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Introduction and Basic Measurement  

You will learn about:

  • Electrical measurement with various instruments
  • The CRO
  • Construction methods.

Introduction and Basic Measurement

Electronics is about getting electrical currents to do more interesting things, beyond heating, lighting, and driving electric motors.  Electronic devices have made a major impact on society in the areas of:

  • Entertainment – TV, Hi-fi, Digital broadcasting
  • Telecommunications – land based and mobile telephony
  • ICT – computers, both in commerce and at home.

Question 2

Symbols

Electronic engineers use standard symbols in their circuit diagrams.  You will need to know these.

Tools of the Trade

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 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.
  • Digital voltmeters have a very high resistance, about 107 W (10 000 000 W  or 10 Megohms), which makes them almost perfect.

Ammeters

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

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.

 

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

The Cathode Ray Oscilloscope

The cathode ray oscilloscope 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.

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

Now try Question 8.

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.

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