Generic Skills for Microelectronic Engineers

Component IdentificationAIM:

To be able to identify common electronic components used in miniature and medium power applications. Methods of labelling, range of values, relevant applications and uses

and methods of construction will also be examined.

This session is available at http://maxwell.me.gu.edu.au/sok/gs/gs.html

Many of the photos in this section are taken from retailers catalogues. Reference is made to the retailer for each photo.

Scope

We will examine the following components

ResistorsCapacitorsInductors

TransformersSwitches

Plugs & SocketsDiodes

TransistorsSpecial devices

Resistors

We already know resistors oppose the flow of current like a hose opposes the flow of water. The most obvious parameter that is important about a resistor is its value of resistance. There are however other things that are important.

• Resistance• Tolerance• Wattage • Voltage rating• Construction

Resistance and tolerance

Resistors are made in specific resistances that repeat every decade. Depending on the tolerance (accuracy with respect to its marked value) there are a different number of values in each decade.

These are referred to as the resistor series

E12 has 12 values per decadeE24 has 24 values per decade etc.

The tolerance of the resistor is quoted in percent and tells of the expected variation in value of the resistor. A 10% resistor marked as 100W may actually have a value that varies by 10%.

It is clear that there is no point making resistors in the higher E series values if they have a large tolerance. The sizes would overlap.

The resistance value for most smaller resistors is indicated by coloured bands on the resistor. Some larger resistors have the values stamped on them. (MEMORISE CODE)

WattageThe power dissipated in a resistor can be calculated as

P = V x I

P = I2R

P =V2/R

The size of the resistor generally determines how much heat it can dissipate. The common sizes are 0.25W, 0.5W, 1W, 5W, 10W.

Voltage rating

Typically 0.25W resistors are rated at a working voltage of 250V. The larger sizes usually have a slightly higher rating, ie. 0.5W resistors are typically rated at 350V.

For special high voltage applications there are special high voltage resistors available.

This is analogous to the pressure applied to a pipe. If you exceed its rated pressure it will burst and the water will flood out. If a resistors voltage is exceed it will breakdown and the electrons will flood out. Usually in an arc!

Resistor construction

Resistors are manufactured in different ways for different applications. The common types are:

• Carbon film

• Metal film

• Wire wound

Carbon film resistorsThese are a general purpose resistor offering good stability, long life and low noise.

Metal film resistors

These are a better quality resistor. They offer very good stability, long life and low noise. They can be manufactured to high tolerance. They are therefore usually used to make the higher tolerance resistor series.

Both leaded carbon film and metal film resistors are useful up to approximately 500 MHz. At higher frequencies the resistors start to exhibit inductive effects and surface mount resistors are commonly used.

Wire wound resistorsThese resistors are used for high power applications. Because of their wound construction they exhibit high inductance which can be a problem at high frequency. Their ceramic construction allows them to operate at high temperatures.

Variable resistors

Resistors can also be made so that they can be adjusted by the user. They can be utilized as controls that the user will operate regularly or as adjustments that might be set when the equipment is built then left alone.

Rotary Potentiometers Slider potentiometers

A potentiometer (pot) is simply a voltage divider that is adjustable.

Potentiometers come in many sizes and also come in a number of tapers. The common ones are

A Logarithmic TaperB Linear Taper

Linear tapers are used for circuits where the output signal is required to change linearly with the movement of the control, ie the voltage from a variable power supply. logarithmic tapers are used for audio equipment where the output should change logarithmically. The human ear receives sound logarithmically, ie double the amplitude does not sound twice as loud. We therefore use A tapers for volume controls etc.

Trimpot

When the variable resistance will be set during manufacture we usually use screwdriver adjustable potentiometers called trimpots. Many different varieties exist.

Capacitors

+

Capacitors come in a huge range of types. Understanding where they are best used is very important. You will learn more about there construction in other subjects. We will examine:

• Polyester• Monolithic• Tantalum• Ceramic• Electrolytic

Polyester (Greencaps)

These non-polarised capacitors are good for general purpose applications where small to medium size low cost capacitors are required. Their tolerance is usually 10% and they typically range in size from about 0.001mF to 0.47 mF. (Special ones are available for speaker crossovers up to 10mF) The capacitance value is normally written on the capacitor using the same notation as for resistors except the base unit is pF.

