The half wave rectifier circuit is the simplest form of rectifier circuit that can be used, and although it may not provide the highest level of performance in some aspects, it is nevertheless very widely used.
Half wave diode rectifier applications
The half wave diode rectifier is used in a variety of ways and in a host of different types of circuit.
- Power rectification: One of the most obvious ways for a half wave diode rectifier to be used is within a power rectifier. A line or mains power input normally passes through a transformer to transform the voltage to the required level.
- Signal demodulation: A simple half wave diode rectifier can be used for signal demodulation of amplitude modulated signals. The rectification process enables the amplitude modulation to be recovered.
- Signal peak detector: The simple half wave diode detector can be used as a peak detector, detecting the peak of an incoming waveform.
Basic half wave rectifier circuit
The basics of the operation of a half wave rectifier circuit are quite straightforward. The incoming signal is passed through a diode. As this can only pass current in one direction, it only passes that part of the waveform that for which the diode is forward biased.
The half wave rectifier circuit normally utilises a single diode. The incoming signal is connected directly to the diode and the diode is in turn connected to a load as shown in the half wave rectifier circuit below.
Half wave diode circuits
Although the half wave diode rectifier circuit basically uses a single diode, there are a few circuit differences around the diode dependent upon the application.
- Power rectifier: When used for power rectification, the half wave rectifier circuit is used with a transformer if it is to be used for powering equipment in any way. Normally in this application the input alternating waveform is provided via a transformer. This is used to provide the required input voltage.
- AM demodulation: When the half wave rectifier is used for amplitude modulation detection, the circuit obviously requires to interface to the other circuits in the radio. A typical circuit might be one like that shown below.
- Peak detector: The half wave diode circuit is often used as a simple voltage peak detector. By placing a capacitor across the output load, the capacitor will charge to the peak voltage ( &sqrt;2 x RMS voltage of a sine wave). Provided the time constant of the CR network, capacitor and load resistance is much longer than the period of the waveform or sufficient to capture the peak of a varying waveform the circuit will hold the peak of the voltage.
- Forward current: It is necessary that the diode is able to handle the levels of average current and peak current flowing through it in a half wave rectifier circuit. The current will peak as a result of the capacitor smoothing circuit. As the current only flows as the capacitor charges up, the current is in short bursts which are much higher than the average current.
- Peak inverse voltage: The diode must be able to reliably withstand the peak reverse or inverse voltages that appear across it. The peak voltages are not just the output voltage, but higher. The peak inverse voltage rating of the diode should be at least 2 x √2 times the RMS voltage of the input. This is because the output is normally smoothed by a capacitor, and this will take a value that is the peak of the input waveform. This will be √2 times the RMS voltage. With this voltage on the output, the input waveform on the "blocked" half of the cycle will fall and reach a peak value at the bottom of the crest of √2 times the RMS value. The maximum reverse value seen across the rectifier diode is the sum of these two voltages.
There should also be a significant margin, especially when used in a mains or line power supply. This is because voltage spikes can appear on the line.
- Diode turn on voltage: All diodes have a forward voltage drop required to turn the diode on. This can be important in some applications. Typically a silicon diode is 0.6V and a germanium diode is 0.2 to 0.3V. A silicon Schottky diode is also about 0.2 to 0.3V. Reducing the forward voltage drop reduces power loss and in some applications like signal detection, it makes the diode rectifier more sensitive.
- Forward voltage drop: Apart from the forward turn on voltage, diodes also have a level of resistance. As the current increase, so does the level of voltage drop. Power diodes normally have a wider area for current conduction and therefore their voltage drop will be less at high current levels.
- Diode capacitance: When the diode is used in a half wave rectifier for signal detection, the capacitance may be an issue because of the frequencies involved. Typically Schottky diodes have a very small junction capacitance.
The transformer provides isolation from the mains or line power and also enables the input voltage to the diode to be at the required level. Note that the peak voltage is equal to &sqrt;2 or 1.414 times the RMS value.
Half wave rectifier diode requirements
When designing a half wave rectifier circuit, it is necessary to ensure that the diode is capable of providing the required performance. While there are very many parameters that define individual diodes, and these may need to be taken into account for a given design, some of the major parameters are detailed below:
Half wave rectifier circuit precautions
When designing a half wave rectifier circuit, it is necessary to make sure there is a DC return in the circuit. Often when using the diode rectifier for signal or peak detection it is easy to omit a DC return. This needs to be included either as a resistor or as part of a transformer or choke. Examples are given below.
The half wave rectifier circuit can often be used to good effect. As a power rectifier it only addresses half of the waveform making smoothing an issue later. As a result a full wave system is normally used for power rectification. The half wave rectifier is often used for signal and peak detection.
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