A quick review of diodes, what are they?
A diode is an electrical component that acts as a one way valve. They can be made to conduct current in one direction or the other depending on the diode’s intended use. If they are not electrically and thermally rated for their intended operating environment, diodes can fail. Failure of a diode causes current to flow in an undesired direction, which could damage any components normally protected by the diode.
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Why are diodes used in circuit protection?
Diodes are used in circuit protection due to their ability to restrict electrical current to flow in only one direction. This trait is useful because some electrical components and devices will be damaged or malfunction if current flows in the wrong direction. For example, current problems can cause microprocessors to experience erroneous calculations or simply short out and fail. One of the reasons that radiation damages microprocessors because it induces stray currents across the microprocessor's contacts.
The diode can be used to not only protect from stray currents, but can be electrically routed to redirect the stray currents into useful current. This is known as shunting, which is dumping current down a very low resistance path. Rerouting the current ensures that the stray power is not simply wasted through grounding.
In systems where multiple power supplies are present, diodes play an extremely important role. The diode can be used to protect against transient overvoltages from the first or second power supply, depending on the system setup. The diode can also protect against transient overvoltage on the load side of the diode or even short circuit and/or overload on the power supply side. Additionally, if current supply to a diode protect load must be shut off, it can be done without taking other loads offline.
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Other uses of diodes
Diodes can be used to convert AC current to DC current. DC current flows like water from a hose, meaning it follows a linear path towards the end of the hose. Devices that function on DC circuits need protection against stray back-flow currents because they are most susceptible to damage from such currents. Other devices function on AC current, which flows back and forth like a zig-zag. Devices use AC current for many reasons, the primary being that it is more power dense, meaning that an AC current flows in and out of both the negative and positive contacts of any device in the system.
This current conversion is known as AC to DC rectification. Because AC flows like a zig-zag, diodes can be used to prevent the negative cycle from flowing down the circuit. Therefore, the top of the wave is allowed through and is electrically converted to the linear flow of DC current. It is also for this reason that diodes are only used in protecting DC circuitry - they only allow current to flow in that single direction and thus do not allow AC current to flow normally.