Diodes

Diodes block current flow in one direction and pass current in the opposite direction. This is accomplished by joining a layer of P type material and a layer of N-type material during manufacturing. Where they meet is called the PN junction. At the PN junction, some of the electrons of the N-type material move into some of the holes in the P-type material and create a neutral area at the junction. Another way of thinking of this is that the positive holes attract the negative electrons leaving no free electrons, so current is unable to flow past that point. This neutral area acts as a barrier, which is called the depletion region.

DIODES

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The depletion region is very thin and responds rapidly to voltage changes. It is here that current is either allowed to pass or is blocked.

When the diode is connected in a circuit where the N-type material is connected to the negative terminal of the battery and the P-type material is connected to the positive terminal, the excess electrons in the N-type material are repelled by the negative potential of the battery. At the same time, the positively charged holes in the P-type material are repelled by the positive potential of the battery, resulting in a concentration of holes and electrons at the depletion region. When voltage applied to the diode is great enough (.5 to .7 volts) electrons in the N type material will move across the depletion region at the junction, filling holes in the P-type material and leaving holes in the N-type material. Electrons move through the diode to the positive terminal of the battery and holes move through the diode to the negative terminal of the battery. When this happens the diode conducts current and is said to be forward biased.

If the connection of the diode in the circuit is reversed, with the N-type material connected to the positive terminal of the battery and the P-type material connected to the negative terminal, the diode is reverse biased. In this case, the electrons in the N-type material are attracted to the positive terminal of the battery and the holes in the P-type material are attracted to the negative terminal of the battery. This results in an increase in the depletion region or neutral zone so no current can flow through the diode. Whether the diode conducts or blocks current flow is determined by the voltage polarity applied to it.

Neutrale Zone Diode

If the reverse bias voltage applied to a diode is great enough, the voltage can overcome the depletion region at the junction and the diode will conduct for a short period before burning open. When this happens the diode is destroyed.

SILICONE DIODE

Reverse breakdown

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Reverse Current fc

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Forward bias voltage

Note: Change of scale from reverse to forward

The three main uses for diodes in the automobile are rectification, de-spiking, and isolation.

Rectification—Since a diode will allow current to flow in one direction and not the other, it can be used to turn alternating current into direct current. This is called rectification. Diodes can provide either full-wave or half-wave rectification, depending on the number of diodes and how they are connected.

A half-wave rectifier consisting of one diode will have an output voltage that is approximately one half of the AC source. Since the output from an AC power source continually changes or alternates from positive to negative, the diode is forward biased for part of the output and reverse biased for the other. The diode will allow current to flow in the circuit when it is forward biased but will block the flow of current when it is reverse biased. The result is that only half of the wave is output while the other half is blocked by the diode. This type of rectifier is not commonly found in an automotive application since it is not an efficient way to rectify AC to DC to charge a battery.

A full-wave rectifier uses a four-diode network to rectify both halves of an AC output. In such a system, current flows from the first half of the phase of the AC power source through the first diode in forward bias, through the external circuit, through the second diode, then completes the circuit. On the second half of the phase, the current flows through the third diode, through the external circuit, through the fourth diode and completes the circuit.

By using four diodes in the full-wave rectifier, all of the current flows to the DC part of the circuit and the current in the DC part always flows the same direction even though the current flow in the AC power source changes directions.

The full-wave, three-phase rectifier found in an automotive alternator goes a step further. Because the alternator uses three coils that produce three overlapping AC sine waves staggered at 120 degree intervals, six diodes are required to achieve full-wave rectification. Each coil uses four of the diodes to rectify the output, achieving full-wave rectification (as in the full-wave, single-phase rectifier discussed earlier). Because the coils and diodes are interconnected, the same diodes are used by different coils at different times. Due to the overlap of the waves, output from each coil in this type of alternator produces a smooth output to the DC system.

The following worksheet shows how the six diodes can rectify the output of all three coils.

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Responses

  • Andrea Austerlitz
    Where is starting diode located on corolla 2010?
    7 months ago
  • adone
    What does the diode block look like on a toyota land cruiser?
    4 months ago

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