Electronics Chapter 1: Passive Components 1.9 1.10

Electromechanical Devices: Switches

• Switches are essential components in electronic equipment, available in various configurations and types.
• Toggle switches are simple and important, with different types including SPST (single-pole single-throw), SPDT (single-pole double-throw), and DPDT (double-pole double-throw).
• Toggle switches can have "center OFF" positions and up to four poles switched simultaneously.
• They are always "break before make," meaning the moving contact never connects to both terminals in an SPDT switch.

Momentary-Contact Switches

• Momentary-contact switches, also known as pushbutton switches, are useful for momentary-contact applications.
• They are drawn schematically with NO (normally open) and NC (normally closed) labels.
• SPDT momentary-contact switches have labeled terminals NO and NC, while SPST types are self-explanatory.
• Momentary-contact switches are always "break before make."

Rotary Switches

• Rotary switches are available with many poles and positions, often as kits with individual wafers and shaft hardware.
• They can be shorting (make-before-break) or non-shorting (break-before-make) types.
• Shorting types are useful to prevent an open circuit between switch positions, while non-shorting types are necessary if separate lines being switched to one common line must not be connected to each other.

PC-Mounting Switches

• PC-mounting switches, often called DIP switches, are commonly used on printed-circuit boards.
• They can be coded rotary switches and are used for set-and-forget internal settings.
• Multipin header blocks with slide-on "shunts" can substitute for DIP switches.

Other Switch Types

• Exotic switches include Hall-effect switches, reed switches, and proximity switches.
• All switches have maximum current and voltage ratings, and operating below these ratings is generally acceptable.
• Some switches rely on substantial current flow to clean away contact oxides, so using a switch designed for "dry switching" is important when switching low-level signals.

Switch Examples

• A simple switch circuit can be used to sound a warning buzzer if the driver of a car is seated and one of the car doors is open.
• The circuit uses normally open switches and requires the seat switch to be closed and one or both door switches to be closed.
• Switch circuits can be used to turn a ceiling lamp on or off from a switch at either of two entrances to a room.
• A generalization of this circuit allows any of N switches to turn a light fixture on or off, requiring two SPDT switches and N-2 DPDT switches.

Electromechanical Devices: Relays

• Relays are electrically controlled switches that can be used to switch high-voltage or high-current circuits.
• Traditional electromechanical relays work by using a coil to pull in an armature, which closes the contacts, when sufficient current flows.
• There are various types of relays available, including:
  • Latching relays
  • Stepping relays
  • Mercury-wetted relays (for high-speed applications)
  • Reed relays (for high-speed applications)
  • Giant relays (used by power companies to switch thousands of amps)

Solid-State Relays (SSRs)

• Solid-state relays use a semiconductor electronic switch turned on by a LED.
• SSRs provide better performance and reliability than mechanical relays, but at a greater cost.
• Advantages of SSRs include:
  • Rapid operation
  • No contact "bounce"
  • Smart switching of AC power (turning on at zero voltage and off at zero current)

Applications of Relays

• Relays are primarily used for remote switching and high-voltage (or high-current) switching, where electrical isolation between the control signal and the circuit being switched is important.
• Transistor switches can also be used for electrically controlled switching of signals within a circuit, eliminating the need for relays (covered in Chapters 2 and 3).

Connectors

• Connectors are essential components in electronic equipment, allowing signals and DC power to be routed between various parts of an instrument.
• They provide flexibility by permitting circuit boards and larger modules to be unplugged and replaced.
• Connectors come in a vast variety of sizes and shapes, with over 43,000 individual varieties available.

Single-Wire Connectors

• Simple pin jacks or banana jacks are used on multimeters, power supplies, etc.
• They are handy and inexpensive but not as useful as shielded-cable or multiwire connectors.
• The humble binding post is another form of single-wire connector, notable for its clumsiness.

Shielded-Cable Connectors

• To prevent capacitive pickup, shielded coaxial cables are used to pipe signals between instruments.
• The most popular connector is the BNC type, which connects with a quarter-turn twist and completes both the shield and inner conductor circuits simultaneously.
• Other connectors for use with coaxial cables include TNC, type N, SMA, SMB, LEMO, SMC, MHV, and SHV.

