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What is a Blocking Diode?

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  • Written By: G.W. Poulos
  • Edited By: Daniel Lindley
  • Last Modified Date: 21 August 2016
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A blocking diode is a diode placed in an electrical circuit for the purpose of preventing electrical current from flowing in a specific direction. By function, all diodes block electrical current; however, blocking diodes are used specifically to prevent a reverse flow of electrical power in order to protect batteries, charging systems, or power generation systems. Blocking diodes are used heavily in green-energy systems such as solar or wind-powered electrical generators.

Diodes are common semiconductor devices, usually made of silicon, that are used to control the flow of electricity in circuits. A diode has two terminals: the positively charged anode and the negatively charged cathode. When electrical power enters a diode through its anode, it is conducted through the diode and passes out through the cathode and then on to the rest of the circuit. When electrical power enters the cathode of a diode, however, it is blocked and cannot pass through the device.

In a solar power generating system, DC electrical energy is generated by the system’s photovoltaic cells. This energy then passes to another part of the system that either converts the DC power to AC or uses the DC power to charge a battery or other power-storage device. When sunlight is striking the solar cells, the power created by the cells passes through the anode of a blocking diode and on to the other systems without interference.

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When night falls, solar cells no longer generate electrical power. The difference in electrical charge between the solar cells and the battery or conversion circuit causes them to try to rebalance the difference in electrical charge by sending power to the solar cells, which can damage or destroy them. When this happens, the power sent to the solar cells enters the cathode of the blocking diode and is stopped before it can reach the solar cells and harm them. Once the sun rises again, and the photovoltaic cells are generating an electrical charge, the power generated can again pass through the blocking diode uninhibited.

A blocking diode performs essentially the same function in a wind-powered system as it does in a solar generator. The difference between the wind system and a solar power system is that a wind system usually creates AC power using a generator driven by blades or a turbine. In this case, the reverse flow of power that occurs when the wind dies down can actually drive the generator as though it were an electric motor. The blocking diode prevents the reverse drive of the generator, which may not only damage the generator but can also consume all the power stored in the system’s batteries.

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