Half wave rectifier circuit Theory & Calculations

Half wave rectifier Definition

The half wave rectifier circuit is constructed simply by connecting a diode between the power supply transformer and the load, as shown in Figure (a). When the secondary ac voltage swings positive, as shown in Figure (b), the anode of the diode is made positive, causing the diode to turn ON and connect the positive half-cycle of the secondary ac voltage across the load (RL). When the secondary ac voltage swings negative, as shown in Figure (c), the anode of the diode is made negative, and therefore the diode will turn OFF. This will prevent any circuit current, and no voltage will be developed across the load (RL).

Half wave rectifier vutput voltage Formula

Figure (d) illustrates the input and output waveforms for the half wave rectifier circuit. The 120 V ac rms input, or 169.7 V ac peak input, is applied to the 17:1 step-down transformer, which produces an output of:

Because the diode will only connect the positive half-cycle of this ac input across the load (Rl), the output voltage (VRL) is a positive pulsating dc waveform of 10 V peak. In this final waveform in Figure (d), you can see that the circuit is called a half wave rectifier because only half of the input wave is connected across the output.
The average value of two half-cycles is equal to 0.637 V peak. Therefore, the average value of one half-cycle is equal to 0.318 V peak voltage of (0.637/2 = 0.318):

Vavg =0.318 x Vs peak

In the example in Figure , the average voltage of the half-wave output will be:

                Vavg = 0.318 X Vs peak
          Vavg = 0.318 X 10 V
Vavg = 3.18 V

To be more accurate, there will, of course, be a small voltage drop across the diode due to its barrier voltage of 0.7 V for silicon and 0.3 V for germanium. The output from the circuit in Figure 1 would actually have a peak of 9.3 V (10 V - 0.7 V), and therefore an average of 2.96 V (0.318 X 9.3 V), as shown in Figure (e).

Vout =Vs - Vdiode 

Half wave rectifier Graph

Another point to consider is the reverse breakdown voltage of the junction or rectifier diode. When the input swings negative, as illustrated in Figure (c), the entire negative supply voltage will appear across the open or OFF diode. The maximum reverse breakdown voltage, or peak inverse voltage (PIV) rating of the rectifier diode, must therefore be larger than the peak of the ac voltage at the diode’s input.

Output Polarity of Half wave rectifier

Output polarity. The half wave rectifier circuit can be arranged to produce either a positive pulsating dc output, as shown in Figure (a), or a negative pulsating dc output, as shown in Figure (b). Studying the difference between these circuits you can see that in Figure (a) the rectifier diode circuit is connected to conduct the positive half-cycles of the ac input, while in Figure (b) the rectifier diode is reversed so that it will conduct the negative half-cycles of the ac input By changing the direction of the diode in this manner, the rectification can be made to produce either a positive or a negative dc output

 

Ripple frequency of Half wave Rectifier

Referring to the input/output waveforms of the half wave rectifier circuit shown in Figure 11-20, you can see that one output ripple is produced for every complete cycle of the ac input. Consequently, if a half wave rectifiers is driven by the 120 V ac 60 Hz line voltage, each complete cycle of the ac input and each complete cycle of the output will last for one-sixtieth or 16.67 ms (1/60 = 16.67 ms). The frequency of the pulsating dc output from a rectifier is called the ripple frequency, and for half wave rectifier circuits, the:

output pulsating DC ripple frequency = AC   frequency

the Transient suppressor diode | Transorb

transient suppressor diode Definition

A device used to protect voltage-sensitive electronic devices in danger of destruction by high-energy voltage transients.

Transorb Definition

Absorb transients. Another name for transient suppressor diode.

Transient Suppressor Diode Uses

Lightning, power line faults, and the switching on and off of motors, air conditioners, and heaters can cause the normal 120 V rms ac line voltage at the wall outlet to contain under-voltage dips and over-voltage spikes. Although these transients only last for a few microseconds, the overvoltage spikes can cause the input line voltage to momentarily increase by 1000 V or more. In sensitive equipment, such as televisions and computers, shunt filtering devices are connected between the ac line input and the primary of the dc power supply’s transformer to eliminate these transients before they get into, and possibly damage, the system.

Transorb Schematic Symbol

One such device that can be used to filter the ac line voltage is the transient suppresion diode. Referring to Figure (a), you can see that this diode contains two zener diodes that are connected back-to-back. The schematic symbol for this diode is shown in Figure (b).
Transient suppressor diodes are also called transorbs because they “absorb transients.” Figure (c) shows how a transorb would be connected across the ac power line input to a dc power supply. Because the zeners within the transient suppresion diode are connected back-to-back, they will operate in either direction (the device is “bi-directional”) and monitor both alternations of the AC input. If a voltage surge occurs that exceeds the Vz (zener voltage) of the diodes, they will break down and shunt the surge away from the power supply.

