Zener Diode Description

ZENER DIODE

A Zener diode is a type of diode that permits current in the forward direction like a normal diode, but also in the reverse direction if the voltage is larger than the breakdown voltage known as "Zener knee voltage" or "Zener voltage". The device was named after Clarence Zener, who discovered this electrical property.

A conventional solid-state diode will not allow significant current if it is reverse-biased below its reverse breakdown voltage. When the reverse bias breakdown voltage is exceeded, a conventional diode is subject to high current due to avalanche breakdown. Unless this current is limited by external circuitry, the diode will be permanently damaged. In case of large forward bias (current in the direction of the arrow), the diode exhibits a voltage drop due to its junction built-in voltage and internal resistance. The amount of the voltage drop depends on the semiconductor material and the doping concentrations. A Zener diode exhibits almost the same properties, except the device is specially designed so as to have a greatly reduced breakdown voltage, the so-called Zener voltage. A Zener diode contains a heavily doped p-n junction allowing electrons to tunnel from the valence band of the p-type material to the conduction band of the n-type material. In the atomic scale, this tunneling corresponds to the transport of valence band electrons into the empty conduction band states; as a result of the reduced barrier between these bands and high electric fields that are induced due to the relatively high levels of dopings on both sides. A reverse-biased Zener diode will exhibit a controlled breakdown and allow the current to keep the voltage across the Zener diode at the Zener voltage. For example, a diode with a Zener breakdown voltage of 3.2 V will exhibit a voltage drop of 3.2 V if reverse bias voltage applied across it is more than its Zener voltage. However, the current is not unlimited, so the Zener diode is typically used to generate a reference voltage for an amplifier stage, or as a voltage stabilizer for low-current applications.

The breakdown voltage can be controlled quite accurately in the doping process. While tolerances within 0.05% are available, the most widely used tolerances are 5% and 10%. Another mechanism that produces a similar effect is the avalanche effect as in the avalanche diode. The two types of diode are in fact constructed the same way and both effects are present in diodes of this type. In silicon diodes up to about 5.6 volts, the Zener effect is the predominant effect and shows a marked negative temperature coefficient. Above 5.6 volts, the avalanche effect becomes predominant and exhibits a positive temperature coefficient.

TC depending on zener voltage In a 5.6 V diode, the two effects occur together and their temperature coefficients neatly cancel each other out, thus the 5.6 V diode is the component of choice in temperaturecritical applications. Modern manufacturing techniques have produced devices with voltages lower than 5.6 V with negligible temperature coefficients, but as higher voltage devices are encountered, the temperature coefficient rises dramatically. A 75 V diode has 10 times the coefficient of a 12 V diode. All such diodes, regardless of breakdown voltage, are usually marketed under the umbrella term of "Zener diode".

Uses Zener diode shown with typical packages. Reverse current −

iZ is shown.

Zener diodes are widely used as voltage references and as shunt regulators to regulate the voltage across small circuits. When connected in parallel with a variable voltage source so that it is reverse biased, a Zener diode conducts when the voltage reaches the diode's reverse breakdown voltage.

POT RESISTOR: The humble potentiometer (or pot, as it is more commonly known) is a simple electro-mechanical transducer. It converts rotary or linear motion from the operator into a change of resistance, and this change is (or can be) used to control anything from the volume of a hi-fi system to the direction of a huge container ship. The pot as we know it was originally known as a rheostat (or reostat in some texts) - essentially a variable wirewound resistor. The array of different types is now quite astonishing, and it can be very difficult for the beginner (in particular) to work out which type is suitable for a given task. The fact that quite a few different pot types can all be used for the same task makes the job that much harder freedom of choice is at best confusing when you don't know what the choices actually are, or why you should make them. This article is not about to cover every aspect of pots, but is an introduction to the subject. For anyone wanting to know more, visit manufacturers' web sites, and have a look at the specifications and available types. The very first variable resistors were either a block of carbon (or some other resistive material) with a sliding contact, or a box full of carbon granules, with a threaded screw to compress the granules. More compression leads to lower resistance, and vice versa. These are rare in modern equipment, so we shall limit ourselves to the more common types :-) Basic Pots It is worthwhile to have a look at a few of the common pot types that are available. Figure shows an array of conventional pots - both PCB and panel mounting.

