© by Mike Tooley, Tony van Roon
Diodes and Diode Data
A diode, or “rectifier,”is any device through which electricity can flow in only one direction. The first diodes were crystals used as rectifiers in home radio kits. A weak radio signal was fed into the crystal through a very fine wire called a cat’s whisker.The crystal removed the high frequency radio carrier signal, allowing the part of the signal with the audio information to come through loud and clear. The crystal was filled with impurities, making some sections more resistant to electrical flow than others. Using the radio required positioning the cat’s whiskers over the right kind of impurity to get electricity to flow through the crystal to the output below it.
At the time, though, no one really understood about the impurities — then in 1939 Russell Ohl accidentally discovered that it was the boundary between sections of different purity that made the crystal work. Now that the way they work is understood, manufacturers make crystal diodes that work much more consistently than the ones in those original radio kits.
A crystal diode is made of two different types of semiconductors right next to each other. One side is easy for electrons to travel through; one side is much tougher. It’s something like trying to swim through a pool filled with water and then a pool filled with mud: swimming through water is easy; swimming through mud is next to impossible. To an electron some semiconductors seem like water, some like mud.
One side of the semiconductor boundary is like mud, one like water. If you try to get electricity to move from the mud side to the water side, there’s no problem. The electrons just jump across the boundary, forming a current. But try to make electricity go the other way and nothing will happen. Electrons that didn’t have to work hard to travel around the water side just don’t have enough energy to make it into the mud side. (In real life, there are always a few electrons that can trickle in the wrong direction, but not enough to make a big difference.)
This boundary has turned out to be crucial for our daily lives. Diodes change the alternating current that comes from your wall outlet into the direct current that most appliances need. And transistors need two such boundaries to work.
Semiconductor diodes are generally a single p-n junction of either Silicon(Si) or Germanium (Ge), although Germanium diodes are seldom used these days.In order tomaintain conduction, the p-type material (the p-type connection constitutes the anode while the n-type connection constitutes the cathode).The direction of current flow is from anode to cathode whom the diode is conducting, as shown in Fig. 1.Very little current (negligible in the case of the most silicon devices) flows in the reverse direction Fig. 2.
Diodes exhibit a low resistance to current flow in one direction and a high resistance to current flow in the other.The direction in which the current flows is referred to as the forward direction while that in which negligible current flows is known as the reverse direction.When a diode is conducting, a diode is said to be forward biased and a small voltage (ideally zero) is dropped across it.This voltage is know as the forward voltage drop.The maximum reverse voltage (Vrrm) or peak inverse voltage (PIV).
Germanium diodes conduct at lower forward voltages than their silicon counterparts, but they tend to exhibit considerable more reverse leakage current.Hence the fact that they are seldom used anymore.
One of the applications they still find a use is battery chargers for 6/12/24V Lead-Acid batteries.
The leakage current to charge those heavy-duty batteries seem not to matter much.
Furthermore, the forward resistance of a conducting silicon diode is much lower than that of a comparable germanium type.
Hence, again, germanium diodes are used primarily for signal detection purposes whereas silicon devices are uses for rectification and for general purpose applications.
Typical forward and reverse characteristics for comparable germanium and silicon diodes are shown in Fig. 3.
Diodes are often divided into signal and rectifier types, according to their principal field of application.Signal diodes require consistent forward characteristics with low forward voltage drop.
Rectifier diodes need to be able to cope with high values of reverse voltage and large values of forward current, consistency of characteristics is of secondary importance in such applications.
Rectifier diodes are often available in the form of a Bridge (see Fig. 4) which provides fullwave rectification.
Various diode encapsulations are illustrated in Fig. 5
The European system for classifying semiconductor diodes involves an alphanumeric code which employs either two letters and three figures (general
purpose diodes) or three letters and two figures (special purpose diodes).The first two letters have the following significance:
First Letter - semiconductor material: AGermanium BSilicon CGallium Arsenide, etc. DPhoto diodes, etc. Second Letter - application: AGeneral Purpose BTuning (varicap) Diode ETunnel Diode PPhotovoltaic Diode QLight Emitting Diode (LED) TControlled Rectifier XVaractor Diode YPower Rectifier ZZener Diode
In the case of diodes for specialized applications, the third letter does not generally have any particular significance.
