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Bredhurst Receiving and Transmitting Society

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3. Technical Basics

3j Measurements

3j.1 Recall the purpose of a multimeter and understand how to set the meter to the correct range and polarity before connecting to the circuit.

There are two types of multimeter :-



Both the meters can carry out much the same function for measuring Volts, Amps Ohms and each has a range of scales.

There are two considerations to make before taking a reading:-

  • What am I trying to measure?

  • What sort of reading do I expect?

The first item should be straightforward else you would not even be using the meter but the second is often misjudged. To avoid the risk of damage to the meter it is always worth while starting at the highest setting of Amps, Volts and Ohms and then work down to a level that gives you a reading that you considered in within the range you had expected.

Analogue meter need correct polarity to meter else can be damaged

It is more important to properly observe polarity of the meter connection in an analogue meter and to connect the wrong way round will mean that the needle is driven hard against the zero end stop. If this occurs then there is also the likelihood of mechanical damage to the meter rendering it useless.

Digital meter need correct polarity to meter else misleading reading result

With a digital meter on the other hand it often copes well and merely displays a minus sign to show the wrong polarity but still give you the reading required.

3j.2 Understand the advantages and disadvantages of analogue and digital displays, and be able to read analogue and digital values.

Whilst both meters can do the same measurements each has advantages that are reflected in disadvantages in the other type of meter.

Digital meter advantages

Simple to read and usually more accurate reading given as the digital display shows the unit and decimal of the unit on many occasions.

Digital meter disadvantages

The Digital meter on the other hand, due to its style of operation does not know, when readings are changing, which value to display nor can it display the rate of change of readings.

Analogue meter advantages

Where there are changes in reading to be determined then the swing of the needle on the analogue ;meter shows the changes and the rate of the change.

Analogue meter disadvantages

The analogue meter by contrast suffered from errors of parallax where you can miss read due to not looking at the meter from right above. Usually there is a mirror behind the scale so that you can see if you are properly lined up. If you can see a reflection of the needle then you are to one side or the other when there is no reflection seen then the reading would be true.

3j.3 Understand that a voltmeter is always connected in parallel with a circuit component, and that an ammeter is always connected in series with a component.

Measuring Voltages

The measurement of a voltage is trying to assess the potential difference between two places, across a load, and can therefore this can be carried out without breaking the circuit. It is using the meter in parallel with the circuit when the switch is closed.

Measuring Current

When it comes to measuring current the only way to do this with a multimeter is to break the circuit at the point of interest and connect the meter in series with the part of circuit if interest, with the switch closed. You could even measure across the switch !!

3j.4 Understand that current in all parts of a series circuit has the same value and that the potential differences across components in parallel are the same.

When you have a series circuit (where all the components are linked one to the next) at all points, with ;the ;switch ;closed, the measurement of current would give you the same answer, or even across the switch itself with the switch open.

With regard to voltage the only time a similar answer would appear is when you were measuring the voltage across components which themselves are connected in parallel. In this case the meter is a voltmeter.

If you want to determine the total current flowing in a circuit however complex then the ammeter must be put in the circuit in a position where all the current in the circuit is flowing. The best place is as close to the source of the current as it enters or leaves the circuit as the same reading would be made.

Would ammeters at A1, A2, A3 all read the same ? NO

A1 and A3 would as they are respectively measuring the current entering and leaving the circuit but A2 is only measuring the current ;through resistors R1 and R3.

Where could you put a volt meter to measure the voltage in the whole circuit ?

The only place to put the volt meter which is in parallel with the whole circuit is across the input +12V and 0V as shown below.

May be your tutor could build us a circuit so that you can see this as it will be needed in the practicals to demonstrate measuring voltages and currents!

3j.5 Understand the use of voltmeters and ammeters to determine the power applied to a circuit.

Recall that the RF output power (of an amplifier) is less that the DC input power.

By the use of measuring voltage and current in a circuit and applying it to the power triangle P = V x I you learned in the FLC you can calculate the power in the circuit.

However if you did this with an RF power amplifier you would find that the DC input power is higher than the RF power out due to inefficiency in the amplifier, losses due to say heat occur.

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