Lecture 1: Electricity and electronics
Reading: Diefenderfer, Principles of Electronic Instrumentation, Ch 1, 3.
1) Introduction
a) Mass 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.
2) Basic physics of electricity
a) Charge (e or q)
i) 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.
b) Current (I)
i) The flow of charged particles (electrons) is called a current, defined as I = dq/dt.
ii) Units are amperes (amps, A); 1 amp = 1 coulomb/second.
iii) 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).
c) Voltage (V)
i) 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.
ii) Units are volts (V), named after physicist Alessandro Volta (first battery). Defined as 1V = 1
joule/coulomb.
iii) 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).
iv) 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.
d) Ground
i) Because 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).
ii) 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.
iii) 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.
iv) 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
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‘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.
e) Resistance (R)
i) Unit is ohms ().
ii) 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).
iii) In an ideal resistor, current flow is directly proportional to voltage, leads to Ohm’s Law:
R=V/I, often written as V = IR.
(1) 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.
(2) Resistors in series: the resistance of each adds linearly, thus Rnet = R1+ R2 + …
(3) Resistors in parallel: resistance adds in inverse, thus 1/Rnet = 1/R1 + 1/R2 + …
iv) 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.
f) Power (P)
i) 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).
ii) For electrical energy, P = V x I. Since V = IR, could also write P = I2R.
(1) 1W = 1V x 1A.
(2) Typical household circuit is 110V and 10A. Maximum power is 1100W, or 1.1 kW.
(3) 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.
iii) 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.
(1) 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.
g) Capacitance (C)
i) 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.
ii) Defined as Q = C x V, where Q is the stored charge (coulombs) and constant C is called the
‘capacitance’
iii) Units are the Farad (F). 1C = 1F x 1V
(1) A Farad is a huge capacitance, and for most applications capacitors have values in the
micro to pico farad range.
iv) 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
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current – capacitance in a circuit will tend to mute that signal, changing its characteristics.
Will talk about this more when we get to amplifiers.
h) Switch
i) Pretty 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.
(1) Mechanical switches are slow, but carry large power (also solid-state relays)
(2) 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).
ii) 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.
i) Circuits
i) Electricity 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).
ii) 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).
iii) Symbology
(1) Power source
(2) Conductors (lines)
(3) Resistors (zig-zags)
(4) Capacitors (parallel lines)
(5) Switches (lever arm)
(6) Ground (inverted triangle, or 3 lines)
j) Real Power supplies
i) Come in lots of forms: batteries, devices, wall plugs, windmills, etc
ii) Typically, then can provide either constant voltage or constant current, not both. We usually
consider constant-voltage power supplies
iii) Real 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 circuitry.
iv) Commonly 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.
k) Real Wires
i) Real conductors exhibit both resistance and capacitance
(1) 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.
(2) Capacitance is proportional to volume, and so leads you to try to make conductors as
small and short as possible.
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(3) 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.
ii)
Power circuitry versus electronics
i) Main 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.
3) AC Power Circuits
a) What is it.
i) Voltage varies in sinusoidal form, with mean of zero and maximum (+ and -) amplitude.
Suddenly becomes harder to describe voltage. Commonly either:
(1) Peak voltage (maximum amplitude); useful in electronics
(2) RMS voltage, defined as the equivalent DC voltage that would dissipate power according
to ohm’s law. Typically used for power applications.
(3) Vrms = 0.707 Vpeak
ii) Frequency 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.
iii) 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).
b) Why AC?
i) 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.
ii) 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.
iii) 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.
c) Power distribution.
i) Power is typically supplied to buildings in one of two ways: single-phase or three-phase.
ii) 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.
(1) For a 120V circuit, you connect hot to neutral wires.
(2) For a 240V circuit, you connect two hot wires (thus doubling voltage). Phasing not an
issue.
(3) Europe uses a single-phase, 2-wire system with the hot wire being 240VAC to neutral.
iii) 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.
(1) 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.
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(2) 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.
(3) 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.
(a) 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.
d) Wire colors
i) US, 120/240V: red/black (hot), white (neutral), green/yellow or bare (ground)
ii) Europe, 240V: brown/black (hot), blue (neutral), green/yellow (ground)
iii) US, 3-phase: red/black/blue (hot), green (ground)
iv) Europe, 3-phase: brown/black/gray (hot), blue (neutral), green/yellow (ground)
e) Clean power characteristics
i) Typical problems. AC voltage is supposed to be a perfect sine wave, but never is. Common
problems include:
(1) 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.
(2) 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.
(3) 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.
(4) 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.
ii) Remedies
(1) Isolation transformer. Designed to catch transients. Doesn’t help with voltage regulation.
(2) Power conditioner. Controls both voltage regulation and transients, not distortion. Can
bridge gaps up to a couple cycles. Most commonly used.
(3) 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.
4) Safety issues
a) If you don’t understand what you are doing, don’t do it.
b) Unplug and lock out plugs if possible. Allow time for capacitors to discharge. Even when
unplugged, measure first before assuming something is dead.
c) Never ground your body (feet in water, knees against metal post, etc). With rubber soles on dry
ground, you can grab a 120V wire.
d) Work with one arm where possible. Much better to be shocked between fingers than across your
heart.
e) 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.
f) 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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