* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Section B7: Filtering
Audio crossover wikipedia , lookup
Oscilloscope types wikipedia , lookup
Analog television wikipedia , lookup
Superheterodyne receiver wikipedia , lookup
Index of electronics articles wikipedia , lookup
Spark-gap transmitter wikipedia , lookup
Regenerative circuit wikipedia , lookup
Josephson voltage standard wikipedia , lookup
Wien bridge oscillator wikipedia , lookup
Power MOSFET wikipedia , lookup
Phase-locked loop wikipedia , lookup
Transistor–transistor logic wikipedia , lookup
Current source wikipedia , lookup
Analog-to-digital converter wikipedia , lookup
Surge protector wikipedia , lookup
RLC circuit wikipedia , lookup
Valve audio amplifier technical specification wikipedia , lookup
Oscilloscope history wikipedia , lookup
Integrating ADC wikipedia , lookup
Operational amplifier wikipedia , lookup
Valve RF amplifier wikipedia , lookup
Radio transmitter design wikipedia , lookup
Resistive opto-isolator wikipedia , lookup
Voltage regulator wikipedia , lookup
Schmitt trigger wikipedia , lookup
Current mirror wikipedia , lookup
Power electronics wikipedia , lookup
Switched-mode power supply wikipedia , lookup
Section B7: Filtering As mentioned at the end of the previous section, simple rectification results in a pulsating dc voltage at the output, also known as output ripple. These deviations from the desired dc may be reduced by the process of filtering. The simplest form of filter uses a single parallel resistor-capacitor combination. Recall, from the olden days of circuit analysis, that this creates an exponential decay curve as the capacitor discharges with a time constant τ=RC. So (using the full-wave rectified output as an example)... what we Therefore, the voltage ripple, vr = ∆V = Vmax − Vmin is significantly reduced by this technique. NOTE: The figures above represent a rectified signal that is only positive and is used to generate a positive dc voltage. A negative dc voltage may also be generated by creating a purely negative signal (flipped about the time axis). Vmax and Vmin indicate the magnitudes of the respective voltages and may be either positive or negative. Since we already have a resistor (the load) in all the rectifying circuits we talked about, the challenge in designing this type of filter is to define an appropriate (and realistic) capacitor to give us an acceptable ripple. This ripple is defined, either explicitly or implicitly, in terms of the parameters Vmax and Vmin. Your text presents a thorough derivation of the development of capacitor criteria in terms of the exponential decay relationship and available discharge time, with the result of a conservative rule of thumb for filter design C = Vmax ∆VfpRL (Equation 3.52) where: Vmax is the maximum voltage magnitude of the rectified signal (positive or negative) ∆V is the ripple (Vmax-Vmin) RL is the load resistor fp is the output fundamental frequency. It’s worthwhile to take a minute on this last term. The output fundamental frequency is simply the number of pulses per second of the rectified output. For half-wave rectification, this is simply the frequency of the input signal (where we’ve just chopped off half of the waveform), but for full-wave rectification this term is double the original frequency (flipped part of the original sinusoid so that everything’s on the same side). Specifically, for a 60-Hz input: fp=60 Hz for half-wave rectification fp=120 Hz for full-wave rectification A note of caution (from one who has been caught many times) -- be careful what numbers you slap in an equation! Equation 3.52 provides a solid estimate of the capacitance required for a given circumstance, but a factor of 2 makes a difference! Just a couple more relationships that you may want to make note of: ¾ The rms (root-mean-square) ripple voltage, Vr(rms) is derived by approximating the average of the ripple as ∆V/2 and the shape of the filtered waveform as a sawtooth, or Vr (rms) = Vmax − Vmin 2 3 (Equation 3.54) ¾ The ripple factor is defined as the ratio of the rms ripple voltage to the dc voltage desired. Ideally, this term would be zero (i.e., no ripple), but practically, this may be another design constraint to satisfy. ripple factor = Vr (rms) Vdc (Equation 3.55)