Multipin Connectors

• Very frequently, electronic instruments demand multiwire cables and connectors.
• There are dozens of different kinds of multipin connectors, including the simple three-wire "IEC" powerline cord connector.
• Popular multipin connectors include type-D subminiature, Winchester MRA series, MS type, and flat ribbon-cable mass-termination connectors.

Card-Edge Connectors

• The most common method used to make connection to printed-circuit cards is the card-edge connector.
• Card-edge connectors mate to a row of gold-plated contacts at the edge of the card.
• They come with different lug styles according to the method of connection and can have from 15 to 100 or more connections.

Indicators

• Meters are used to read out voltage or current values, and they come in two types: moving-pointer type and digital-readout meters.
• Digital-readout meters are more expensive and more accurate than moving-pointer type meters.
• Both types of meters are available in various voltage and current ranges.
• There are also exotic panel meters that read out specific values such as VUs (volume units), temperature, and percentage motor load.

Lamps, LEDs, and Displays

• Small incandescent lamps used to be standard for front-panel indicators, but they have been replaced with LEDs.
• LEDs behave electrically like ordinary diodes, but with a forward voltage drop in the range of 1.5 to 2 volts.
• LEDs light up when current flows in the forward direction, and they typically require 2 mA to 10 mA to produce adequate brightness.
• LEDs are cheaper than incandescent lamps, they last longer, and they come in four standard colors as well as "white".
• LEDs can also be used for digital displays, such as 7-segment numeric displays or dot-matrix displays.
• LCDs are generally preferred for displaying more than a few digits or characters, and they come in line-oriented arrays with a simple interface.
• LCDs are inexpensive, low power, and visible even in sunlight, but back-lighted versions are not low power.

Variable Components

• Variable resistors, also known as potentiometers (pots, trimmers), are used as panel controls or internal adjustments in circuits.
• They have a rotatable "wiper" contact and are available in different types, including multiturn (3, 5, or 10 turns) with counting dials for improved resolution and linearity.
• Ganged pots (several independent sections on one shaft) are also available, but limited in variety.

Symbol and Electronic Version

• The symbol for a variable resistor (pot) is shown in Figure 1.127, sometimes with CW and CCW to indicate clockwise and counterclockwise ends.
• An all-electronic version of a potentiometer can be made with an array of electronic switches that select a tap in a long chain of fixed resistors.

Digital Potentiometers

• Digital potentiometers are available as ICs, with up to 1024 steps, and come as single or dual units, some being nonvolatile (remembering their last setting even when power is off).
• They are used in consumer electronics, such as TVs and stereos, for adjusting volume using infrared remote control.

Important Points

• Don't use a potentiometer as a substitute for a precise resistor value in a circuit, as they are not as stable as good (1%) resistors and may not have good resolution.
• Use a combination of a 1% precision resistor and a potentiometer, or a series combination of several precision resistors, for a precise and settable resistor value.

Other Variable Components

Variable Capacitors

• Variable capacitors are primarily used in RF circuits, are confined to smaller capacitance values (up to about 1000 pF), and are available as trimmers for in-circuit adjustments and panel types for user tuning.
• The symbol for a variable capacitor is shown in Figure 1.128.

Variable Inductors

• Variable inductors are made by arranging to move a piece of core material in a fixed coil, and are available with inductances ranging from microhenrys to henrys, typically with a 2:1 tuning range for any given inductor.
• Rotary inductors (coreless coils with a rolling contact) are also available.

Variable Transformers

• Variable transformers are handy devices, especially the ones operated from the 115-volt ac line, and are configured as "autotransformers" with a sliding contact.
• They are commonly called Variacs and are made by General Radio, Technipower, and others, providing 0 to 135 volts ac output when operated from 115 volts.
• Important warning: the output is not electrically isolated from the power line, unlike a transformer.

Component Markings and Confusion

• Capacitors are notorious for lacking unit specifications, spanning 12 orders of magnitude (picofarads to farads).
• Ceramic SMT capacitors often have no markings whatsoever.
• Capacitor values are sometimes represented as integers (e.g., "470" meaning 470 pF) and sometimes in exponent notation (e.g., "470" meaning 47 × 10⁰, or 47 pF).

Date-Code Gotcha

• Date-codes (yydd) can be mistaken for part numbers, leading to confusion.
• Examples include the four date-code examples in the photo.