- Some Examples of Pots

Note that these are not to scale, although the relative sizes are passably close. Apart from the different body shapes and sizes, there are also many "standard" mounting hole and shaft sizes. Probably the most common of all is the one in the centre of the picture. A panel mount, 25 millimetre (1") diameter pot. This uses a 10mm (3/8") mounting hole, and has a 6.35mm (1/4") shaft. These pots have been with us almost unchanged for 40 years or more. The remainder show a few of the many variations available. The fluted shaft types are commonly referred to as "metric", but will accept a standard 1/4" knob albeit with a little play (it is less than a perfect fit, but is acceptable if the grub screw is tight enough). Metric pots are also available in 16mm round and 25mm round formats. Most rotary pots have 270 degrees of rotation from one extreme to the other. A "single turn" pot is therefore really only a 3/4 turn device, despite the name. There are some other rotary types with only 200 degrees or so, and some specialty types may have less than that again. The standard schematic symbol for a pot is shown to the right(although some people insist on using zig-zag lines for resistors and pots

Power and Voltage Ratings For most audio applications, these are of little on no consequence. In many other applications however, exceeding the specified ratings could lead to the destruction of the pot or yourself! Neither can be considered a good thing. Power - A pot with a power rating of (say) 0.5W will have a maximum voltage that can exist across the pot before the rating is exceeded. All power ratings are with the entire resistance element in circuit, so maximum dissipation reduces as the resistance is reduced (assuming series or "two terminal" rheostat wiring). Let's look at the 0.5W pot, and 10k is a good value to start with for explanation. If the maximum dissipation is 0.5W and the resistance is 10k, then the maximum current that may flow through the entire resistance element is determined by ... P = I² * R ... therefore I = √P / R ... so I = 7mA

In fact, 7mA is the maximum current that can flow in any part of the resistance element, so if the 10k pot were set to a resistance of 1k, current is still 7mA, and maximum power is now only 50mW, and not the 500mW we had before. Voltage - Two separate issues here. One is directly related (in part, at least) to the power rating, and is important to ensure that the life of the pot is not reduced. Knowing about the other might save your life. Voltage across resistance element - The maximum voltage across the example pot from above is 7mA * 10k, or 70V. This will rarely (if ever) be achieved in an audio system, but is easy with many other designs. As the resistance increases, so does the voltage - a 0.5W 1M pot will pass only 700uA at maximum power rating, but the voltage needed to create this current is 700V. Unless the pot is actually rated to withstand 700V across the resistance element (rather unlikely), it will fail - maybe not today, or tomorrow, but it will fail eventually. Special pots are made (custom jobs, of course) for high voltages, and standard pots should never be used beyond their rating - assuming that you can find out what the rating is, of course. Dielectric Voltage - The dielectric (insulation of pot "guts" to the body) rating is especially important if the pot is connected to mains operated, non-isolated equipment. Wall mounted lamp dimmers and such are typical examples. This is not commonly specified, but for safety, should be at least 2.5kV. A common way to achieve this is to use a plastic shaft, with the body of the pot insulated from the chassis, and inaccessible by the user (even if the knob falls off or is removed!) This point cannot be stressed highly enough. Most standard pots will safely withstand (maybe) 100V or so between the resistance element and terminals, and the body and shaft. Miniature types will usually be less than this. Never, ever, use a standard pot with a metal shaft to control direct mains operated equipment. Potentiometer Types "But we already covered that, didn't we?" Not really - I merely glossed over the basics. Now, we shall look at a few examples of pots you may come across. Firstly, there is the resistive material and some typical characteristics ... Material

Manufacturing Method

Common uses

Power (Typ) 0.1 to 0.5W

Carbon

Deposited as a carbon composition ink on an insulating (usually a phenolic resin) body

Cermet

Ceramic/metal composite, using a metallic resistance

Most common material, especially for cheap to average quality pots. Has a reasonable life, and noise level is quite acceptable in most cases. (DC should not be allowed to flow through any pot used for audio control) High quality trimpots, and some conventional 0.25 to panel mount types (not very common). Low 2W

element on a ceramic substrate Conductive Special impregnated plastic Plastic material with well controlled resistance characteristics Wire wound Insulating former, with resistance wire wound around it, and bound with adhesive to prevent movement

noise, and high stability. Relatively limited life (or (200 operations typical for trimpots) more) High quality (audiophile and professional) 0.25 to pots, both rotary and linear (slide). Excellent 0.5W life, low noise and very good mechanical feel High power and almost indefinite life. 5 to Resistance is "granular", with discrete small 50W steps rather than a completely smooth (or transition from one resistance winding to the more) next. Low noise, usually a rough mechanical feel.

FULL WAVE RECTIFIER CIRCUIT:.

