Okay, rectifier diodes are very basic and important components in electronic circuits, and their core function is to utilize unidirectional conductivity (allowing only current to flow from the anode to the cathode). The following are typical situations that require the use of rectifier diodes:
AC to DC Conversion - Core Applications
Situation description: This is the most fundamental and common use of rectifier diodes. When a circuit or device requires a DC power source (such as battery power or most electronic components), but the input is AC power (such as mains 220V/110V).
Why use: The direction of alternating current is periodically changing. A rectifier diode acts like a "one-way valve", allowing only the positive half cycle (or negative half cycle, depending on the circuit connection) of alternating current to pass through while blocking reverse current. By using specific circuit structures such as half wave rectification, full wave rectification, and bridge rectification, AC power can be converted into pulsating DC power, which is then smoothed by filtering capacitors to obtain usable DC voltage.
Typical applications: mobile phone chargers, laptop power adapters, desktop computer power supplies (internal), home appliance control board power supply parts, industrial equipment power modules, electric vehicle chargers, and any other places that require the conversion of mains power into the required DC voltage inside the device. The bridge rectifier circuit (consisting of four rectifier diodes) is the most common implementation method.
Prevent power reverse connection (reverse polarity protection)
Situation description: When the device is powered by a battery or an external DC power source, if the operator accidentally reverses the positive and negative poles of the power source.
Why use: Reverse connected power supplies can cause serious damage or even burning to components in the circuit that are not resistant to reverse voltage, such as electrolytic capacitors and IC chips. Connect a rectifier diode in series at the power input terminal (with the anode connected to the positive input point of the power supply and the cathode connected to the positive terminal of the circuit), and conduct when current flows in normally; Once the power supply is reversed, the diode will cut off due to reverse bias, like disconnecting a circuit, effectively protecting the subsequent circuit.
Typical applications: In the input protection circuit of battery powered portable devices (toys, handheld instruments) and devices that require external DC power supply (such as development boards, certain motor drivers).
Continuous Current in Switching Power Supply
Situation description: In switching power supplies (such as Buck buck and Boost boost circuits) and motor drives (such as H-bridge), when the control switch (usually MOSFET) is turned off.
Why use: There are inductive components (inductors, motor coils) in the circuit. According to Lenz's law, when the current attempts to decrease, the inductive component will generate a self induced electromotive force (reverse voltage) in an attempt to maintain the original direction of the current. This reverse high voltage may break through the switch tube. The rectifier diode (called freewheeling diode or flywheel diode) connected in parallel at both ends of the inductor or coil provides a low impedance discharge circuit for this reverse current, protects the switching transistor, and feeds back the stored energy to the load or power supply.
Typical applications: DC-DC switching regulator module, stepper motor/DC motor driver, relay coil protection circuit.
Signal peak detection and demodulation
Situation description: It is necessary to extract the peak voltage of AC signals (such as audio signals, radio signals) or demodulate (detect) amplitude modulated signals.
Why use it: By utilizing its unidirectional conductivity, only the positive part of the signal (the part exceeding the diode conduction voltage drop) is allowed to pass through and charge the capacitor. The voltage on the capacitor approximately reflects the peak value of the input signal (peak detection). In amplitude modulation radios, diode detectors use this principle to extract audio signals (demodulation) from the carrier wave.
Typical applications: Simple peak detection circuits, amplitude modulation radio detection circuits, and some sensor signal conditioning circuits.
Voltage isolation (in specific circuits)
Situation description: In certain circuits that require voltage reversal prevention or unidirectional isolation.
Why use: Although not as thoroughly isolated as optocouplers or relays, in some scenarios where isolation requirements are not high or cost sensitive, series diodes can prevent reverse voltage or current from flowing from one circuit part to another, serving as a simple voltage blocking or "OR gate" function (such as redundant power backup circuits).
Typical applications: simple logic level isolation, redundant power input selection, and prevention of signal feedback.
Provide DC output in the generator/AC power supply
Situation description: Small generators (such as bicycle generators, car AC generators), solar panels (essentially DC, but some controllers or connection methods may involve it) need to output DC power.
Why use it: Generators usually generate alternating current, while solar panels are direct current but may need to prevent backflow when connected in series. Rectifying diodes are used to rectify the alternating current generated by a generator into direct current for battery charging or equipment use (there is a rectifier bridge inside the car generator). In the series circuit of solar panels, bypass diodes are used to prevent a board from becoming a load and consuming energy when it is obstructed, while blocking diodes prevent battery current from flowing back into the panel at night.
Typical applications: automotive and motorcycle generator rectifiers, small wind/hydro generator output rectifiers, bypass diodes and blocking diodes in solar photovoltaic systems.
Summarize key points (when to consider using):
When you need to convert alternating current into direct current.
You need to prevent the reverse flow of current to protect the circuit from damage caused by reverse connection.
There are inductors/coils in the circuit that need to provide a continuous current path for the current when the switch is turned off to protect the switch tube.
You need to detect the peak value of the AC signal or demodulate the amplitude modulation signal.
When you need to isolate voltage or current in one direction (for simple applications).
When you need to extract direct current from a generator or similar AC source.
When selecting rectifier diodes, key parameters such as maximum forward current, maximum reverse withstand voltage, forward voltage drop, and reverse recovery time should also be considered to ensure that they can meet the requirements of specific circuits.