Introductory Electronics Tutorial  -  Basic Measurement

Learning Objectives

• Use and interpret circuit diagrams using conventional symbols;

• Use meters to measure electrical quantities;

• Use the CRO to measure electrical signals;

Symbols

Electronic engineers use standard symbols in their circuit diagrams.  You will need to know these.  Click on the button to take you to the Components page.

Circuit diagrams are essential to communicate your design with other engineers.  They are often called schematics.   Although you don't have to be a good artist to do good circuit diagrams, it is essential that they are clear.  You can buys specialist circuit design software, which is eye-wateringly expensive, but will design printed circuit boards.  You can draw circuit symbols and diagrams using graphics from MS Word, a graphics package like Adobe Photoshop.  This circuit diagram for a logic tutor kit was made from MS Word graphics.

Although it's usual to use software nowadays, you may need to do it the old-fashioned, low-tech, way.

• Use a sharp pencil;

• Use a ruler;

• Conductors meet at right-angles;

• Junctions have black circles;

• Put down the values of the components.

A bad circuit diagram is use to neither man nor beast.

A good circuit diagram is one that another electronic engineer can pick up and build the circuit that you have designed.  And it will work in exactly the same way as your original circuit.

Some books (or software) will have symbols that are different to the ones you see on the sheet.  If you are not sure, ask.

 Question 1 Fill in the words for the interactive cloze activity.

You will be familiar with the use of voltmeters and ammeters in circuits, in that the ammeter is wired in series with the component, while the voltmeter is wired in parallel.

We have always treated ammeters and voltmeters as perfect.

• A perfect voltmeter has an infinite resistance so takes no current.

• A perfect ammeter has zero resistance.  Therefore there is no voltage drop across it.

However, real voltmeters and ammeters are not perfect.

Voltmeters

·   Voltmeters should have a high value of resistance, but this is not always the case.  Some moving coil or analogue meters (the kind with a needle and a scale) have quite a low resistance, about 10 000 W.  This can lead to serious reading errors when we measure high resistance circuits.  The picture on the left shows a demonstration meter used for teaching electricity in Physics lessons.

Picture by Hannes Grobe, Wikimedia Commons                                         Picture by Ravn, Wikimedia Commons

·

A digital voltmeter is shown in the picture on the right.   Digital voltmeters have a very high resistance, about 107 W (10 000 000 W  or 10 Megohms), which makes them almost perfect.  The picture shows a commercial digital voltmeter, with a home-made instrument built by students of the Technical University of Berlin.

Whatever the type of instrument, it is vital that the range selected is appropriate to the voltage measured.  The picture below shows the mess that can be made when a 30 volt instrument was connected to a 230 V supply.  One of the multiplier resistors has completely blown apart.

Picture by Daan Berg, Wikimedia Commons

Ammeters

Ammeters have a very low value of, but quite definite, resistance.   The meter has a very low value resistor called a shunt wired in parallel.

Many schools and colleges buy analogue meters that can be turned into voltmeters, ammeters, or ac meters by plugging appropriate multipliers and shunts onto the instrument.

When you use a meter like this, you need to know what voltages or currents you are using.  It would be no good measuring a current of 50 mA (milliamps) using the 10 A shunt.  If you were measuring a voltage of 30 V, you would use the 50 V range on the multiplier.  You also need to read the correct scale.  If you are using the 50 V range on the multiplier, you need to use the bottom scale.  A needle deflection to 1 would be 10 V.

If you use the 10 A range on the shunt, 6 on the top scale will be 6 A.  However, if you were using the 2 A range, you need to have to multiply the top scale by 0.2 - if you have a reading of 6 on the top scale, the current is 6 × 0.2 = 1.2 A.

Multimeters

The Multimeter is a combined instrument that can:

• Measure voltage

• Measure current

• Measure resistance

• Measure frequency in some instruments.

• Test diodes and transistors in some instruments.

1. Function/Range Switch: selects the function (voltmeter, ammeter, or ohmmeter) and the range for the measurement.

2. COM Input Terminal: Common ground, used in ALL measurements.

3. V Input Terminal: for voltage or resistance measurements.

4. 200 mA Input Terminal: for small current measurements.

5. 10 A Input Terminal: for large current measurements.

6. Low Battery LCD: appears when the battery needs replacement.

There may be an internal fuse or a cut out to prevent excessive currents in ammeter mode, which otherwise might damage the instrument.

The digital multimeter is very close to being a perfect voltmeter, with a very high input resistance, with a very low input current.

Digital multimeters have functions whereby they can test capacitors, diodes, and transistors.  They can also display frequency.

