A diode is an electronic component that permits current flow in only one direction. The 1N4148 is a general-purpose discrete diode with high switching speed and modest maximum current and reverse voltage ratings. As it is a low-cost two-lead part, hobbyists use it for building project circuits, including those for audio, radio and digital applications.
A diode conducts electricity if the voltage across it is positive on the anode end and negative on the cathode. If you reverse this polarity, the diode blocks current flow. However, a large enough voltage will overwhelm the diode's ability to block current. This voltage, called peak inverse voltage (PIV) or peak reverse voltage (PRV), is an important diode rating. As the PIV of the 1N4148 is a relatively low 100 volts, it restricts the diode to low-level signal processing applications, such as sound processing or digital circuits.
A diode conducts current in the "forward" direction, blocking it in the "reverse" direction. Its ability to conduct current is limited by its size and construction. A circuit that puts too much current through a diode will burn it up. The 1N4148 has a maximum continuous current of 200 milliamps and a pulsed maximum current of 450 milliamps.
The 1N4148 has a glass body with two leads for connecting it to circuits. This format is handy for solderless breadboard, perfboard and printed circuit boards. Generally, this is a good diode for hobbyists and engineers looking to make small numbers of circuits. In 2011, mass-produced circuits use surface-mount components. They are physically smaller, lack leads and are optimised for machine handling.
The 1N4148's high switching speed of 4 nanoseconds makes it useful for audio, radio and some computer circuits. This is the time the diode takes to permit current flow after a signal changes its polarity from reverse to forward. A 4-nanosecond switching speed corresponds with a frequency of about 125 megahertz, but individual diodes may have much faster speeds. The 4-nanosecond figure is the maximum switching time, so some circumstances allow for higher speeds.