Editing Category:Common Issues (section)

==== Active Components ====
Components designed to exploit the physical properties of both conductive materials and semi-conductive materials, such as carefully crafted and organized silicon and other semiconducting elements, are ''active'' components.  These components use sub-atomic physical properties to control the flow of electrons in highly orchestrated (e.g. processor/digital logic) and controlled (e.g. [[wikipedia:Field-effect_transistor|FET]]/power/signal control). Such components and circuits may become so sophisticated, "integrated", and functional, that as part of their design a "power rail" or "power supply" (or multiple) are required inputs.

Active components are a bit more advanced and can be very difficult to make accurate measurements on while in circuit. Often times, the best way to measure these components is to turn the device on and measure their behavior (with a DMM in DCV mode or an oscilloscope) while the system is trying to work.
[[File:Diode.png|thumb|Diode symbol.]]

* '''Diodes''' (component designation D) are composed of a P{{--}}N junction. This doesn't mean anything to you and doesn't have to. The way they work is they will pass current in one direction, and not in the other, and they create a voltage drop across them in the process of conducting. The side of the diode that is marked with a stripe is the output side of the diode. You actually can test these with the device turned off if your meter has a diode mode. The meter will display a voltage{{---}}this is the voltage drop across the diode. The voltage drop varies depending on the type of diode, normal diodes have 0.6{{--}}0.7{{nbsp}}V, schottky diodes 0.3{{--}}0.5{{nbsp}}V and LEDs anywhere from 1{{--}}4{{nbsp}}V, depending on their color. If you measure a diode in the reverse direction (with the probes facing against the conduction direction), then your multimeter should show a high voltage drop (>1{{nbsp}}V) or "open" (depending on your model). This is because no current can flow and the shown voltage is the applied voltage from the meter. If you measure 0.3{{nbsp}}V or less, then your diode has likely shorted and should be replaced. When measuring a diode, it is always recommended to measure in both directions across it{{---}}this will guarantee that you haven't made a mistake in your measurement and will also make confirm that there is nothing else in the circuit (like another diode, facing the reverse way!) messing up your measurement. Keep in mind that LEDs (light emitting diodes) and laser diodes are both types of diodes and share the same characteristics{{---}}they can fail just like any normal diode can.[[File:NPNPNP.png|thumb|NPN and PNP symbols.]]
* '''Transistors''' (component designation Q, usually) are composed of two diode junctions and therefore come in two flavors: NPN and PNP. The difference between these is that one of them requires sourcing current to the base in order to conduct and the other requires sinking current from the base in order to conduct. These are generally used in digital signal switching and amplification circuits. They are a bit tricky to measure in circuit unless you have a good handle on your theory. The easiest way to diagnose if these are healthy or not is to turn the device off, and measure the resistance between each set of leads (B{{--}}E, B{{--}}C, C{{--}}E). If you measure anything below a few k{{ohm}}, it could possibly be bad. However, there are transistor circuits that involve feedback resistors with low values, so you should try to verify if that type of topology is present before declaring the part is bad. You could also just remove the part and measure the pads on the board to see if the resistance is the same with the part removed{{---}}this would indicate that the part itself is fine. Note that their behavior is driven by current and not voltage, so you will have a very difficult time measuring these in circuit unless you lift up traces, It's generally not worth the hassle{{---}}just verify they look OK with the resistance measurement.
* '''Integrated Circuits''' (also known as ICs, with component designation U) are collections of all of the above parts into a subsystem packaged into a single chip. If you suspect that you have an issue with an IC, you should find the datasheet and look at the pinout. The datasheet will describe what the chip does, as well as how to integrate it into a circuit. From this, you can also find what voltages and signals should be present on all the pins. Sometimes you will be able to read the part number off the chip, and in other situations, you may have to refer to the schematics in order to see what part is used. You cannot generalize that ICs do a certain thing, as their functionality varies widely from power supplies to microprocessors to amplifiers.