A Diode allows flow of current in one direction and prevents (blocks) it in opposite direction. In other words, diode allows current flow from higher potential to lower potential only.
Diodes are available in different sizes and shapes. Normally a diode is identified with 1N as prefix and four digit number as suffix. The prefix 1N indicates one P-N junction.
P-N junction means Positive(Anode) [marked as P in figures] and Negative(Cathode) [marked as N in figures] junction with two connections (conductors) connected to Anode[P] and Cathode [N] sides. The Negative (Cathode) is marked with a band, like white band on black surface or black band on glass surface.
If the positive terminal of DC power supply is connected to Anode of the diode and negative terminal of the DC power supply is connected to Cathode of the diode through some load (or resistance), the current flows through the diode. This is called Forwad bias connection of a diode. Only current (Amps) is the limiting factor, which flows through the diode, in case of forward bias. About 0.7V will be dropped across the Silicon diode and about 0.3V will dropped across the Germanium diode.
If the positive terminal of DC power supply is connected to Cathode of the diode and negative terminal of the DC power supply is connected to Anode of the diode through some load (or resistance), the flow of current is blocked through the diode. This is called Reverse bias connection of a diode. The Voltage applied across the diode should be less than breakdown voltage of the diode, in case of Reverse bias
The main specifications of a diode are BREAKDOWN VOLTAGE, CURRENT CARRYING CAPACITY and RESPONSE TIME ON VOLTAGE REVERSAL.
Breakdown Voltage is the maximum voltage, that the diode withstands, before damage, applied across the diode in reverse biased.
The size (i.e, diameter or cross-sectional area) of a diode indicates current carrying capacity. Higher diameter or higher cross-sectional area of diode, means more current allowed to flow through the diode.
The minimum time taken by a Diode to allow flow and prevention of current is called Response time. The less response time means, works better with high frequencies in rectifier circuits.
The basic data of highly used Diodes are shown below for quick reference.(refer DATASHEET for exact full details)
Diode as Rectifier:
Diode is frequently used to convert AC power supply to DC power supply. When a diode is connected in series with AC power supply with some load, either positive half wave or negative half wave is appeared across the load due to forward bias. The other half wave will be blocked by the diode due to reverse bias. This is called Half-wave rectifier
Let us understand all four possible cases as shown in the above figure.
Case 1A : when AC supply is +ve in upper conductor , -ve in lower conductor and the diode’s P is connected to upper conductor, then the diode is in forward bias and the half cycle of AC supply is appeared across the load.
Case 1B : when AC supply is -ve in upper conductor , +ve in lower conductor and the diode’s P is connected to upper conductor, then the diode is in reverse bias and the diode blocks the half cycle of AC supply. Which means, there is no supply across the load.
Case 2A : when AC supply is +ve in upper conductor , -ve in lower conductor and the diode’s N is connected to upper conductor, then the diode is again in reverse bias and the diode blocks the half cycle of AC supply. Which means, there is no supply across the load in this case also.
Case 2B : when AC supply is -ve in upper conductor , +ve in lower conductor and the diode’s N is connected to upper conductor, then the diode is in forward bias again and the half cycle of AC supply is appeared across the load
Diodes in Bridge Rectifier:
If single diode is used as rectifier in AC circuit, half of the AC cycle is lost in the output (load). So, to overcome this problem, four diodes are used for full wave rectification, which is called as Bridge rectifier.
In a Bridge Rectifier, four diodes are arranged in a particular format. Anodes of two diodes are connected together and Cathodes of other two diodes are connected together. The left out connectors of two sets of diodes connected in a Diamond form with free Anode of one diode connected to Cathode of another free diode. AC signal is applied at anode-cathode junctions and the rectified output of Positive and Negative voltages are obtained at Anodes junction and Cathodes junction respectively
When an AC signal is applied at Anode-Cathode junctions of bridge rectifier, the output is always positive at Anode junctions and the output is always negative at Cathode junctions, which is shown here as graphical representation.
Considering first case, as the connections of AC supply may be positive and negative on the two conductors, the working of the diodes in bridge rectifier may be understand from the above figure WORKING CONDITION-1. In case, the AC supply is negative and positive (reverses) on the same two conductors, the working of the diodes in the same bridge rectifier may be understand from the figure WORKING CONDITION-2.
A practical circuit for AC to DC conversion circuit using bridge rectifier is shown here. The obtained output is full wave DC.
Full Wave Rectifier using TWO Diodes:
Some transformers have center tapping in secondary winding. In that case, the middle tapping may be used as ground and the extreme tappings may be connected to two diodes for full wave rectification as shown here.
Zener Diodes are used to have fixed voltage reference. When a zener diode is used in reverse bias, the PN junction maintains certain voltage across the PN junction and it will have fixed value. So, zener diode is used in reverse bias only. Cathode is identified by a black band on zener diode
In practice a resistance is used in series with a zener diode in reverse bias mode and the voltage across the zener diode is used as reference voltage.
Zener diodes are available in various voltages(2.7V, 3V, 3.3V, 4.7V 12V, 15V etc.) and various wattages (025W, 0.5W, 1W etc.).
The wattage may be identified by the diameter (or size), where as it is some what difficult to read the voltage marked on the zener diode. Some times it is difficult differentiate zener diode with signal diode (1N4148).