Ideal diode
The key function of an ideal diode is to control the direction of current flow. Current passing through a diode can only go in the one direction, called the forward direction. Current trying to flow the reverse direction is blocked. They're like the one way valve of electronics.If the voltage across a diode is negative , no current can flow , and the ideal diode looks like an open circuit. In such a situation , the diode is said to be off or reverse biased.
As long as the voltage across the diode isn't negative, it'll "turn on" and conduct current . Ideally a diode would act like a short circuit ,if it was conducting current. When a diode is conducting current it's forward biased.
Circuit Symbol
Every diode has two terminals connections on each end of the components and those terminals are polarized, meaning the two terminals are distinctly different . Its important not to mix the connections on a diode up. The positive end of a diode is called the anode , and the negative end is called the cathode. Current can flow from the anode end to cathode , but not the other direction. If you forget which way current flows through a diode , try to remember the mnemonic ACID . "anode current is diode".
The circuit symbol of a standard diode is a triangle butting up against line.
Above are a couple simple diode circuit examples. On the left side diode D is forward biased and allowing current to flow through the circuit. In essence it looks like a short circuit. On the right Diode D2 is reverse biased. Current cannot flow through the circuit, and it essentially looks like an open circuit.
Depending on the voltage applied across it, a diode will operate in one of three regions:
1. Forward Bias: When the applied voltage across the diode is positive the diode is "ON" and current can run through . The voltage should be greater than the forward voltage in order for the current to be anything significant.
2. Reverse Bias: This is the "OFF" mode of the diode , where the voltage is less then Vf but greater than voltage breakdown, in this mode current flow is blocked, and the diode is off, A very small amount of current called reverse saturation current is able to flow in reverse through the diode.
3.Breakdown: When the voltage applied across the diode is very large and negative , lots of current will be able to flow in the reverse direction , from cathode to anode.
Forward
Voltage
Breakdown
Voltage
For normal diodes this breakdown voltage is around -50V to -100V, or even
more negative
2. Light-Emitting Diodes (LEDs!)
Like a normal diodes, LEDs only allow current through one direction. They also have a forward voltage rating. Which is the voltage required for them to light up. The Vf rating of an LED is usually larger than that of a normal diode and it depends on the color the LED emits.
3. Zener Diode
It is very useful diode in all types , it use for stable reference voltage. It runs under reverse bias condition, in forward bias it behaves like ordinary diode. Figure below shows symbol of a zener diode.
another figure for zener diode characteristic ,
The above diagram shows V-I characteristic of a zener diode.
In a forward voltage mode it characteristic is like a ordinary diode, when the reverse bias voltage is greater than a predetermined voltage zener breakdown voltage takes place. To make the breakdown voltage sharp and distinct , the doping is controlled and the surface imperfection are avoided. In the V-I characteristics above Vz is the zener voltage. We can say . It is also the knee voltage because at this point the current is the current is very rapid.
fig below shows circuit diagram of a zener diode as voltage regulator.
Above are a couple simple diode circuit examples. On the left side diode D is forward biased and allowing current to flow through the circuit. In essence it looks like a short circuit. On the right Diode D2 is reverse biased. Current cannot flow through the circuit, and it essentially looks like an open circuit.
Real Diode Characteristics
Current -Voltage Relationship
The most important diode characteristic is its current voltage relationship. This defines what the
current running through a component is, given what voltage is measured across it. Resistors, for example , have a simple , linear i-v relationship ohm's law . The i-v curve of a diode , through is entirely non linear .Depending on the voltage applied across it, a diode will operate in one of three regions:
1. Forward Bias: When the applied voltage across the diode is positive the diode is "ON" and current can run through . The voltage should be greater than the forward voltage in order for the current to be anything significant.
2. Reverse Bias: This is the "OFF" mode of the diode , where the voltage is less then Vf but greater than voltage breakdown, in this mode current flow is blocked, and the diode is off, A very small amount of current called reverse saturation current is able to flow in reverse through the diode.
3.Breakdown: When the voltage applied across the diode is very large and negative , lots of current will be able to flow in the reverse direction , from cathode to anode.
Forward
Voltage
In order to “turn on” and conduct
current in the forward direction, a diode requires a certain amount of positive
voltage to be applied across it. The typical voltage required to turn the diode
on is called the forward voltage (VF).
It might also be called either the cut-in voltage or on-voltage.
As we know from the i-v curve, the current through and voltage
across a diode are interdependent. More current means more voltage, less
voltage means less current. Once the voltage gets to about the forward voltage
rating, though, large increases in current should still only mean a very small
increase in voltage. If a diode is fully conducting, it can usually be assumed
that the voltage across it is the forward voltage rating.
A specific diode’s VF depends on what semiconductor material
it’s made out of. Typically, a silicon diode will have a VF around 0.6-1V.
A germanium-based diode might be lower, around 0.3V. Thetype of diode also has some importance in
defining the forward voltage drop; light-emitting diodes can have a much larger
VF, while Schottky diodes are designed specifically to have a much
lower-than-usual forward voltage.
Breakdown
Voltage
If a large enough negative voltage is
applied to the diode, it will give in and allow current to flow in the reverse
direction. This large negative voltage is called the breakdown
voltage. Some diodes are actually designed to operate in the
breakdown region, but for most normal diodes it’s not very healthy for them to
be subjected to large negative voltages.
Diode Application
1.Normal diodes
Standard signal diodes are the most basic, average, no-frills members of the diode family.
They usually have a medium high forward voltage drop and a low maximum current rating. A common example of a signal diode is the 1N4148. Very general purpose. It's got typical forward voltage drop of 0.72 volt and 300mA maximum forward current rating.
A rectifier or power diode is a standard diode with a much higher maximum current rating. This higher current rating usually comes at the cost of a larger forward voltage . 1N4001, for example has a current rating of 1A and a forward voltage of 1.1V.
2. Light-Emitting Diodes (LEDs!)
Like a normal diodes, LEDs only allow current through one direction. They also have a forward voltage rating. Which is the voltage required for them to light up. The Vf rating of an LED is usually larger than that of a normal diode and it depends on the color the LED emits.
3. Zener Diode
It is very useful diode in all types , it use for stable reference voltage. It runs under reverse bias condition, in forward bias it behaves like ordinary diode. Figure below shows symbol of a zener diode.
another figure for zener diode characteristic ,
The above diagram shows V-I characteristic of a zener diode.
In a forward voltage mode it characteristic is like a ordinary diode, when the reverse bias voltage is greater than a predetermined voltage zener breakdown voltage takes place. To make the breakdown voltage sharp and distinct , the doping is controlled and the surface imperfection are avoided. In the V-I characteristics above Vz is the zener voltage. We can say . It is also the knee voltage because at this point the current is the current is very rapid.
fig below shows circuit diagram of a zener diode as voltage regulator.
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