What are the disadvantages of bridge rectifiers
Bridge rectifier circuit
The bridge rectifier circuit is also referred to as a two-pulse bridge rectifier circuit B2. It consists of two pairs of diodes connected in parallel. The AC voltage input is located between the diode pairs.
The arrangement of the semiconductor diodes in the circuit means that the alternating current flows through the circuit in two different ways.
The consumer is only ever traversed by electricity in one direction.
Alternative representation of the bridge rectifier circuit
Oscilloscope image of the input voltage Ue
A very normal sinusoidal alternating voltage is applied to the input of the bridge rectifier circuit.
Oscilloscope image of the output voltage Ua
The current flow of the second half-cycle of the input voltage Ue folded up. This causes the output voltage U to pulsatea. It is also referred to as pulsating (fully rectified) DC voltage.
An ohmic load reduces the pulsating DC voltage, given by the source resistance of the transformer output. The form of the voltage remains unchanged, unless the output is loaded significantly more than the specified nominal load (manufacturer's specification) allows.
The pulsating DC voltage is stabilized with subsequent smoothing and sieving.
Bridge rectifier (component)
The bridge rectifier circuit is available ready-made as a component. To rectify alternating voltages, the finished bridge rectifier must be preferred to individual rectifier diodes. The integrated silicon diodes are specially designed for rectifying AC voltage.
Marking of rectifiers
Example: B 250 C 1000
|250||max. supply voltage (rms value)|
|1000||max.current consumption in mA (capacitor load)|
|800||0.8 A (2500 µF)|
|1000||1 A (2500 µF)|
|1500||1.5 A (2500 µF)|
|3200||3.5 A (5000 µF)|
|5000||5 A (10000 µF)|
Bridge rectifier with capacitors
If you use 4 individual diodes instead of a bridge rectifier, you should ensure that suitable diodes are used for the rectification. So special rectifier diodes or signal diodes.
If universal diodes are used, 4 capacitors connected in parallel must also be used. They are used to protect the rectifier in order to suppress high voltage transients.
1N4148 or 1N914 are not suitable as rectifier diodes. They are primarily used for signal processing when high switching speeds or high frequencies are required.
Diodes between 1N4001 (max. 50 V) and 1N4007 (max. 1,000 V) are better. The 1N4004 (max. 400 V) is recommended for low-voltage rectification and 1N4007 (max. 1,000 V) for 230V rectification. This large voltage reserve usually prevents the diodes from breaking due to peak voltage without capacitors in parallel with the diodes.
In addition to the 1-ampere diodes 1N4001 to 1N4007, there are also 3-ampere diodes 1N5400 to 1N5408, depending on the current intensity.
Steep voltage peaks can occur in the voltage curve of a bridge rectifier circuit with simple diodes. These voltage peaks can be observed especially with slow diodes with an analog oscilloscope. In relation to the voltage period of the pulsating DC voltage, the voltage peak has a larger pulse width.
Note: These voltage spikes have a detrimental effect on devices that are nearby. For example, radio receivers.
What can also be added: The equivalent circuit of the bridge with the transformer when the diodes are blocked (junction capacitance) form an oscillating circuit if the diodes are slow enough for an oscillation to build up.
The resonant circuit is excited by the voltage when it is blocked. Depending on the combination of diode and transformer, vibrations can form which are radiated and thus interfere with other electronic devices. With a resistance one could dampen these vibrations. But that would put unnecessary stress on the transformer.
This can be remedied by 4 ceramic capacitors with approx. 100nF parallel to the rectifier diodes. As a result, voltage peaks are short-circuited and any oscillating circuit that may have arisen is detuned and thus damped.
Note: Additional capacitors for the rectifier are usually not necessary.
But from a certain interaction of resistance, inductance and capacitance you are always dealing with an HF network. This circuit measure can therefore be ineffective.
The capacitors selected must be able to withstand the full AC voltage plus transients and spikes. More like that. This requires ceramic capacitors and a dielectric strength at least as high as the supply voltage. Better higher.
Furthermore, the fine fuse should be correctly dimensioned so that a lightning strike in the vicinity cannot cause damage through these ceramic capacitors.
Dimensioning of the bridge rectifier
When switched on, the electrolytic capacitors (by smoothing and sieving) act like a short circuit. The current through the diodes is only limited by the internal resistance of the upstream transformer. The peak voltage can be well above what is expected of the transformer as the secondary voltage.
Therefore, each individual diode must be able to withstand about twice the possible input voltage. So it still has reserves when the input voltage shows voltage peaks.
The current carrying capacity of the diodes is based on 1 ampere per millifarad of the subsequent charging capacity of the electrolytic capacitors (smoothing and sieving).
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