Transorb Circuit

Most manufacturers’ transorbs have a high power dissipation rating because they may have to handle momentary power line surges in the hundreds of watts. For example, the Motorola 1N5908 1N6389 series of transorbs can dissipate 1.5 kW for a period of approximately 10 ms (most surges last for a few milliseconds). The devices must also have a fast turn-on time so that they can limit or clamp any voltage spikes. For example, the Motorola P6KE6.8 series has a response time of less than 1 ns.
In DC applications, a single unidirectional (one-direction) transient suppressor can be used instead of a bidirectional (two-direction) transient suppressor. These single transorbs have the same schematic symbol as a zener.

Metal Oxide Varistors MOVs

Metal oxide varistors (MOVs) are currently replacing zener-diode and transient-diode suppressors because they are able to shunt a much higher current surge and are cheaper. These are not semiconductor devices in fact, they contain a zinc-oxide and bismuth-oxide compound in a ceramic body but are connected in the same way as a transient suppressor diode. They are called varistors because they operate as a “voltage dependent resistor” that will have a very low resistance at a certain breakdown voltage. The MOV’s schematic symbol, typical appearance, and construction are shown in Figure 2.

Zener diode lesson

Definition of zener diode 

Zener Diode : is a Diodes constructed to operate at voltages that are equal to or greater than the reverse breakdown voltage rating.

Zener diode principle

Figure 1(a) shows the two schematic symbols used to represent the zener diode. As you can see, the zener diode symbol resembles the basic P-N junction diode symbol in appearance; however, the zener diode symbol has a zig-zag bar instead of the straight bar. This zig-zag bar at the cathode terminal is included as a memory aid since it is “Z” shaped and will always remind us of zener.
Figure 1(b) shows two typical low-power zener diode packages, and one high- power zener diode package. The surface mount low-power zener package has two metal pads for direct mounting to the surface of a circuit board, while the axial lead low-power zener package has the zener mounted in a glass or epoxy case. The high-power zener package is generally stud mounted and contained in a metal case. These packages are.

identical to the basic P-N junction diode low-power and high-power packages. Once again, a band or stripe is used to identify the cathode end of the zener diode in the low- power packages, whereas the threaded terminal of a high-power package is generally always the cathode.

Characteristics voltage current 

Figure 2 shows the V-1 (voltage-current) characteristic curve of a typical zener diode. This characteristic curve is almost identical to the basic P-N junction diode’s characteristic curve. For example, when forward biased at or beyond 0.7 V, the zener diode will turn ON and be equivalent to a closed switch; whereas, when reverse biased, the zener diode will turn OFF and be equivalent to an open switch. The main difference, however, is that the zener diode has been specifically designed to operate in the reverse breakdown region of the curve. This is achieved, as can be seen in the inset in Figure 2, by making sure that the external bias voltage applied to a zener diode will not only reverse bias the zener diode (+ —» cathode, -  —» anode) but also be large enough to drive the zener diode into its reverse breakdown region.

As the reverse voltage across the zener diode is increased from the graph origin (which represents 0 volts), the value of reverse leakage current (Ir) begins to increase. Comparing the voltage developed across the zener (Vz) to the value of current through the zener (Iz), you may have noticed that the voltage drop across a zener diode (Vz) remains almost constant when it is operated in the reverse zener breakdown region, even though current through the zener (Iz) can vary considerably. This ability of the zener diode to maintain a relatively constant voltage regardless of variations in zener current is the key characteristic of the zener diode.

Generally, manufacturers rate zener diodes based on their zener voltage (Vz) rather than their breakdown voltage (VBr). A wide variety of zener diode voltage ratings are available ranging from 1.8 V to several hundred volts. For example, many of the frequently used low-voltage zener diodes have ratings of 3.3 V, 4.7 V, 5.1 V, 5.6 V, 6.2 V, and 9.1 V.

How to test zener diode with multimeter

Because a zener diode is designed to conduct in both directions, we cannot test it with the ohmmeter as we did the basic P-N junction diode. The best way to test a zener diode is to connect the voltmeter across the zener while it is in circuit and power is applied, as seen in Figure 3. If the voltage across the zener is at its specified voltage, then the zener is functioning properly. If the voltage across the zener is not at the nominal value, then the following checks should be made:

1. Check the source input voltage. If this voltage (Vin) does not exceed the zener voltage (Vz), the zener diode will not be at fault because the source voltage is not large enough to send the zener into its reverse breakdown region.
2. Check the series resistor (Rs) to determine that it has not opened or shorted. An open series resistor will have all of the input voltage developed across it and there will be no voltage across the zener. A shorted series resistor will not provide any current-limiting capability and the zener could possibly bum out.
3. Check that there is not a short across the load because this would show up as 0 V across the zener and make the zener look faulty. To isolate this problem, disconnect the load and see if the zener functions normally.

If these three tests check out okay, the zener diode is probably at fault and should be replaced.