3. Power supply filter
4. Applications of diodes
← Signal rectifier
← Diode gate
← Diode clamps
Some elements are linear (resistors, capacitors, inductors), which means that doubling the applied signal (let us say a voltage) produces a doubling of the response (let us say a current). They are also passive – they do not have built-in source of power. They are two-terminal devices, (which is self-explanatory). Diode is also two-terminal, passive but non-linear a device. Figure 1 shows the diode. | |[pic] | | | | |[pic] | | |Fig.1. Diode. |Fig.2. Diode voltage-current curve, U-I curve. |
In Fig. 2 there is U-I (voltage-current) curve (characteristic). The diode arrow, anode terminal, shows the direction of forward current flow. If the diode is in a circuit in which a current of 10mA=10*10-3A is flowing from anode to cathode, then the anode is approximately 0.5 volt more positive than cathode. We call it the forward voltage drop. The reverse current is measured in nanoampers and 1nA=1*10-9A. It is so small in comparison to mA, that can be neglected until we reach the reverse breakdown voltage. Typically it is approximately 75V and normally we never subject a diode to voltage large enough to cause reverse breakdown.
Similarly, the forward voltage drop, which is about 0.5 or 0.8 V, is of little concern. For these reasons we treat the diode as a good approximation of an ideal one-way conductor.
Commercially available diodes are described also by other important characteristics, e.g.: maximum forward current, capacitance (measured in pF), leaking current, reverse recovery time (measured in nanoseconds, 0-2-4-5000).
A rectifier changes ac (alternating current) to dc (direct current). This is the most important application of diodes. Diodes are sometimes called rectifiers. The basic circuit is shown in Fig. 3.
[pic]Fig.3. Half-wave rectifier.
The ac represents a source of ac voltage. It can be a transformer or just ac sine-wave power line. For sine-wave input, of amplitude much larger than forward voltage drop, the output will look like it is shown in Fig. 4. [pic]Fig.4. Voltage across Rload in Fig.3.
The process and the circuit we call a half-wave rectifier, because only half of the input waveform is used. In Fig. 5 is shown a full-wave rectifier and Fig. 6 shows the voltage across the load. The small gaps across zero voltage occur because of the forward voltage drop. |[pic] | | | |[pic] | |Fig.5. Full-wave bridge rectifier. |Fig.6. Voltage across Rload in Fig.5. |
Power supply filtering
The rectified wave from Fig.6 is not good for application: it is dc only in the sense that it does not change polarity. But it does not have constant value and has plenty of ripples i.e. small waves or undulations (wave like forms). It has to be smoothed out in order to obtained authentic direct current. This can be done by means of a low-pass filter, which is shown in Fig. 7.
[pic]Fig.7. Full-wave bridge with RC filter.
The full-wave bridge diodes prevent flow of...