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.
The multimeter is an essential piece of measuring equipment. They are available in analogue form
with a needle that moves across a number of different scales and also
digital form with a display based on a LED display. Typically they
measure:
Resistance
DC Voltage
AC Voltage
Current
Some can check transistors and sometimes there is a check for inductance and capacitance.
Readings are divided into ranges to make for more accurate readings. For example the resistance ranges could be:
0-200 ohms
0-2000 (2K)ohms
0-20K ohms
0-200K ohms
0-2M
0-20M
Setting the scale to use, is achieved by a rotary switch.
The two leads are coloured black and red, it is important that
the red lead is plugged into the one of the positive sockets and the
black lead into the socket labelled Common or just Com.
When measuring voltage and current it is best to set the scale to a
high range first and then move to a lower range. This will prevent
accidentally damaging the meter by using a voltage or current that is too
high.
Current measurement often involves moving the leads to a different socket for higher current readings.
3j.2 Understand the advantages and disadvantages of analogue and
digital displays, and be able to read analogue and digital values.
Comparing digital and analogue meters.
Digital meters are usually cheaper than analogue.
Analogue meters are often easier to read, particularly if the voltage /
current is changing slowly. You do, however, have to learn which scale
to use and what the subdivisions of the scale are equal to.
Digital meters can be more robust as they do not have a large scale and pointer with bearings to reduce friction.
Some of the cheaper analogue meters have a fairly low impedance (e.g.
10K). If used in-circuit this can alter the actual reading. Digital
meters have a much higher impedance in the order of Mega ohms. They are
less likely to change the reading.
Both types of meter require batteries. In the analogue circuit this is
to operate the resistance scales. In a digital meter power is required
for the LED readout and the operation of the measuring circuit as well as reading resistance.
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.
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. Current and voltage readings
To measure current the meter has to be placed in series with the component.
To measure voltage the meter is placed across the component
The current reading is the same in all parts of a series circuit
although in complex circuits different currents may be present in
different parts of the circuit.
In the drawing opposite, the current should read the same when measured
anywhere in the circuit. So, for example, cutting the wire between R3
and the battery and then inserting the ammeter will give the same
reading inserting the meter between R1 and R2.
When components are connected in parallel the voltage will be the same across each component.
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 than the DC input power.
Calculating the input power of a transmitter.
We already know that power in Watts used by a circuit is equal to the voltage multiplied by the current.
So. to measure the total power used by a radio, turn the set on. Measure
the potential difference across the input from the power supply
and measure the current flowing through the power supply leads (either
positive or negative) by disconnecting one of the power leads and
placing the meter in series with the power supply. Make sure the
positive lead is connected to the positive terminal of the supply.
Multiply the current in amps by the potential difference in volts. This will give you the power in Watts.
For example a receiver has a supply of 12.2 volts and draws a current of 0.5amps
The power consumed = 12.2 x 0.5 = 6.1Watts.
Calculating the output power of a transmitter.
If you find out the power consumed by a transmitter using the above
technique you cannot assume that this is power output to the antenna.
For example a transmitter at 45v draws 1.5 amps of current
P=VI = 45 x 1.5=67.5Watts
When you measure the power output to the antenna this may be only 33Watts. So, what has happened to the 34.4 watts (67.5-33)?
The answer is that transmitters are not 100% efficient. Some of the
power is converted to heat. This is why modern transistor amplifiers
have large heatsinks and fans. Both to remove the heat from the
transistors.
