Source: Ali Bazzi, Department of Electrical Engineering, University of Connecticut, Storrs, CT.
A DC power supply is generally considered to be a device that supplies DC, or unidirectional, voltage and current. Batteries are one such power supply, however, they are limited in terms of lifetime and expense. An alternative method to providing unidirectional power is to transform AC line power to DC power using a rectifier.
A rectifier is a device that passes current in one direction, and blocks it in the other direction, enabling the transformation of AC to DC. Rectifiers are important in electronic circuits as they only allow current in a certain direction after a certain threshold forward voltage across them is overcome. A rectifier can be a diode, a silicon controller rectifier, or other types of silicon P-N junctions. Diodes have two terminals, the anode and the cathode, where current flows from anode to cathode. Rectifier circuits use one or more diodes that change AC voltages and currents, which are bipolar, to unipolar voltages and currents that can be easily filtered to achieve DC voltages and currents.
It is expected that a resistive load coupled to a half-wave rectifier will only see the positive half-cycle of the input AC voltage since the diode rectifier can pass current in one direction. With a full-bridge rectifier, the input positive and negative half-cycles are rectified to be positive, but adding a capacitor will filter out most of the voltage ripple and provide the load with a clean DC voltage.
When an inductor is added in series with the load, it is expected that the diode turn off will be delayed. This can be explained as follows: Diodes turn off under two conditions (that are required to coexist) 1) the current in the diode has to go to zero, and 2) the voltage across the diode (Anode-to-cathode voltage) is below the turn-on threshold. When an inductor is in series with the load, it stores energy and will act as a current source when the source is not available or is going negative at the diode's anode side. Therefore, inductor current will maintain the diode as forward biased until the inductor energy is dissipated. Essential equations that govern basic rectifier circuits with input Vin=V0cos(ωt):
Single diode and resistive load: <Vout>=V0/π (1)
Diode bridge and resistive load: <Vout>=2V0/π (2)
Diode bridge, current source load: <Vout>=2V0/π (3)
Diode rectifiers are almost in every power supply, charger, variable frequency drive, and in many protection circuits. Most DC power supplies or adjustable AC power supplies use diode rectifiers to convert AC to DC, and then to adjustable AC if needed as in AC power supplies and variable frequency drives. Applications in power electronic converters are common for voltage blocking, and for freewheeling energy in inductors, electro-mechanical relays, and motor windings. Diode applications extend beyond power electronics applications to low power electronics, communication systems, and lighting applications.
Single-phase rectifiers are used to convert AC supply voltage and current to DC, as required for powering equipment and digital electronic devices. Standard mains power supplied to homes and commerce is AC. However, most digital electronics are designed to run on DC power. Rectifiers are devices that can be used to transform AC electricity to a compatible DC supply. A rectifier passes current in only one direction, thereby transforming bipolar AC input to a unipolar rectified output. Rectifier circuits use one or more diodes to pass only positive or negative AC power, resulting in a pulsating source, which is then filtered to achieve smooth, consistent, DC voltage and current. This video introduces fundamental rectifier and diode circuitry concepts, demonstrates several common rectifier circuits, and tests the voltage output of rectifier circuits with variations in voltage input and loading configuration.
Rectifiers are devices used in electronic circuits to pass current in one direction and block it in the other direction. Rectifiers allow the passage of current only when a threshold forward voltage is surpassed. Diode rectifiers have two terminals, the anode and the cathode, with current flowing from anode to cathode and blocked from cathode to anode. Single-phase half-wave rectifiers pass voltage across across a single diode. In this circuit, only the positive half of the AC input voltage transmits to the output across the load resistor. If the diode were reversed, only the negative half of the AC input voltage would appear across the resistor. Voltage for the negative half of the AC cycle is blocked. With only one polarity, the RMS, or root mean squared, output voltage is reduced compared to that of the bi-polar input voltage. Full-wave rectifiers pass both half cycles of the AC input voltage across a four diode bridge circuit, as shown. Flipping the polarity of the negative half and yielding a higher average output voltage across the load resistor. Rectifiers result in a unidirectional, but pulsating current with the effect more apparent in half-wave rectifiers. However, the output of the rectifier is typically filtered by the addition of an inductor in series with the load resistance. In the full-wave rectifier, a capacitor assembled in parallel to the load resistor serves the same purpose. This video illustrates half and full-wave single phase rectifier operation with different output loads, diode turn off characteristics, and filtering of the DC output voltage using different circuits.