Zener diodes have an additional letter (which appears after the numbers) which denotes the tolerance of the zener voltage.
The following letters are used:
A1% C5% B2% D10%
Zener diodes also have additional characters which indicate the zener voltage (e.g. 9V1 denotes 9.1V).
Example 1 Identify each of the following diodes: (i) AA113(ii) BB105 (iii) BZY88C4V7Diode (i) is a general purpose germanium diode.Diode (ii) is a silicon diode for tuning applications (sometimes referredto as a ’varicap’.Diode (iii) is a silicon zener diode having 5% tolerance and 4.7V zener voltage
The following tables summarize the characteristics of a variety of popular semiconductor diodes.
Note: Most of the bridge rectifiers listed are available in 200V, 400V and 600V versions.It is important to ensure that manufacturers’ voltage ratings are not exceeded.
Hints and Tips:
When designing power supply circuits (in which appreciable currents are present) it is important to allow for the forward voltage drop associated with each rectifier diode.In a bridge rectifier, for example, two diodes will conducting any one time.The total forward voltage drop associated with these diodes can approach 2V and this should be allowed for when determining the AC input voltage to the rectifier.
Zener diodes are silicon diodes which are specially designed to exhibit consistent reverse breakdown characteristics.Zener diodes are available in various families (according to their general characteristics, encapsulation(casing) and power ratings) with reverse breakdown (zener) voltages in the E12 and E24 series (ranging from 2.4V to 91V).
A typical characteristic for a 5.1V zener diode is shown in Fig. 6. The following series of zener diodes are commonly available:
BZX88 series: Miniature glass encapsulated diodes rated at 500mW (at 25 °C).Zenervoltages range from 2.7V to 15V (voltages are quoted for 5mA reverse currentat 25 °C. BZX55 series: Low-power diodes rated at 500mW and offering zener voltages in the range of2.4V to 91V. BZX61 series: Encapsulated alloy junction rated at 1.3Watt (25 °C ambient).Zenervoltages range from 7.5V to 72V. BZX85 series: Medium-power glass-encapsulated diodes rated at 1.3Watt and offering zenervoltages in the range of 5.1V to 62V. BZX93 series: High power diodes in stud mounting encapsulation.Rated at 200Watt forambient temperatures up to 75 °C.Zener voltages range from 9.1V to 75V. BZX97 series: Medium power wire-ended diodes rated at 1.5W and offering zener voltages inthe range of 9.1V to 37V. 1N5333 series: Plastic encapsulated diodes rated at 5W. Zener Voltages range from 3.3V to24V.
The slope resistance of a zener diode is the rate of change of reverse voltage (zener voltage) with diode current.Slope resistance is measured in the breakdown region and expressed in ohms.An ideal zener diode would have zero slope resistance (i.e. the diode would conduct perfectly at its rated zener voltage).In practice, values of 200 or less can be achieved.
The temperature coefficient of zener voltage is the change of zener voltage (from its rated value) which results from a temperature change of 1°C.Temperature coefficient (which should ideally be zero) is expressed in mV/deg.C.In many voltage reference applications, it is essential for the reference diode to exhibit a zener voltage which does not vary with temperature.The following data (for the BZX55 series) is typical of most low-power zener diodes.
A zener diode is to be used as a voltage reference. The diode has the following specifications:
Zener voltage (at 20 °C): 9.1VTemperature coefficient: +4mV/°C
If the equipment is designed to operate over the range -10°C to +40°C, determine the extreme values of reference voltage and the percentage change in the reference voltage over the working range.The temperature coefficient is positive and thus the zener voltage will increase with temperature.
