Topic 1 - Basic Electrical Principles

In the exam you are expected to:

• Be able to discuss multimeters: digital and analogue, relative advantages and disadvantages.

• Know that Impedance Z = V/I

Measurement of Voltage, Current, and Resistance.

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 by any means.

Voltmeters

·        Voltmeters should have a high value of resistance, but this is not always the case.  Some moving iron 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, which makes them almost perfect.

Ammeters

·        Ammeters have a very low value, but quite definite resistance.

Multimeters

Electrical and electronic engineers do not carry separate voltmeters and ammeters with them to where they are working.  Instead they will carry about a multimeter which is a combined instrument that can:

·        Measure voltage

·        Measure current

·        Measure resistance

·        Measure frequency in some instruments.

·        Test diodes and transistors in some instruments.

 
A typical digital multimeter looks like this. 

An analogue meter is shown below:  

Question 1  What is the most obvious difference between the two types of meter? ANSWER

The circuit diagram for an analogue meter is shown below:

You can tell an analogue meter by the fact that it has a needle and several scales.  The needle often has a mirror behind it to reduce reading errors by parallax.  The analogue multimeter has a moving coil meter with several multipliers and shuts that can be switched in as required.  Cheap multimeters may have the several sockets to make the selection, whereas in more expensive instruments you will get a rotary switch for this purpose.

There may be an internal fuse or a cut out to prevent excessive currents in ammeter mode, which otherwise might damage the instrument.  We can see this in the highlighted region of the picture below.

The analogue meter scale is linear with several ranges for both current and voltage.  You have to know what range you are reading from.

In resistance mode the internal battery supplies a current.  At zero resistance the meter reads at full-scale deflection (its maximum reading).  The variable resistor is used to set the full-scale deflection reading at zero resistance.  At infinite resistance, at open circuit, there is no reading.  In between, the higher the resistance, the lower the current.  The ohmmeter has a non-linear scale[1].

For AC use, a diode is placed in the circuit to rectify the AC to DC to which the meter responds.  In cheaper meters, the meter itself is calibrated with a separate AC scale.  In the more expensive meters, there is a separate range of shunts and multipliers whose values are adjusted for the voltage drop across the diode.  In AC mode, the meter reads rms values.

Digital Multimeter

These instruments have more complicated electronics in then at the heart of which is an analogue to digital converter.  A simplified layout is shown below:

Low voltages are measured directly, high voltages by a potential divider.  Currents are measured by measuring the voltage across a resistor.  To measure resistance a known stable and constant current is produced.  The voltage across the outside resistance is measured.  Since the current is constant, the voltage is proportional to the resistance, so the scale is linear.  Several ranges are included.

The digital multimeter is very close to being a perfect voltmeter, with a very high input resistance, with a very low input current.  We can treat the open circuit terminal voltage of a battery measured by a digital meter as being the emf.[1]

A digital multimeter is not a perfect ammeter.  Like an analogue meter, there is a definite resistance, so there is a voltage drop.

Digital meters have come down in price, and you can buy one for £5, although a very high quality instrument might cost as much as £500.  The cheapest meters are not always reliable, nor are they particularly accurate.  For work that requires the measurement of voltages down to three or four decimal places, an expensive research multimeter is needed.

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

Comparing Multimeters

For very small changes in resistance, neither type of meter is really suitable.  Instead we would have to use a Wheatstone Bridge, the function of which we need not consider here.  Very precise results can be obtained, but the Wheatstone Bridge is quite a fiddle to use.  It is widely used in many electronic measuring instruments.

Question 2  Give two reasons why a digital multimeter is used in preference to an analogue in a school physics lab.

ANSWER

Question 3   Give one advantage and one disadvantage of using an analogue multimeter. ANSWER

Impedance

In AC, we do not use the term resistance.  Instead we use the term impedance.  This is because in AC circuits there are resistive components and reactive components.  If we think of something like the element of a kettle, it is just a thin bit of resistance wire, so it is entirely a resistive load.

If there are capacitors in the circuit, or inductors, then either of these components will tend to oppose the flow of the current.  However they do this in a completely different way to resistors.  The opposition to the flow of current caused by capacitors and inductors is called reactance.  We also can describe capacitors and inductors as reactive components. The combined effect of reactance and resistance is called impedance.

The impedance is defined as the ratio between the rms voltage and the rms current. A simpler way of looking at it is the “effective resistance” of an AC circuit, but be careful when using this.  It is given the symbol Z, and its units are ohms (W).

            Impedance =   rms voltage

                                    rms current

                        Z = V_rms

                               I_rms

We will see in a later example that the square of the impedance can be worked out from the sum of square of the reactance and the square of the resistance.

Impedance ² = reactance² + resistance²

Z² = X_C²+ R²

Question 4   What is the difference between impedance and resistance?   ANSWER

Click HERE to see the Circuit Symbols.  You need to know these.

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