 Question 2 Do the matching exercise Question 3 Multiple choice question on ammeter Question 4 Multiple choice question on meters

### Comparing Multimeters

 Property Analogue Digital Reading errors Can occur, especially when the pointer off marks. Less likely Input resistance as a voltmeter Moderate, varies with range, about 20 kW/V High, about 10 MW on all ranges. Scale/display Continuous In steps of 1 digit Response to input Continuous Samples taken at intervals, about every microsecond Power used None except when used as an ohmmeter Battery needed, LCD instruments take a very small power. Cost £10 - £200 £5 - £500

Have a go at these questions:

 Question 5 Interactive question on using multimeters compared to separate ammeters and voltmeters Question 6 Interactive question on analogue or digital?

The Cathode Ray Oscilloscope

The cathode ray oscilloscope (CRO) is a particularly useful tool for the electronic engineer.

The most important controls that we use are:

• The vertical sensitivity setting, calibrated in V/cm.

• The time base, in s/cm.

The CRO is a perfect voltmeter as its input resistance is very high indeed.

Remember:

• We measure the voltage on the vertical  axis.  We can adjust the sensitivity by turning the knob marked y-gain or voltage gain.

• The horizontal direction is determined by the time base setting.   We can change this by using the time base knob.

As well as analysing the waveform, there are two measurements we can make with the CRO:

• We can determine the peak voltage of the AC waveform shown below.

• We can also read the period, which in turn allows us to work out its frequency.

For a more detailed look at the CRO, go to www.doctronics.co.uk.

Notice that:

• The peak to peak voltage is 12.8 V.  Often engineers read the peak to peak voltage off the CRO as the determination of the 0 level is not always easy.  The peak voltage is half of the peak to peak voltage.

• The root mean square voltage, which we use in electrical calculations, is the peak voltage divided by Ö(2)

• Therefore the Vrms = 6.4 ÷ Ö2 = 4.5 V

 Question 7 Interactive question on CRO compared with voltmeter

We can use the CRO as a voltmeter by placing it in parallel with a component.  We can also use it as an ammeter by placing it across a resistor of known resistance.

The normal CRO displays wave-forms in real time.  When the wave-form stops, the display stops as well.  However it is possible to buy a storage scope, which stores the image of the waveform for more detailed analysis.  They are expensive.

A more recent innovation has been the oscilloscope that can be connected to a computer.  The computer makes the display.

Building Circuits

Electronic engineers test their circuits on breadboard or prototype board.  It used to be called breadboard, because the early engineers used the breadboard from the kitchen, sticking nails in it.  What other family members had to say is not recorded.

The horizontal lines take the power supplies.  The components need to go horizontally between the vertical lines.  If you use the same vertical line, you will short the component out.  You can put components vertically across the the channel in the middle.

It is worth planning how you are going to arrange your components.  You can get proformas to help you to do this.

When we have got our circuit working, we can solder it onto strip board.  It is helpful to draw out the circuit on a strip board planner.  In your project, you are NOT expected to do any soldering.

Be careful with soldering:

·        Heat both the wire and the board sufficiently so that the solder runs in.

·        Be careful not to overheat the component.  Some components are easily damaged.  Use a crocodile clip as a heatsink if necessary.

·        Do not apply too much solder, otherwise you could bridge the tracks.

For more help with soldering, go to the video clip on How to Solder.

Once we have found that the circuit works properly, we can design a printed circuit board (PCB), especially if we are going to construct several copies of the board.

It is possible to get software that will design the PCB.  In large factories PCBs are printed in large quantities and components are placed by computer-controlled assembly machines which can operate at very high speed, producing circuit boards very cheaply.

Printed circuit boards can only carry a limited current.  Excessive currents will cause overheating.  Large currents need thick wires.

Electronic components are sensitive to high voltages and strong electric fields.  If you are handling circuit chips, you need to earth yourself, using an earthing strap.  Otherwise static electricity could wreck the chips.  A metal case will protect electronic components from high electric fields.  Whenever I do the Van der Graaff generator in Physics, I always get my students to remove their mobiles, I-pods, etc and put them in a metal cabinet.  I also turn off the computer, the whiteboard, and the projector.

 Question 8 Interactive question on prototyping

Summary

• A perfect voltmeter has an infinite resistance so takes no current.

• A perfect ammeter has zero resistance.

• ##### Need a battery

Multimeters

• Can be analogue or digital.

• Measure voltage, current, and resistance.

• Digital multimeters have functions with which they can test capacitors, diodes, and transistors.

CRO

• Displays waveforms.

• Needs calculations to read voltage and time period.

Circuit Boards

• Electronic engineers test their circuits on breadboard.

• Then they use strip-board.

• PCBs are used to mount mass produced circuits.

 Using the multimeter How to solder Electronics 2000 Presentation on CRO