474 = 470000pF = 0.47mF

The MKT type (above right) have fixed leg spacing to make circuit board layout more precise. The main usage is for bypass or coupling in low to medium frequency circuits.

Different voltage ratings are available (100v, 250v and 630v)

Monolithic

These non-polarised capacitors have small size for their capacitance value. They are labelled similarly to polyester caps. Size ranges from 0.001mF to 1.0 mF.

The main use for these capacitors is for power supply filtering on digital electronic circuit boards, ie computers. Their small size allows less wasted space.

They typically have a 10% tolerance and voltage rating of 50V.

Tantalum

These caps are polarised (they have a +ve and -ve leg). They have very large capacitance for their size. They can only be used in circuits where one leg stays more positive always. (usually marked by +ve sign)

Typical range of 0.10mF to 47.0 mF with voltage ratings up to 35V.

Applications include power supply filtering or bypassing where a larger capacitance is required. Typical tolerance is 10%.

Ceramic

Non-polarised capacitors with low values of capacitance, typically 1pF to 0.1mF. Typically 50V rated but higher available (3kV). Tolerance is usually low, 20%, some higher tolerance ones available.

Main application is for low capacitance values especially suited to Radio Frequency (RF) circuits or for high voltage applications.

Electrolytic

These capacitors are the largest valued capacitors available. They range typically from 1.0mF to 10,000mF. They are polarised and have voltage ratings up to around 63V. Some special non-polarised ones are available. They come in axial (RT) or single ended (RB) lead configurations.

They must have +ve DC voltage on the +ve leg. The -ve leg is usually marked with arrows. They are typically used as the large filter capacitors in power supplies.

TrimCaps

Sometimes an adjustable capacitor is required. These are called trimcaps and are usually only available in small sizes 1pF to 100pF. They are normally used to fine tune a capacitor that might be performing a timing or filtering function. Typically found in radio

receivers.

Inductors

Simple chokes are just coils of wire on a former of plastic or iron based material. Large chokes are used in power supplies to stop AC signals but let DC signals pass.

Small chokes are used at higher frequencies for the same purpose in radio circuits. they are often mistaken for resistors.

Transformers

Transformers are used to change the voltage of an AC signal to a new voltage. We commonly use these to change 240V from the mains to a lower voltage suitable to power our circuits. They are made of two coils wound onto the same former.

Plugs and Sockets

There are hundreds of plugs and connectors in use today. You will learn many over time but should know the following

• Phono (2.5mm, 3.5mm, 6.5mm) - audio for speakers, mics, headphones etc• RCA - line level audio• BNC - radio frequency signals, test equipment• Banana - general purpose power and low freq signals• Crocodile (alligator) - general purpose power and low freq signals testing

Diodes

Diodes are like valves that only allow electrons to pass in one direction. the current will flow from Anode to Cathode only. The Cathode is marked always with a stripe.

They are very often used to turn AC signals into DC signals in power supplies. We often use 4 diodes to do this and they are available in a package called a bridge rectifier especially for this purpose.

Light Emitting Diodes

Special Light Emitting Diodes (LEDs) used for indicator lights etc. Usually require about 20 mA to run. Available in different physical sizes.

Transistors

Many thousands of transistors are available for many different applications. At this stage it is important for you to know some of the common packages that transistors are made into.

The higher power devices often need to be attached to a heat sink as they get quite hot. These packages have some form of mounting holes etc.

Special Devices

Integrated circuits

Voltage Regulators

Switches

Number of poles (3)

Numberofthrows (2)

Switches are classified into their electrical operation and mechanical type. The number of poles, number of throws and whether it is a momentary switch or not is described. A momentary switch does not sit at either throw in the rest position and does not connect to either throw until operated.

Eg. A DPDT switch is a double pole double throw switch. Many combinations are available.

SPDT, 4PDT, SPST etc.

Toggle switch Rocker Switch

Slider switches are also available but less common.

Fuse Holders

To provide protection to the circuitry due to a fault condition we often use fuses. They are wire devices designed to melt when excess current flows through them. This can prevent a fire.

Fuse holders are used to hold the fuse.

Surface Mount ComponentsWith electronic circuits needing to become much smaller and frequencies of operation becoming much higher we see a lot of surface mount design being done.