Editing Category:Common Issues (section)

=== Basic Electronic Components ===
[[File:Resistor Schematic Symbol .png|thumb|Resistor symbol.]]
[[File:Capacitors in parallel.png|thumb|Capacitors in parallel. This highlights exactly why you can't tell which one has failed when there is a short from one capacitor to ground.]]
Modern electronics are composed of a few types of devices, arranged in a schematic to create functional systems (blah blah, I know). However, a fundamental understanding of how these electronic components work can help you more efficiently find and resolve issues, as well as throw out red herrings if you're wiling to give practical theory a bit of thought when you're working on something. 

==== Passive components ====
Resistors, capacitors, and inductors are all common ''passive'' components. This means that they are not powered, and rather are strategically placed around the schematic to create the desired effect. These components can all be diagnosed with the power OFF and your meter in resistance mode (yes, even for capacitors). You cannot probe with a meter in resistance mode while the circuit is powered{{---}}your measurements will be meaningless.

Capacitors and inductors are ''reactive'' components, as their design exploits electromagnetic properties allowing energy storage as electricity (electric fields: capacitance; electrons/charge) and magnetism (magnetic fields: inductance; magnetic flux/"magic").  Resistors are designed to avoid these reactive properties, instead to impede the flow of charge in a static, ''non-reactive'', consistent manner, limiting flow proportional to the voltage. To do so, resistors dissipate as heat the energy absorbed from the electrons flowing through, again proportional to the (the square of the) voltage.

Passive components are generally constructed of "normal" materials, classified as conductive (conductors) and non-conductive (insulators).

* '''Resistors''' (component designation R) can be used to create voltage drops and condition signals. Typically, when they fail, they will burn on the inside and either become an open circuit ("OL" on your multimeter) or short circuit (0{{nbsp}}{{ohm}}), although partial failures are possible. it is not always easy to measure the resistance in circuit because of other nearby components (like capacitors) which charge up and will distort the measurement. However, measuring them with the resistance mode should give you an idea of whether the part is in good health or not most of the time.

* '''Capacitors''' (component designation C) act as small "tanks" of energy. They can be used for a variety of things, including signal filtering, power supply decoupling (reducing ripple), and AC coupling signals. You can't measure capacitance in circuit because of the parasitic capacitance in many of the traces as well as the behavior of other components. However, this doesn't really matter because their failure mode is much more straightforward{{---}}they just get shorted. Often times, this causes a problem because they are used as power supply decoupling, meaning they are connected from the supply rail to ground{{---}}meaning a short basically dumps the power supply entirely.  They do not always appear burned or broken when failed. A trickier issue with these is that all decoupling capacitors are connected between the supply and ground, effectively putting them in parallel{{---}}meaning if there is one failure, it will appear as all of them failing (until you remove the specifically broken part). This can be tedious to track down.[[File:Inductor.png|thumb|Inductor symbol.]]

* '''Inductors''' (component designation L) are small turns of wire. They work based on the electromagnetic field produced by current flowing through the wire. Inductors are generally used power supply filtering to remove high frequency content (like in switching power supplies) and oscillator circuits. Since they literally are just turns of wire, they will look like a short on your meter. However, this also means that they generally do not change value and when they fail, they will just fail open (OL). These generally will cause you the least amount of problems.

==== 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.

==== Miscellaneous Components ====

* '''Relays''' (component designation K, sometimes) are isolator switches. These are generally usually used to protect a subsystem from some signals (power rails or high voltage) until some criteria has been met (like transient voltage spikes during startup). These have a normally open (NO) and normally closed (NC) state. Verifying these work is a matter of checking the input signal and seeing if the output is switching between the open and closed states.[[File:Fuses.png|thumb|Fuse symbols.]]
* '''Fuses''' (component designation F) are used to protect the circuit in case of failure from over-current. They are made from a small film or wire that has a current rating matching the rating of the fuse, and the part will fail open circuit (OL) if the current through the part is exceeded. These are generally located around power supplies, laser diodes, and backlight drivers. The only measurement you need to take to verify that the part is working as normal is to make sure it reads under a few ohms.
* '''Connectors''' (component designation J) are physical connectors. when these fail it usually is from a pin getting bent, or something like that. It's different from connector to connector. It is also possible in the case of chemical spills for the plastic in the connector to become slightly conductive and leak signals from one pin to another, although this is a bit more uncommon. Usually a visual inspection is enough to tell if the part is good or not.