Rectifiers convert an alternating voltage into a direct voltage. They are usually found in power supply units, generators and frequency converters. In general, a distinction can be made between uncontrolled rectifiers, which use diodes, and controlled rectifiers, which use thyristors (also named as Silicon Controlled Rectifier SCR). A rough distinction can also be made between single-phase rectifiers for alternating voltage and three-phase rectifiers for three-phase current.
Half Wave Rectifier
This circuit is the simplest rectifier circuit consisting of only one diode. Only the positive half-wave of the AC voltage is utilized; the negative half-wave is blocked. This circuit has the following properties:
- High ripple voltage
- Asymmetrical load on the AC source
- low efficiency
Single phase Diode bridge rectifier B2U
The Bridge rectifier B2U uses both AC voltage half-waves and is therefore the most commonly used rectifier circuit. Since both half-waves of the alternating voltage are used, the arithmetic mean of the open-circuit voltage on the output side is twice as much as of the single-pulse circuit. In addition, the ripple voltage on the output side is low.
Note especially for low voltages: When rectifying, 2 diodes are always conductive, i.e. the output voltage is reduced by the threshold voltage of two diodes (Si diodes approx. 0.7V). This has a negative effect, especially for low voltages:
The following applies: U2max = U1max - 2 x UF
Smooth the output voltage
In order to smooth the ripple voltage of the output voltage, a so-called smoothing capacitor can be connected in parallel. Electrolytic capacitors are usually used here (they are polarized), which can provide the required capacity.
In general, the larger the load and the larger the ripple voltage, the higher the capacity of the smoothing capacitor must be. We use the following rule of thumb:
C = I * Δt
C the capacitance of the capacitor to be calculated
I Load current
Δt Period duration in ms (10 ms at 50 Hz)
ΔV ripple voltage