A full-wave rectifier circuit is a circuit that rectifies the entire cycle of the AC sine-wave. A basic full-wave rectifier uses two diodes together with a transformer whose secondary winding is split equally into two and has a common centre tapped connection, (C). Now each diode conducts in turn when its Anode terminal is positive with respect to the centre point C as shown below. The circuit consists of two Half-wave rectifiers connected to a single load resistance with each diode taking it in turn to supply current to the load. When point A is positive with respect to point B, diode D1 conducts in the forward direction as indicated by the arrows. When point B is positive (in the negative half of the cycle) with respect to point A, diode D2 conducts in the forward direction and the current flowing through resistor R is in the same direction for both circuits. As the output voltage across the resistor R is the sum of the two waveforms, this type of circuit is also known as a "bi-phase" circuit. As the spaces between each half-wave developed by each diode is now being filled in by the other diode the average DC output voltage across the load resistor is now double that of the single half-wave rectifier circuit and is about 0.637Vmax of the peak voltage, assuming no losses.

The peak voltage of the output waveform is the same as before for the half-wave rectifier provided each half of the transformer windings have the same rms voltage value. To obtain a different d.c. voltage output different transformer ratios can be used, but one main disadvantage of this type of rectifier is that having a larger transformer for a given power output with two separate windings makes this type of circuit costly compared to a "Bridge Rectifier" circuit equivalent. FULL WAVE BRIDGE RECTIFIER:

The Bridge Rectifier

Another type of circuit that produces the same output as a full-wave rectifier is that of the Bridge Rectifier. This type of single phase rectifier uses 4 individual rectifying diodes connected in a "bridged" configuration to produce the desired output but does not require a special centre tapped transformer, thereby reducing its size and cost. The single secondary winding is connected to one side of the diode bridge network and the load to the other side as shown below.

The Diode Bridge Rectifier

The 4 diodes labelled D1 to D4 are arranged in "series pairs" with only two diodes conducting current during each half cycle. During the positive half cycle of the supply, diodes D1 and D2 conduct in series while diodes D3 and D4 are reverse biased and the current flows through the load as shown below.

The Positive Half-cycle

During the negative half cycle of the supply, diodes D3 and D4 conduct in series, but diodes D1 and D2 switch of as they are now reverse biased. The current flowing through the load is the same direction as before.

The Negative Half-cycle

As the current flowing through the load is unidirectional, so the voltage developed across the load is also unidirectional the same as for the previous two diode full-wave rectifier, therefore the average DC voltage across the load is 0.637Vmax and the ripple frequency is now twice the supply frequency (e.g. 100Hz for a 50Hz supply).

The Smoothing Capacitor We saw in the previous section that the single phase half-wave rectifier produces an output wave every half cycle and that it was not practical to use this type of circuit to produce a steady DC supply. The full-wave bridge rectifier however, gives us a greater mean DC value (0.637 Vmax) with less superimposed ripple while the output waveform is twice that of the frequency of the input supply frequency. We can therefore increase its average DC output level even higher by connecting a suitable smoothing capacitor across the output of the bridge circuit as shown below.

Full-wave Rectifier with Smoothing Capacitor

The smoothing capacitor converts the full-wave rippled output of the rectifier into a smooth DC output voltage. Two important parameters to consider when choosing a suitable a capacitor are its Working Voltage, which must be higher than the no-load output value of the rectifier and its Capacitance Value, which determines the amount of ripple that will appear superimposed on top of the DC voltage. Too low a value

and the capacitor has little effect. As a general rule of thumb, we are looking to have a ripple voltage of less than 100mV peak to peak. The main advantages of a full-wave bridge rectifier is that it has a smaller AC ripple value for a given load and a smaller reservoir or smoothing capacitor than an equivalent half-wave rectifier. Therefore, the fundamental frequency of the ripple voltage is twice that of the AC supply frequency (100Hz) where for the half-wave rectifier it is exactly equal to the supply frequency (50Hz). The amount of ripple voltage that is superimposed on top of the DC supply voltage by the diodes can be virtually eliminated by adding a much improved π-filter (pi-filter) to the output terminals of the bridge rectifier. This type of low-pass filter consists of two smoothing capacitors, usually of the same value and a choke or inductance across them to introduce a high impedance path to the alternating ripple component. Another more practical and cheaper alternative is to use a 3-terminal voltage regulator IC, such as a LM7805 which can reduce the ripple by more than 70dB (Datasheet) while delivering over 1amp of output current. DELTA-WYE TRANSFORMER:

A delta-wye (Δ-Y) transformer is an electrical device that converts three-phase electric power without a neutral wire into 3-phase power with a neutral wire. It is generally built from 3 independent transformers. The term Delta-Wye transformer is used in North America, and Delta-Star system in Europe.

Delta Wye Transformer In the United States, Delta-wye transformers are common in commercial, industrial, and high-density residential locations, to supply three different types of power from one power source: • • •

Three-phase power, for 480 V motors Single-phase leg-to-leg, for 208 V motors and high-current loads Split-phase leg-to-neutral, for 120 V lights and appliances

An older method of providing these three different forms of power from three-phase was with the high-leg delta transformer, but these have fallen out of use due to complex application rules and hazards