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Circuits #3 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 Digital Electronics xxxand currenxxxxxt 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