Download Circuits #3 - Electro Tech Online

Circuits #3
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DC and AC Current
o
DC current is the flow of current in one direction only. This means that the polarity
(+ and -) stay the same in a circuit
o
DC current is a constant flow, unlike static electricity,(which is a quick spark)
o
Batteries produce direct current
o
A battery producing a Direct Current will produce a constant voltage. Figure 1 shows
a graph of the voltage from a 9v battery
o
The flow of DC current is analogous to the flow of water through a hose
o
AC is the better choice to be transmitted from power plants to your home, because
it can be ‘stepped up’ and ‘stepped down’ by a transformer at either end
o
AC does not have a constant voltage, but rather it goes up and down like a wave
(called a sine wave – Figure 2)
o As the voltage goes up and down, it goes into positive and into negative, meaning
that the direction of the current also changes
o
AC is the type of power that comes out of a wall socket at home
o
Quite a lot of appliances will have power supplies in them to convert the AC from
the wall outlet, to DC that goes through the circuits inside them (such as a TV or DVD
player), and sometimes it will be in an adaptor that plugs into the wall (such as a
cordless phone)
o
An audio signal is an AC signal, and can also be represented by a sine wave (and
other wave forms). This is the type of signal that will be flowing to speakers
o
Capacitors will handle AC and DC
voltage
differently. Capacitors will block DC
voltage. as soon as they’re charged, but they will not impede the flow of
AC????????????????
o
Capacitors are often in series with a speaker, so they allow the audio signal (sine
wave) to pass, but filter out any
xxxxaccidentalxxxx DC voltage,
o
o
o
o xxxxxxsuch as interference from magnetic
fieldsxxxxxxxxxxxx
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Digital Electronics
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o
The voltage
in a circuit can be used to form a
signal. For example, +5v could be used to represent the binary value ‘1’, while 0v
could be used to represent the binary value ‘0’
o
0 and 1 could also be called ‘low’ (meaning the low voltage) or ‘high’ (the high
voltage)
o
To make this kind of signal, DC
would be used, as it doesn’t go up and
down like AC does. This would be called a digital signal
o
An analogue signal is variable, so it would use AC. It would be used to produce audio
signals
o
Digital signals can be used for logic. This means using the two states, high and low,
to make decisions
o
Eg, a circuit could decide that if signal input A is high, and signal input B is also high,
then the output signal C will be high
o
This output signal could then be used to turn on a LED, or start a motor, etc
voltage
Voltage Names
o
Some circuit diagrams will have ‘Vss’ (source), ‘Vdd’ (drain), ‘Vcc’ (collector) or ‘Vee’
(emitter) written on them
o
They are referring to the positive and negative voltage in the circuit
o
Vcc and Vdd mean positive voltage
o
Vss and Vee mean negative, or ground
Filtering Electrical Interference
o
Electricity and magnetism will affect each other. The flow of current affects
magnetic fields, and magnetic fields affect current flow
o
Because some components use magnetic fields (such as motors, inductors and
solenoids), or create magnetic fields as a side effect, it is easy for those fields to
cause interference or fluctuations to the current flow in other parts of the circuit
o
Disturbance to the flow of electricity will cause unwanted or unpredictable effects,
such as the timing in a timing circuit being inaccurate
o
In a sound circuit, interference could cause problems in the signal, which would
produce bad quality sound and distortion
o
Interference can also be caused by the power supply, particularly power supplies
that convert AC to DC, as they produce magnetic fields
o
The way to filter this out, or at least minimize the noise is to use a capacitor
o
The capacitor would normally be fairly small, but if there is any ‘spikes’ or ‘drops’ in
the current flow, the capacitor will filter it out
Fig. 1 – DC
Fig. 2 – AC