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BTC108 4 AC Electronics


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BTC108 4 AC Electronics

  1. 1. Lecture 4 AC Electronics James Uren BTC108 Electronics
  2. 2. Lecture 4 AC Electronics DC vs. AC All the currents and voltages we have worked with so far are constant positive voltage and DC (Direct Current). Power and signals are often delivered using an oscillating current that changes direction, AC (Alternating Current) and voltage that changes from positive to negative. Mains voltage is delivered at 230V AC in this country. When this is generated at the power station, the generating motors’ magnets produce power in AC form, and it is then easy to convert between voltages through transformers to the home. Analogue signals are modulated on to radio frequency (RF) AC for transmission. In Amplitude Modulation (AM) radio for example the amplitude of the wave is changed to carry the information. Waveform The AC voltage or current can be viewed as a sine wave (the solid line): Amplitude sine cosine A P Time T Where: Lecture 4: AC Electronics BTC108: Electronics – James Uren 2
  3. 3. A is the amplitude of the waveform. It is measured from zero to the peak of the wave. P is the peak-to-peak (p-p) amplitude. It is measured from the peak to the trough of the wave. T is the time period of the wave – the time it takes for the wave to go through one cycle. The dashed line in the waveform diagram above is a cosine wave. The difference in starting points of the waves is a phase difference, and is measured in seconds, or sometimes in degrees (°) where one cycle is 360°. What is the phase difference in degrees between sine and cosine? Other Types of Waveforms Frequency Domain Lecture 4: AC Electronics BTC108: Electronics – James Uren 3
  4. 4. A useful way of representing AC signals is in the frequency domain, instead of the time domain, for example for viewing the spectrum of a signal with a spectrometer. In this domain the Y-axis represents the voltage gain and the X-axis the frequency. The frequency of an AC signal is directly related to the period of the wave: Where: f is the frequency in Hertz (Hz) T is the time period of one wave in seconds In the frequency domain, the X and Y axes are both log scales (logarithmic) where an equal division represents an increase of x10. The gain is measured in decibels (dB) and is found using the log function: Where: G is the gain in decibels (dB) Vout is the amplitude of the output in Volts Vin is the amplitude of the input in Volts Gain G Frequency Draw a 1kHz sine wave on the above graph, where Vout = Vin. What is the time period of this wave? Filters Lecture 4: AC Electronics BTC108: Electronics – James Uren 4
  5. 5. In order to isolate AC signals, it is necessary to filter out areas of the spectrum. There are three main types of filters: • LPF (Low-Pass Filter). Allows low frequencies to pass, but filters high frequencies. • HPF (High-Pass Filter). Allows high frequencies to pass, but filters low frequencies. • BPF (Band-Pass Filter). Allows a mid-band of frequencies to pass, but filters lower frequencies and higher frequencies. An ideal filter would provide a sharp cut-off at the required frequency, but realistically filters have a roll-off. The point at which the filter has a gain of -3dBs is the -3dB frequency, f-3dB, and is used to describe the filter. Gain (dB) 0 -3 Roll-off f-3dB Frequency (Hz) What is the percentage of Vout / Vin at a gain of -3dBs? Lecture 4: AC Electronics BTC108: Electronics – James Uren 5
  6. 6. Draw the frequency responses of the other types of filters and mark the -3db frequency(s): HPF Gain Frequency BPF Gain Frequency Lecture 4: AC Electronics BTC108: Electronics – James Uren 6
  7. 7. RC Filters The simplest kind of filter is constructed from a single resistor and capacitor. In the low pass filter configuration: R C Vin Vout From capacitor theory we know that the time constant for this RC combination is: Where: τ, (‘tau’) is the time constant in seconds R is the resistance in Ohms C is the capacitance in Farads The resistor and capacitor combine to make a filter, and the -3dB point of the filter can be found from the time constant: What is the capacitance in a RC Low Pass Filter with resistance 1k and a -3dB frequency of 3.4kHz? Lecture 4: AC Electronics BTC108: Electronics – James Uren 7
  8. 8. Practical: Build an RC Filter Read the Health and Safety Information on page 5. • Build the following circuit: R C Vin Vout Use a signal generator for the AC source, Vin and an oscilloscope to measure Vout. • Draw the frequency response of your filter. • Calculate the -3dB frequency. How does this compare with your measurements? Extended Practical: Other RC Filters • Repeat the above practical for a High Pass Filter, where the capacitor and resistor are swapped over. • Design a bandpass filter with two stages, one LPF and one HPF. Lecture 4: AC Electronics BTC108: Electronics – James Uren 8
  9. 9. Health & Safety Considerations Soldering and de-soldering: Solder melts at between 180 and 200°C. Soldering irons will heat up to between 250 and 400°C. Be extremely careful when soldering and take the following precautions: • Switch off the soldering iron at the mains when not in use • Always keep the iron in its stand • Make sure your workspace is clear, well lit and well ventilated • Never solder while your circuit is powered up • Never solder without tutor supervision • Only apply the soldering iron for the minimum amount of time • Keep your soldering tidy and use the minimum amount of solder • Avoid breathing in solder fumes • You must only use the lead-free solder provided • You must use tools e.g. pliers to support components that are being soldered and ensure the board is secure. Switching it on: Powering up a circuit that is incorrectly connected can cause components or equipment to get extremely hot or even ‘blow’. A short circuit (where unintended electrical connections are made) for example may damage equipment or blow components causing them to behave in an unpredictable way. • Before powering up your circuit you MUST have it checked by the tutor • Have your neighbour physically inspect your work before powering on • If your circuit does not behave as you expect, switch it off immediately • Use your nose! A faulty circuit with hot components will often smell or smoke Lecture 4: AC Electronics BTC108: Electronics – James Uren 9
  10. 10. If your circuit does not behave as you expect: • With the power off, confirm by eye that your circuit is connected correctly and that you are using all the correct components and mounted with the correct polarities • Inspect your circuit closely for short circuits, soldering faults and dry joints: • Do all the testing on your circuit that you can with it powered off. • Be extremely careful when probing your circuit live as the probe itself can cause short circuits • When probing with an oscilloscope ensure the earth connection is applied safely Lecture 4: AC Electronics BTC108: Electronics – James Uren 10