Concise Electronics for Geeks > 자유게시판

This helps ensure proper polarity when wiring up speakers. Any new electrical installation requires new wiring that conforms to local building codes. Sometimes, windings may have multiple taps, or additional windings can be provided as a feedback mechanism for building high voltage flyback transformers that exploit resonant frequencies of the ferrite core. Lightbulbs are fairly unremarkable electrically, other than their interesting non-linear current characteristics: while the wire is cold, a significant current can flow, but only for a limited time. This negative voltage will prevent that MOSFET from turning on until the charge is dissipated through the voltage divider (which takes time proportional to the capacitances and resistances involved) - and the gate is positive enough again. Now that we have the basic characteristics of electronic circuits, and the common components, sorted out, it's time to see what it's good for - starting with traditional, analog circuitry. A good overview of possible 555 circuits can be found on this webpage. Electrolytic capacitors enjoy some of the highest capacitances in proportion to their cost and size - but need to be polarized, work well only for fairly low voltages, have some leakage current, and tend to exhibit non-trivial resistance (denoted as ESR, what are electric cables and limiting their ability to deal with high-frequency signals); so avoiding electrolytics as long as possible is generally a good idea (cheap multi-layer ceramic capacitors - MLCC - are available up to at least 10 µF).

Wound-coil inductors usually perform well to around 10 MHz; chip inductors tend to be fine to 1 GHz or so. Standard ceramic capacitors usually perform as expected to around 100 MHz; past that point, parasitic inductance tends to take hold and makes them perform far worse than expected. For example, 1 kΩ may become "1k", while 100 Ω may become "100" or "100R". Commonly used resistances span from 1 Ω to 10 MΩ; stocking anything outside these limits is probably not very useful in everyday work. Because of the length of the conductive path needed to achieve marked inductance, many small inductors will have a noticeable resistance - usually between 0.5 and 20 Ω. As the wire heats up, its resistance tends to rise - resulting in a self-limiting current flow that prevents the device from overheating and being destroyed within the (usually very generous) range of voltages it is designed for.

In particular, parasitic capacitance as low as 0.1 pF can cause a significant drop in apparent resistance at signal frequencies of 1 MHz or so. You can see them installed on some USB cables, power supply cables, and so on. While very useful for controlling high-impedance signals, the diode simply serves as a "crowbar" across the supply terminals - and therefore, for input voltage sources that can source a significant current, this arrangement gets dangerously inefficient; a resistor can be used to limit supply current, of course - but this simply takes you back to the high-impedance scenario - not very useful for, say, driving motors. Therefore, the resistors need to be picked with the expected loads - and the acceptable voltage swings - in mind. First, we need to find an equivalent resistor that, when placed across the terminals of our 9V battery, would conduct just 20 mA. A more interesting case is what happens when both switches are switched on at the same time: assuming the non-linearity of the lightbulb is negligible (which is fair if the capacitor is large enough), initially, the capacitor will offer a low-impedance path, with very little current flowing through the bulb; but as the charge builds up, the voltage across its terminals will begin to rise - and more current will flow through the bulb.

This operating principle is reversible: when rotated by an external force, motors serve as generators, producing a voltage across their terminals. These principles are used in a number of piezoelectric devices, the most common of which are crystal oscillators - where the crystal vibrates at a resonant frequency when subjected to an external current, similar to some capacitor-inductor arrangements we will discuss later on. Other than the power rating, their most important parameters are the number of actual switch positions, the number of "throws" (signal outputs the switch can alternate between - this must obviously be equal or smaller than the number of positions allowed), the number of "poles" (separate switching pathways put in a single package), and the type of switching action (sustained / latching or momentary; with momentary switches further divided between normally closed or normally open designs - "NC" and "NO"). Resistors: most of the general-purpose resistors consist of nickel-chromium metal films, and are manufactured with a power rating of 0.125 or 0.25W, accuracy of 5% or 1%, and a temperature dependence under 0.01% per kelvin (thermistors are designed with a much higher temperature coefficient); different tolerances, power ratings, and designs are available at a premium, but seldom necessary.