At 40°C the zener voltage will be given by:
Hints and Tips:
Thyristors, (or silicon controlled rectifiers) are three-terminal devices which can be used for switching and AC power control.Thyristors can switch very rapidly from a non-conducting state to a conducting state.In the off-state, the thyristor exhibits negligible leakage current while, in the on-state the device exhibits very low resistance.This results in very little power loss within the thyristor even when appreciable power levels are being controlled.Once switched into the conducting state, the thyristor will remain conducting (e.i. is latched in the on-state) until the forward currents is removed from the device.In DC applications this necessitates the interruption (or disconnection) of the supply before the device can be reset into its nonconducting state.Where the device is used with an alternating supply, the device will automatically become reset whenever the main supply reverses.The device can then be triggered on the next half-cycle having correct polarity to permit conduction.Like their conventional silicon diode counter parts, thyristors have anode and cathode connections; control is applied by means of gate terminal Fig. 8.The device is triggered into the conducting (on-state) by means of the application of a current pulse to this material.
The table on the left summarizes the characteristics of a variety of popular thyristors.
Hints and Tips:
Care should, however, be taken to ensure that the peak value of gate does not exceed the rated value for the device.
Triacs are a refinement of the thyristor which, when triggered, conduct on both positive and negative half-cycles of the applied voltage.Triacs have three terminals known as the main terminal one (MT1), main terminal two (MT2), and the gate (G) as shown in Fig. 10.Triacs can be triggered by both positive and negative voltages present at the gate.Triacs thus provide full-wave control and offer superior performance in AC power control applications when compared with thyristors which only provide half-wave control.In order to simplify the design of triggering circuits, triacs are often used in conjunction with diacs (equivalent to a bi-directional zener diode).A typical diac conducts heavily when the applied voltage exceeds approximately +/- 32V.Once in the conducting state, the resistance of the diac falls to a very low value and thus a large value current will flow.The characteristic of a typical diac is shown in Fig. 11.
Hints and Tips:
Light Emitting Diodes (LEDs) can be used as general purpose indicators and, compared with conventional filament lamps, operate from significantly smaller voltages and currents.LEDs are also very much more reliable than filament lamps.Most regular LEDs will provide a reasonable level of light output when a forward current of between 5mA and 20mA is applied.
Light Emitting Diodes are available in various formats, with the round types being most popular.Round LEDs are commonly available in the 3mm and 5mm (0.2 inch) diameter plastic packages (see Fig. 14 and also in 5mm x 2mm rectangular format.The viewing angle for round LEDs tends to be in the region of 20° to 40° whereas for rectangular types this is increased to around 100°.The last couple years we have seen other formats as well like the 1mm and 10mm types.More of those details are mentioned in the LED tutorial.
In order to limit the forward current to an appropriate value, it is usually necessary to include a fixed resistor in series with a LED indicator.The value of the resistor may be calculated from:
R = (V - Vf) / I
where Vf is the forward voltage drop produced by the LED and V is applied voltage.It is usually safe to assume that Vf will be 2V and choose the nearest preferred value for R.
An LED is to be used to indicate the presence of 21V DC supply rail.If the LED has a nominal forward voltage of 2.2V, and its rated at a current of 15mA determine the value of the required series resistor.
Here we can use the formula:
R = (V - Vf) / I = (21 - 2.2V) / .015 = 1.25K
The nearest standard value is 1K2 or 1200 ohm.The power dissipated in the resistor will be 0.015 times 18.8V, or 280mW.
Hints and Tips:
A forward current of 5mA (per LED) will be perfectly adequate in many applications.This is of course for the standard low light LED types.The high- and ultra-bright types may goes as much as 18mA.
– “The Way Things Work”, by David Macaulay
– “Physics for Scientists and Engineers”, by Paul Fishbane, Stephen Gasiorowicz, and Stephen Thornton
– “Feynman’s Lectures on Physics”, by Richard Feynman
Copyright and Credits:
Mike Tooley, “Diode Data”, ET&T May 1990.
Tony van Roon, editor, graphics and photos, new material, September 2010.
(Hugo) Gernsback Publishing is (sadly) out of business since January 2000.
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