For this demonstration of rectifier operation, two different AC sources are used, high frequency, one kilohertz input, is produced using a function generator with 10 volt peak sinusoidal output. Low frequency 60 hertz input is supplied by a variac. Do not touch any part of the circuit while energized. When using the function generator source, the circuits are grounded as shown. Do not ground the variac supply. To set up the function generator for high frequency output, connect the differential probe to a oscilloscope channel one and a 10x probe to channel two. Adjust the scaling factors to 20x on the differential probe and 10x on the 10x probe. On the scope channel menus, set both probes to 10x. For the differential probe, manually multiply measurements by two to reach the 20x desired output. Next, connect a BNC to alligator cable to the 50 ohm output of the function generator and connect the alligator clips to the 10x scope probe. Set the output to a 10 volt peak and 1,000 hertz sinusoidal wave form with zero DC offset. Once the signal is set accordingly, disconnect the bnc connector and scope probe, but keep the function generator on to maintain its settings. To set up the variac for low frequency output, make sure that the output receptacle is disconnected, and that it is off with it’s knob set to zero. Next, slowly adjust the variac knob to five percent output to achieve 10 volt peak.
First, test the half-wave rectifier with a high frequency input voltage and a resistive load. Build the circuit as shown, using a 51 ohm load resistor and a diode rated for 50 volts and two amps. Diode polarity is labeled with a dash symbol on the cathode end. Before connecting the differential probe to the circuit, connect the probe’s terminals together and adjust the wave form to zero offset voltage. Then, connect the differential voltage probe across the load resistor to observe the output voltage and the 10x probe across the AC side to observe the input voltage. Next, adjust the time base on the scope to show input and output voltage for four cycles of input voltage. Disconnect the function generator and remove the differential probe from the circuit before making any modifications. Next, test the half-wave rectifier with high frequency input and a resistive inductive load. Reuse the circuit, adding an inductor in series with the resistor, as shown. As described previously, connect probes to the circuit and display wave forms of input and output voltage. Turn off the function generator, disconnect the differential probe, and remove the inductor from the circuit. Last, test the half-wave rectifier with low frequency input and a resistive load. Connect the differential probe across the variac and turn it on. Adjust the variac to obtain a 10 volt peak output, then turn the variac off without changing its voltage setting. Attach the variac output to the resistive circuit, as shown. Then, connect the differential voltage probe across the load resistor to observe the output voltage. Turn the variac on. Do not touch the circuit with the variac power connected and on. As described previously, display wave forms of input and output voltage.
First, test the full-wave rectifier with a resistive load. Build the circuit as shown, and connect the probes and the variac output to the circuit. As described previously, display wave forms of input and output voltage and measure the peak to peak voltages. Retaining the probe connections, turn off the variac and connect an electrolytic capacitor in parallel with the resistive load. Then observe the input and output voltage.
The first figure shows four cycles of an AC supply voltage and the output from a resistive load coupled to a half wave rectifier. Only the positive half cycle of the input AC voltage passes across the diode rectifier. If the input voltage of the half-wave rectifier circuit is sinusoidal, then the mean voltage output for a single diode with a resistive load is the input peak voltage divided by pi. When an inductor is added in series with the load resistor, the diode turn off region is delayed. This combination of inductor and resistor is a low pass filter. When the value of the inductor is sufficiently large, the oscillatory component of the output is blocked, leaving only the constant DC component. For a full-bridge rectifier, the input positive half cycles pass through the circuit and the negative half cycles are rectified to positive. Adding a sufficiently large capacitor filters out most of the voltage ripple, and provides the load with a consistent DC voltage.
Diode rectifiers are in most power supplies, chargers, variable frequency drives, and in many protection circuits. First, AC power adapters are used to convert power for DC supplied machines or to recharge DC batteries contained within devices. The adaptor can be as simple as a circuit consisting of a transformer to step down the voltage from the 120 volt wall supply, a four diode bridge full wave rectifier, and a capacitor to smooth the DC output voltage. Thyristors are silicone controlled rectifiers commonly used in light dimmers, motor speed controllers, and voltage regulators. By design, the thyristor is for alternating layers of P and N type semiconductors used to create an anode at the P type end, a cathode at the N type end, and a gate leap connected to the P type layer next to the cathode. Above a latching threshold, a current pulse into the gate switches the thyristor from off to on, allowing forward current flow from anode to cathode. This rectifies current flow in one direction and regulates output power with an integrated switching mechanism.
You’ve just watched JoVE’s introduction to single-phase rectifiers. You should now understand how single-phase rectifiers work, common rectifier circuits and their output, and some common rectifier applications. Thanks for watching.
