A diode is a semiconductor device that lets current pass much more easily in one direction than the other, which makes it act like a one-way electrical valve. This simple behavior is essential in power supplies, signal protection, voltage regulation, and digital electronics. Rectifiers use diodes to convert alternating current, AC, into a one-direction current that can be filtered into direct current, DC.
Understanding diodes helps connect circuit diagrams to the real waveforms seen in electronic devices.
Key Facts
- Forward bias means the diode is connected so it can conduct current after its turn-on voltage is reached.
- Reverse bias means the diode blocks current except for tiny leakage current until breakdown occurs.
- For a silicon diode, a common approximate forward voltage is V_D = 0.7 V.
- Ohm's law still applies to the rest of the circuit: V = IR.
- In a half-wave rectifier, one diode passes only one half of the AC cycle, so the output is pulsed DC.
- In a full-wave bridge rectifier, two diodes conduct at a time, so the approximate peak output is V_out,peak = V_in,peak - 2V_D.
Vocabulary
- Diode
- A two-terminal semiconductor component that conducts current mainly in one direction.
- Forward bias
- The condition when a diode is connected with its anode at a higher voltage than its cathode so it can conduct.
- Reverse bias
- The condition when a diode is connected with its cathode at a higher voltage than its anode so it mostly blocks current.
- Rectifier
- A circuit that converts AC into a one-direction output using one or more diodes.
- Filter capacitor
- A capacitor placed across a rectifier output to store charge and reduce voltage ripple.
Common Mistakes to Avoid
- Treating a diode like a perfect wire, because a real silicon diode usually drops about 0.7 V when conducting.
- Reversing the diode direction in a rectifier, because current flows from anode to cathode during forward conduction and the output polarity will be wrong if the diode is flipped.
- Assuming a half-wave rectifier gives smooth DC, because it only passes one half-cycle and produces large gaps unless filtering is added.
- Forgetting the two diode drops in a bridge rectifier, because current passes through two conducting diodes on each half-cycle and the output peak is reduced by about 1.4 V for silicon diodes.
Practice Questions
- 1 A 9.0 V battery is connected in series with a silicon diode and a 470 ohm resistor. If the diode drop is 0.7 V, what is the current through the resistor?
- 2 A full-wave bridge rectifier is fed by a transformer with a peak secondary voltage of 18 V. Using silicon diodes with V_D = 0.7 V, what is the approximate peak output voltage before capacitor filtering?
- 3 Explain why adding a capacitor across the output of a rectifier reduces ripple, and describe what happens to the capacitor during the rising and falling parts of the rectified waveform.