We learned about the electronic component of diodes in junior high school, which has an important characteristic: unidirectional conductivity. Figure 1-21 shows a schematic diagram of a diode and a light-emitting diode (LED). When using a diode, it is important to pay attention to its direction. If it is soldered incorrectly, the diode will not function as expected.
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Figure 1-21 Diode |
A diode is composed of a P-type
semiconductor and an N-type semiconductor, as shown in Figure 1-22. When the
P-type and N-type semiconductors are combined, they form the well-known PN
junction. This junction has the characteristic of allowing current to flow in
the forward direction and blocking it in the reverse direction. Specifically,
the anode voltage must be higher than the cathode voltage for current to flow
from the anode to the cathode. Conversely, current cannot flow from the cathode
to the anode.
| Figure 1-22 Diode and PN junction |
The right graph in Figure 1-23 shows the
voltage-current characteristic curve of a diode. In the first quadrant, it can
be seen that when the anode voltage is only slightly higher than the cathode
voltage, almost no current flows through the diode. However, when the anode
voltage is higher than the cathode voltage by VF, the diode current instantly
rises and the voltage is "clamped" at VF. "Clamping" means
that the voltage is locked at VF, which is the forward voltage drop, usually
0.7V, although the forward voltage drop of a Schottky diode may be lower. In
the third quadrant, when a reverse voltage is applied to the diode, there is
almost no current and it is in the cut-off state. The diode is only turned on
when the reverse voltage exceeds a certain value, generating a large current.
This reverse application of the diode is often used for voltage stabilization,
which is commonly known as a voltage regulator diode.
Figure 1-23 Voltage-current Characteristic
The unidirectional conductivity of a diode
can be compared to a unidirectional water pipe switch. In the left graph of
Figure 1-23, due to the presence of a one-way baffle, water can only flow from
left to right and not from right to left. (Tip: If some concepts or
characteristics are difficult to remember, they can be strengthened by analogy
with examples from everyday life, of course, diodes are very simple).
There are countless circuit applications around the forward and reverse voltage of a diode, and clamping circuits and voltage stabilization circuits are used here as examples. Figure 1-24 shows a simulation of a diode clamping circuit. The 1KΩ resistor is a current limiting resistor. It can be seen that during the process of the black input voltage signal changing from 0V to 10V, the circuit only outputs (green output curve) when the input voltage of the black curve exceeds 0.7V, and is clamped at 0.7V.
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| Figure 1-24 Diode clamp circuit simulation |
Figure 1-25 shows a simulation circuit and
waveform for voltage stabilization (for convenience, the author uses a signal
source, and an actual circuit should use a power supply). Under the premise of
severe fluctuations in the black curve input voltage, the dynamic extraction of
current by adjusting the resistance value of the load resistor R2 results in a
green load voltage curve that is stable at 2.2V overall, achieving the function
of voltage stabilization. Note that although the voltage regulator diode is
labeled as 2.2V, it does not mean that it can be directly connected to the
input voltage. The current limiting resistor R3 must be added. We can estimate
the resistance value of the current limiting resistor based on the three
parameters of the load voltage of 2.2V, load current ranging from about 28mA to
2mA, and average input voltage of 5V. The voltage across the current limiting
resistor is 2.8V (5V - 2.2V), and the current fluctuates between 28mA to 2mA.
Therefore, the resistance value can be 2.8/0.0282.8/0.002=1001400 ohms. The
larger the resistance value, the more the circuit is affected by the load,
which is commonly known as the load effect. Therefore, the resistance value
should be chosen towards the small resistor direction, which is 100Ω in this
case.
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| Figure 1-25 Diode voltage regulator circuit simulation |
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