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Electronic circuit theory - rectifier diode

Now we get to look at our first semiconductor, which is a device that behaves differently depending upon certain conditions. In this case we are going to look at the most basic semiconductor device called a diode, and specifically a rectifier diode.

The rectifier diode acts like a one-way valve allowing current to flow in one direction, but not the other. This will accept an AC input but give a DC output. In this particular circuit the current will only flow during the positive phase of the AC input and so this is known as a half-wave rectifier circuit.


The diode will only conduct when it's anode (left-hand side on the diagram - the start of the arrow) is at a higher voltage than the cathode (right-hand side on the diagram - after the arrow). The diode will need to be inserted into the circuit the correct way around. Normally the cathode (negative end) is indicated by a line around the diode or a negative sign '-'.

For a diode there is a minimum forward voltage below which it won't conduct. Typically this is 0.6v, although the actual figure depends upon the particular diode. It is not obvious to see, but the 0.6v is reflected in the output waveform of the circuit demonstration above.

This circuit is known as a half-wave rectifier as only the positive half of the input voltage is allowed to pass through to the output. If smoothed through a capacitor then the output voltage would be only about half of that as if it was a full-wave rectifier (also known as a bridge rectifier).

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