The diode is a electronic version of the non-return valve. It allows electron flow in one direction but not the other. It should be noted that the symbol is pointing to the conventional flow' where current is said to flow from the positive to the negative. The reality is that electron flow is from the negative to the positive and understanding on how the diode functions requires investigating this

When the voltage is applied across the diode so that electron flow is allowed this is referred to as Forward-Bias, when the voltage is applied so that electron flow is blocked this is referred to as Reverse Bias .

There is a small volt drop when the diode is Forward Biased which remains virtually constant independent of Current flow. This is about 0.7v for Silicon and 0.3v for Germanium.

In Reverse bias electron flow other than a small Leakage current is blocked up until the Diode breaksdown. The voltage this breakdown occurs is called the Peak Inverse Voltage (PIV), when the PIV is exceeded the diode will generally have failed although there are specialist diodes (called Zener Diodes) used in voltage regulation which are designed to normally work in Reverse Bias.

Working Principal

A diode consists of P-type (material with slight electron deficit sometimes imagined as having positive holes) and N-type (material with slight electron excess). This may be manufactured by the use of doping agents in semiconductor material such as Germanium or Silicon. The juncture of the two materials is called the depletion zone as it contains niether excess electrons or postivie holes into which the electrons can enter.

When a forward bias voltage is applied there is a tendency for the electrons and positive holes to be drawn into the depletion layer collapsing it until current can flow freely. When a reverse bias is applied the electrons and positive holes are drawn away from the depletion layer expanding it and preventing current flow

Although for most circumstances the voltage drop may be considered fixed there are a few instances were the slight increase in voltage drop with increasing current is part of the design of the circuit so must be understood. The actual current flow for a given voltage drop is give by the Diode Equation and contains variables such as Saturation current, applied voltage and diode temperature.


In the following the red lead on a meter is considered to be postive and the black negative. Although this may seen obvious it should be noted that for some older type analogue meters this polarity is reversed when it is set to read Resistance

Most modern meters are able to forward bias a diode when set to measure resistance. No meaningful information can be read off the meter however as it is an ohmic value dependent on variables of the meter itself

Some meters have a Diode Check facility. Use of this will in Forward bias will give a Voltage value equivalent to the nominal forward bias voltage.

It is unlikely that the meter will be able to generate the 50v required to exceed the Peak Inverse Voltage to cause the diode to breakdown. As for most Diodes this would caue there destruction this becomes unecessary. However, devices such as an Insulation Tester ( Megger) can easily exceed this and for this reason all diodes should be disconnected from a device before they are used.

Specialist Types

Zener Diode- these are diodes which may be used in the Reverse biased mode where they breakdown at a known voltage and allow current flow. They are used primarily in voltage stabilising circuits

The stabiliser circuit consists of the Zener diode and a Dropper Resistor across which takes the excess voltage.

Free Wheeling Diodes - sometimes referred to as Inductor Commuting are a standard diode placed across the coil of a coil. These are typically found on relays.

When the supply is turned off the coil the collapsing magentic field causes a high voltage to be generated which can cause arcing at the switch. The diode disapates the current slowing down the collapse of the field ( and thereby slowing the repsonse of the relay which may cause problems in some electronic circuits)

Light Emitting Diode- Electrons flowing through a PN junction experience a change in energy level emitting radiant energy when doing so. Constructing diodes containing elements gallium, Arsenic and phosphorus creates radiant energy at visible wavelengths. A Limited number of Colors can be created by changing the chemical constituency, more colors can be created by adding more than on LED using the same Lens.

Constant Current Diode- These limit the current flowing a circuit. These are seen in LED circuits and charging devices for Secondary Batteries