Transcript Technical Basics - 2

Chelmsford Amateur Radio Society
Intermediate Course
(3) Technical Basics - 2
AC & Impedance
Chelmsford Amateur Radio Society
Intermediate Licence Course
Murray Niman G6JYB
Slide Set 17: v1.0, 25-May-2009
(3) Technical Basics -2: AC & Impedance
1
DC & AC

DC - Direct Current
•
Cells/Batteries provide a source of DC power
•
Direct Current flows in a single direction

AC - Alternating Current
+
•
AC is easier to generate and transform
-
•
Mains is 50Hz AC. Radio Frequencies (RF) use High Frequency AC
•
Simple items such as Filament Light Bulbs work with AC and DC, but
many electronic components are sensitive to the direction of current
Chelmsford Amateur Radio Society
Intermediate Licence Course
Murray Niman G6JYB
Slide Set 17: v1.0, 25-May-2009
(3) Technical Basics -2: AC & Impedance
2
AC Frequency & Period
• The Foundation course just described the shape of a sine wave
whilst Intermediate requires a deeper understanding
Amplitude
Time
One Cycle
• The Period, T of one cycle, in seconds is equal to 1/f, where f is in Hertz
Frequency, f = 1 / T
Chelmsford Amateur Radio Society
Intermediate Licence Course
or
Murray Niman G6JYB
Period, T = 1 / f
Slide Set 17: v1.0, 25-May-2009
(3) Technical Basics -2: AC & Impedance
3
An AC Cycle
• One way of looking at the sine wave is as a cycle of 360 degrees
• The voltage or current has a complete rotation (like an alternator)
+Vpeak
270°
N
S
0°
90°
Time
Slipring
Brush
AC Volts Output
-Vpeak
180°
360°
• The use of degrees indicates the time or ‘phase’ within one cycle
• Unlike a constant DC source, volts/current vary from zero to a +/- peak
and back to zero, so we need a way of describing the average
Chelmsford Amateur Radio Society
Intermediate Licence Course
Murray Niman G6JYB
Slide Set 17: v1.0, 25-May-2009
(3) Technical Basics -2: AC & Impedance
4
Peak & Peak-Peak
• Sine waves and other waveforms have varying amplitude with time
• The variation has positive and negative values during the cycle
• The peak value is the level of a positive or negative peak
• The peak-peak is the difference between the negative and positive peaks
Amplitude
+
Chelmsford Amateur Radio Society
Intermediate Licence Course
Vpk
Time
Vpk-pk
One Period, T
Murray Niman G6JYB
Slide Set 17: v1.0, 25-May-2009
(3) Technical Basics -2: AC & Impedance
5
RMS Value
• RMS = Root Mean Square, which is a form of averaging
• The RMS value of any varying waveform is the equivalent of the
constant DC Voltage that would have the same power or heating effect
• For a sine wave, the RMS value is equal to 1/2 of the peak value.
Vrms = Vpk/2
or
Vrms = 0.707 x Vpk
Vpk +
Vrms
Time
Chelmsford Amateur Radio Society
Intermediate Licence Course
Vpk-pk
Example:
AC Mains is 230Vrms
So…
Vpk = 230/0.707 = 325V
Vpk-pk = 2xVpk= 650V
One Period, T
Murray Niman G6JYB
Slide Set 17: v1.0, 25-May-2009
(3) Technical Basics -2: AC & Impedance
6
Frequency & Wavelength
• In air the velocity, v of radio waves is a constant ( ~3x108m/s)
• So if the frequency increases, the wavelength decreases, and
vice versa, determined by: v = f x 
• Example 7MHz=40m, 10MHz=30m, 14MHz=20m approx
Frequency,
v
f
Wavelength,
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Intermediate Licence Course
v m/s
f Hertz

Murray Niman G6JYB
 metres
Slide Set 17: v1.0, 25-May-2009
(3) Technical Basics -2: AC & Impedance
7
AC & Transformers
• Transformers consist of coils of wire sharing the same magnetic
field and may have an iron or ferrite core to concentrate the field
• Energy is transferred from one coil to the other by AC changing
the magnetic field - which can not happen with constant DC
• Voltages (such as AC Mains) can be stepped down to a lower
level if fewer turns of wire are on the secondary coil than on the
primary - or can be stepped up if the secondary has more turns
Basic
Transformer
Symbol
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Intermediate Licence Course
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General
Transformer
Primary
Murray Niman G6JYB
Secondary
Slide Set 17: v1.0, 25-May-2009
(3) Technical Basics -2: AC & Impedance
8
AC & Components
• Components that store energy in electric or magnetic fields
have a finite reaction time before they reach a steady state or
the stored energy may oppose the change being applied
– Most prominent in Capacitors and Inductors/Transformers
• This behaviour is different from simple Resistance
• If the input is changing (ie AC) then a time difference occurs
between the current flowing and the voltage being applied
– Charge rushes in or out of capacitor plates
– Magnetic fields in coils create back EMFs which oppose current
• You can still apply Ohms law (‘R’=V/I) to such situations to
assess this form of AC Resistance, known as Reactance, X
Chelmsford Amateur Radio Society
Intermediate Licence Course
Murray Niman G6JYB
Slide Set 17: v1.0, 25-May-2009
(3) Technical Basics -2: AC & Impedance
9
Reactance in Inductors
• As AC current flows (or tries to) Magnetic fields create
‘back EMFs’ which oppose the input current
• This AC Resistance, is termed Reactance
• For an inductor, the Inductive Reactance has symbol XL
• Applying Ohms law:XL = Vrms / Irms
Irms
Vrms
L
AC Source
Freq, Hz
Chelmsford Amateur Radio Society
Intermediate Licence Course
Murray Niman G6JYB
Slide Set 17: v1.0, 25-May-2009
(3) Technical Basics -2: AC & Impedance
10
Reactance in Capacitors
• When an AC Voltage is applied, charge rushes
in/out of a capacitor plate, attracting/moving
charge on the opposite plate
• This induced charge effectively enables
an AC current to flow (unlike DC)
• So at AC, we have Volts and Current
and its AC resistance is termed
Capacitive Reactance, XC
• Applying Ohms law:XC = Vrms / Irms
Chelmsford Amateur Radio Society
Intermediate Licence Course
Irms
Vrms
C
AC Source
Freq, Hz
Murray Niman G6JYB
Slide Set 17: v1.0, 25-May-2009
(3) Technical Basics -2: AC & Impedance
11
Impedance
• In combinations of capacitors, resistors, or
inductors, current will result in energy
transfer (into heat) in the resistors and
energy storage and release in the
capacitors or inductors.
R
C
Vrms
• R and X are in Ohms, but distinct in nature
• When correctly combined the overall term
used for the resistance is Impedance, Z
(as the components impede current flow)
• In such a circuit Ohms Law applies to the
ratio of the overall potential difference to
current so we have: Z = Vrms / Irms
Chelmsford Amateur Radio Society
Intermediate Licence Course
Murray Niman G6JYB
R
L
Vrms
Slide Set 17: v1.0, 25-May-2009
(3) Technical Basics -2: AC & Impedance
12