I, often written as V = IR.Basic calculation: if we apply a known voltage across a resistor (or multiple resistors), what is the current that flows? Solve using V = IR. For multiple elements, need to find the net resistance.Resistors in series: the resistance of each adds linearly, thus Rnet = R1+ R2 + …Resistors in parallel: resistance adds in inverse, thus 1/Rnet = 1/R1 + 1/R2 + …Variable resistors. Often need the ability to fine-tune a circuit by varying resistance. Specific elements are called ‘variable resistors’, or potentiometers (pots). Typically have a small screw on top that varies resistance by turning.Power (P)When current flows through a resistor, potential energy is converted to heat. Other types of elements can convert potential energy to mechanical work (like a motor). The rate of energy dissipation per unit time is called Power (P), with units watts (W).For electrical energy, P = V x I. Since V = IR, could also write P = I2R.1W = 1V x 1A.Typical household circuit is 110V and 10A. Maximum power is 1100W, or 1.1 kW.Energy is a measure of total power over a period of time (E = P x t). For our circuit above, running for 1 hour would give energy of 1.1 kw-hrs. SI unit is joules, seldom used.A simple example is resistive electrical heaters. These are comprised of a resitive element (usually a long, skinny wire) which converts electrical potential energy into heat energy.A 100W heater would be one that allows a current of 1A to flow given a 100V potential. Resistance is thus 100?. When examining heaters, need to consider supply voltage and total power dissipation.Capacitance (C)When two conductors are separated by a thin resistor, they have the ability to store charge, just like a water tank stores water. Amount of charge varies linearly with the applied voltage.Defined as Q = C x V, where Q is the stored charge (coulombs) and constant C is called the ‘capacitance’Units are the Farad (F). 1C = 1F x 1VA Farad is a huge capacitance, and for most applications capacitors have values in the micro to pico farad range.Capacitance tends not to matter for steady-state characteristics (with some caveats for AC power circuits), but has a big impact on time-varying properties. Think of a quickly rising ion current – capacitance in a circuit will tend to mute that signal, changing its characteristics. Will talk about this more when we get to amplifiers.SwitchPretty intuitive. Basically, any device that allows you to break the conductance of a circuit. Can be mechanical (like a lever that closes) or electronic (semiconductor). Key attributes are speed and power.Mechanical switches are slow, but carry large power (also solid-state relays)Electronic switches are fast, but carry little power. Most basic electronic switch is the transistor, in which applying a voltage to the ‘base’ (gate) allows current to flow between emitter (source) and collector (sink).Relays. To allow electronic switching of large currents, the two are often combined into a ‘relay’ which is a mechanical switch that can be opened/closed by application of a small voltage with little current. Lots of different physical manifestations.CircuitsElectricity is the flow of electrons, and they must flow somewhere (not just disappear). Thus circuits much generally comprise closed loops. It is possible to construct circuits in which electricity flows into the ground, but this is bad practice (just as it is to have plumbing in which water flows out onto the ground).In general, circuits are closed loops in which current flows back to the power source.The power source then determines the overall voltage in the circuit, and the net resistance of the circuit determines current (and thus power).SymbologyPower sourceConductors (lines)Resistors (zig-zags)Capacitors (parallel lines)Switches (lever arm)Ground (inverted triangle, or 3 lines)Real Power suppliesCome in lots of forms: batteries, devices, wall plugs, windmills, etcTypically, then can provide either constant voltage or constant current, not both. We usually consider constant-voltage power suppliesReal power supplies have limited power (=V x I). If you try to draw too much current from them, they are ‘unable to keep up’ and so voltage drops, usually with bad consequences. Can be approximated by an ideal voltage source in series with a resistance (usually called ‘internal resistance of power supply’). What happens as the voltage supply tries to source more current? Voltage drop across that internal resistance increases, along with heat dissipation. Thus power supplies usually have a power rating, below which internal resistance is negligible. Needs to be matched to the demands of the monly run into this situation when you have a control circuit (DC, low power) that needs to do something like heat a filament, close a valve, etc. This is where a relay would typically be used.Real WiresReal conductors exhibit both resistance and capacitance in a metal wire, resistance is inversely proportional to cross-sectional area of the conductor. This is why wires carrying larger currents are generally thicker, to lower their resistance (and limit heating). Also why your house is divided into circuits, so that wiring does not overheat.Capacitance is proportional to volume, and so leads you to try to make conductors as small and short as possible. When dealing with very small currents (eg ion currents), need to make sure connections do not have appreciable resistance, thus focus on very solid contacts, often gold-plated. Connections are a common point of failure because if they are loose, resistance goes way up. This is why circuit boards are soldered. Measuring with a sharp probe can exhibit significant resistance.Power circuitry versus electronicsMain difference is that power usually employs AC currents, electronics DC currents. Follow the same rules of physics, but behave differently in many ways. Typical to consider them separately.AC Power CircuitsWhat is it. Voltage varies in sinusoidal form, with mean of zero and maximum (+ and -) amplitude. Suddenly becomes harder to describe voltage. Commonly either:Peak voltage (maximum amplitude); useful in electronicsRMS voltage, defined as the equivalent DC voltage that would dissipate power according to ohm’s law. Typically used for power applications.Vrms = 0.707 VpeakFrequency of the variation. In N/S America, we use 60Hz. Europe and Asia use 50Hz. For power applications, this doesn’t matter much but for electronic ones it can. Modern instruments generally rated for either 50/60Hz, but older ones not always. Pay attention to this. Particularly problematic for older computers and other electronics.Phase – the timing of the sinusoid. For a single power source this is irrelevant, but when connecting multiple power sources you have to worry about this (e.g., multiple windmills all connected together).Why AC?First reason is that power often generated by a turbine, which is a conductor spinning inside a fixed magnetic field (work is done by flowing water or steam). Automatically generates an alternating current in the conductor, so very convenient. Note that this is one of the big headaches with solar, because PV cells generate DC voltage, have to be converted to AC with the same frequency and timing.A second benefit is that the average line voltage is always zero (ground), so leakage of current into insulators does not cause them to be charged.Also provides benefits for transmission over long distances, where the speed of electrons matters and could cause different regions to be charged relative to each other.Power distribution.Power is typically supplied to buildings in one of two ways: single-phase or three-phase.Single-phase. In US delivered with 2 conductors (hot) and 1 neutral; hot wires are 120V to ground, 180° out of phase with each other. Called a ‘split-phase’ system.For a 120V circuit, you connect hot to neutral wires.For a 240V circuit, you connect two hot wires (thus doubling voltage). Phasing not an issue.Europe uses a single-phase, 2-wire system with the hot wire being 240VAC to neutral.Three-phase. This system uses 3 hot wires, each 120° out of phase with each other. Main reasons have to do with efficiency of transmission, and of electric motors (this is ideal for transferring power to a rotating stator). Normally we never see this unless you work in a factory, but our instruments use it so we need to deal with it.In US, system uses 3 hot wires, with each wire 120V phase-to-ground. Because there is no neutral wire, single-phase loads are connected phase-to-phase which results in 208V (x square root of 3). Thus this is often called “3phase, 208V delta” configuration.In Europe, they use 4 wires, with the third being neutral. Each phase is 230V relative to ground, or 400V phase-to-phase. In the US this is often called “3phase, 400V wye” configuration.All Thermo IRMS instruments are designed to use the latter, and are not compatible with standard US 3-phase power. You need a transformer to convert the two. This is reported to be the most common problem with installation.The IRMS does not have electric motors, so this is not done for efficiency. Instead it appears to be used to split out 3 independent 230V circuits, with on used for ‘noisy’ equipment like pumps, and others used for ‘clean’ circuits like electronics. We should try to figure out the details of how this works in our instrument.Wire colorsUS, 120/240V: red/black (hot), white (neutral), green/yellow or bare (ground)Europe, 240V: brown/black (hot), blue (neutral), green/yellow (ground)US, 3-phase: red/black/blue (hot), green (ground)Europe, 3-phase: brown/black/gray (hot), blue (neutral), green/yellow (ground)Clean power characteristicsTypical problems. AC voltage is supposed to be a perfect sine wave, but never is. Common problems include:Fluctuations. Voltage may be higher or lower than nominal. Typically caused by power distribution (ie, the utility level), e.g. when everyone turns on their air conditioners during the day. 10% variance is usually acceptable.Sags and surges. Changes in voltage lasting for a few seconds. Caused by turning on or off large loads on the same circuit (remember ideal power supplies). Important reason to have isolated circuits.Transients. Spikes (up to thousands of volts) and dips lasting only microseconds. Caused by motors, compressors, mechanical switches, etc. These can be hard to measure, and the most damaging to electronics. For us in the US, this is usually the biggest worry.Distortion. Basically the shape of the sine wave is incorrect. Not much of a problem for power, can be problematic for DC power supplies. High-end stereos worry about this a lot.RemediesIsolation transformer. Designed to catch transients. Doesn’t help with voltage regulation.Power conditioner. Controls both voltage regulation and transients, not distortion. Can bridge gaps up to a couple cycles. Most commonly used.UPS. Inverts AC to DC, charges batteries, then uses batteries to power AC conversion to get pure sine wave. Deals with all problems, and can bridge gaps of minutes to hours (depending on batteries). Vary expensive, inefficient (typically ~80%), require battery changes. Cost $15-20k for a typical IRMS.Safety issuesIf you don’t understand what you are doing, don’t do it.Unplug and lock out plugs if possible. Allow time for capacitors to discharge. Even when unplugged, measure first before assuming something is dead.Never ground your body (feet in water, knees against metal post, etc). With rubber soles on dry ground, you can grab a 120V wire.Work with one arm where possible. Much better to be shocked between fingers than across your heart.Be careful where you put voltmeter probes. Very easy to bridge two components and short them out. Never measure wall voltage by pulling a plug halfway out.Be extremely careful around high voltages, they can arc across large distances. No pointing, no metal objects in pockets, long shielded probes, etc. ................
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