Q&A for How to Tell Which Way Round a Diode Should Be

The line on a diode typically indicates the end known as the cathode (the "minus" end).

The motor is composed of coils. You start with voltage being generated in one direction, which includes a magnetic field around the coil. When the current is interrupted, the magnetic field collapses, and the collapsing magnetic field creates a voltage in the opposite polarity to when the current was flowing. That voltage is not only opposite in polarity; it's also a very High Voltage, which is a recipe for frying any circuitry within the first cycle. However, if you put in that diode to forward-bias consistent with the voltage of the spike, it will divert the power from the circuitry, and also limit the spike to 0.7 volts.

The band on a diode designates the cathode or negative end of the device.

That's not a problem. In electronic circuits the negative side of a diode is often attached to ground.

If it's a lithium-ion pack, it probably already has its own protection, since those have a tendency to explode when reversed. However, if you want extra protection, the diode(s) would "point" the cathode (-) end toward the positive terminal of the battery being charged, so it acts like a "short circuit" when operated properly from a charged jump-pack and "open" when reversed, or when the car's alternator voltage suddenly become higher than that of the jump pack (when the car starts).

This isn't a concern. The negative side of a diode is often intentionally attached directly to the ground potential.

Generally, nothing happens. A 12v DC adapter output should be designed to not back-feed any dangerous voltages to the AC plug if 12v is applied to the output terminals. An internal transformer, for example, won't "pass" the low-DC voltage to the high-voltage AC side.

Yes, it matters. First you have to identify which lead is the Anode and which is the Cathode. The terminal that is the cathode is identified by a marking. Do an "image search" for "LED diode marking" and you'll see plenty of examples of how LEDs are marked. The first requirement for an LED to light up is to connect it between two points in a circuit with the anode connected to the higher voltage, the cathode connected to the lower voltage. There must be a resistor in series with the LED. Sometimes the resistor comes pre-attached to one lead of a new LED, which assumes a pre-determined voltage between the two points of the circuit it will be going into.

You generally need a resistor in series with an LED to limit the amount of current pulled by the LED, according to its specifications. Otherwise, the LED functions as a "short circuit" and may burn up instantly, if enough power is available from the power supply. The proper resistance is in the range of dividing the required voltage drop (Vsupply - Vled.forward) by the maximum current for the LED.