Tiny Component Markings

• As components shrink in size, markings become increasingly brief.
• Manufacturers are adopting alphanumeric codes for each component, similar to the pharmaceutical industry.
• Examples include National's LMV981 op-amp, marked "A78A" for SOT23, "A77" for SC70, and a single letter "A" (or "H" for lead-free) for microSMD.

Surface-mount Technology (SMT)

• SMT components are excellent from an electrical point of view: low inductance and compact.
• However, they are difficult to wire up in prototype breadboard fashion, unlike "through-hole" (or "leaded") components.
• Examples of "through-hole" components include resistors with axial leads and integrated circuits in DIP (dual in-line) cases.

Challenges of SMT

• SMT components are nearly impossible to work with due to their small size, making experimenters feel "all thumbs".
• The smallest SMT components, such as the "01005" size, measure 0.016′′ × 0.008′′ (0.4mm × 0.2mm) and are barely thicker than a human hair.
• These tiny components can be indistinguishable from dust.

Adapting SMT Components

• One way to work with SMT components is to use adapter carriers that convert an SMT integrated circuit to a fake DIP.
• Companies like Bellin Dynamic Systems, Capital Advanced Technologies, and Aries offer these adapter carriers.

Characteristics of SMT Packages

• Dense SMT packages have no leads, only an array of bumps (up to several thousand) on the underside.
• These packages require "reflow" equipment before they can be used.

Impact of SMT

• The majority of new components are offered only in surface-mount packages.
• This trend is a challenge for lone basement experimenters and inventors.

Variety of SMT Components

• SMT configurations are available for a wide range of passive component types, including connectors, switches, trimmer pots, inductors, resistors, capacitors, crystals, and fuses.

Electronics Fundamentals

• Electronic circuits consist of components connected together with wires.
• Current (I) is the rate of flow of charge through some point in these connections; it’s measured in amperes (or milliamps, microamps, etc.).
• Voltage (V) between two points in a circuit can be viewed as an applied driving “force” that causes currents to flow between them; voltage is measured in volts (or kilovolts, millivolts, etc.).

Signal Types and Amplitude

• Digital electronics deals with pulses, which are signals that bounce around between two voltages (e.g., +5 V and ground).
• In the analog world, sinewaves are popular; a periodic signal is characterized by its frequency f (units of Hz, MHz, etc.) or period T (units of ms, μs, etc.).
• For sinewaves, it’s often more convenient to use angular frequency (radians/s), given by ω = 2πf.

The Relationship Between Current and Voltage

• The simplest linear device is the resistor, for which I = V/R (Ohm’s Law).
• The term “linear” means that the response (e.g., current) to a combined sum of inputs (i.e., voltages) is equal to the sum of the responses that each input would produce.

Resistors, Capacitors, and Inductors

• The resistor is clearly linear, but it is not the only linear two-terminal component; linearity does not require I ∝ V.
• Capacitors (C) and inductors (L) are also linear components, for which there is a time-dependent relationship between voltage and current: I = C dV/dt and V = L dI/dt, respectively.
• These components are described by their impedances, the ratio of voltage to current (as a function of frequency) when driven with a sinewave.

Series and Parallel

• The impedance of components connected in series is the sum of their impedances.
• When connected in parallel, it’s the admittances (inverse of impedance) that add.
• For capacitors in parallel, the formula looks like the formula for resistors in series, Cparallel = C1 + C2 + …; and vice versa for resistors and inductors.

Basic Circuits with R, L, and C

• Resistors are everywhere and can be used to set an operating current or convert a current to a voltage.
• The voltage divider is an important circuit fragment, whose unloaded output voltage is Vout = Vin R2 / (R1 + R2).
• By replacing one of the resistors in a voltage divider with a capacitor, you get a simple filter: lowpass if the lower leg is a capacitor, highpass if the upper leg is a capacitor.

Loading; Thévenin Equivalent Circuit

• Connecting a load (e.g., a resistor) to the output of a circuit causes the unloaded output voltage to drop.
• The signal source is modeled as a perfect voltage source Vsig in series with a resistor Rsig.

The Diode, a Nonlinear Component

• The ideal diode conducts in one direction only; it is a “one-way valve.”
• The onset of conduction in real diodes is roughly at 0.5 V in the “forward” direction, and there is some small leakage current in the “reverse” direction.