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Transistors

March 14, 2011 Leave a comment

Transistors are usually silicon types and are available in either NPN or PNP forms. The abbreviations refer to the sandwiching or junctions of the semiconductor materials used inside the device. Catalog specification sheets or schematics will differentiate the two different types. An NPN or PNP is selected on the basis of how it will be used in the circuit and it is important to use the one specified.

Transistors can have a wide range of applications, but these transistors will tend to cover most categories:

Linear: Transistors designed for linear applications such as low-level voltage application.

Switching: Designed to switch applications.

Power: Transistors which operate at significant power levels, usually divided into audio frequency at radio frequency power types.

Radio frequency: Transistors designed for specifically high frequency applications.

High voltage: Transistors specifically designed for high voltage.

The 2 most common ways to use a transistor are to amplify a signal or to switch a signal on and off. There are literally thousands of different types of transistor and they are rated on an extremely extensive list of criteria. They all have a unique code such as 2N2222 and the number of the transistor will be listed by the manufacturer if I am designing a specific circuit.

If the correct transistor is not available then a substitute can usually be used and the individual manufacturers provide a guide that will help match a transistor with similar parts that can be used as a substitute. The ratings do not appear on the transistor itself and to determine its characteristics I will have to look up the specific characteristics in a specifications book or look at the technical documentation on the manufacturer’s website.

The semiconductor material in a transistor is minute and for the principle of soldering the semiconductor is placed in plastic or metal cases. Signal transistors come in either of these types of cases. The plastic variety will work for most uses, but for a more precise application it may be necessary to use metal ones as they are less susceptible to stray radio frequency interference.

Signal transistors nearly always have three lead connections but sometimes have four. Power transistors can come in either metal or plastic cases and physically they are larger than signal transistors. Transistors will typically have three wire leads being the base, emitter and collector. A base is wired to a voltage or current and turns the transistor on or off. The emitter and collector lead connect to a positive or negative voltage source or ground and the position of the lead will vary on the circuit. Transistors both bipolar and unipolar are ideal for use as switches and these are sometimes known as FET switches and will feature a fourth lead, this lead grounds the case to the chassis of the circuit.

The positions of the leads means that it is vital that the transistor is placed the right way round in the circuit. Any other way will damage the transistor and may damage the other components on the circuit. Bipolar transistors are probably the most common type of transistor. A small input current is applied to the base of the transistor and this will change the amount of current that flows between the collector and the emitter. FETs ( field effect transistors), these transistors also have three connections called gate, source and drain, rather than base, collector and emitter. By applying a voltage, the gate controls the current between the source and the drain. FETs come in two types N-channel which are similar to NPN and P-channel similar to PNP. These FETS come in two sub types known as MOSFET and JFET, and should be stored in anti-static foam as static can damage them.

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Categories: Components

Diodes

March 14, 2011 Leave a comment

Diodes are two terminal devices that exhibit low resistance to current flow in one direction and high resistance to current flow in the other.  The direction in which the current flows is often referred to as the forward direction whilst in the negligible current flows is known as the reverse direction.  When the diode is conducting a small voltage is dropped across it and this is known as the forward voltage drop.  The diode is one of the simplest forms of semiconductor and it is used to control the flow of electrons.

A variety of applications use diodes and they are classed in different subtypes.

Zener diodes:  Zener diodes  limit voltage to a pre-determined amount.  A zener diode can be used to build a voltage regulator into a circuit.

Zener diode

Light emitting diodes, LED‘s: All semiconductors emit infrared light when they conduct current.  LEDs make this light visible. 

Silicon-controlled rectifier (SCR):  The SCR is a type of switch used to control AC or DC currents and are commonly used in light dimmers.

Rectifier:  This basic diode transforms AC current to provide DC current only.  The DC current does not alternate and is only positive or negative.  Diodes can be referred to as rectifiers because they perform this rectifying function, a thyristor is a type of rectifier.

rectifier diode

Bridge rectifier:  This component is made up of four diodes and rectifies AC to DC with great efficiency.

bridge rectifier diode

 Ac inputs and dc inputs:

 A diode does not have a value (apart from the zeners), they are simply used to control the flow of electrons.  Diodes are distinguished by two main criteria their PIV (peak inverse voltage) and current.  These criteria indicate which type of diode should be used in a specific circuit.

 The PIV rating indicates the maximum working voltage for the diode.  For instance if it is rated at 100 volts, you would not use it in a circuit that would apply more than 100 volts to the diode.

 The current rating is the maximum amount of current the diode can handle.  If it was to attempt to conduct more amps than its rating it would overheat and become damaged.

 Diodes are identified by a numerical system which is industry standard.   A typical example would be a 1N4001 rectifier diode, which is rated at 1.0 PIV and 50 volts.  A 1N4002 is rated at 100 volts and a 1N4003 is rated at 200 volts and this pattern continues.

 A rectifier diode rated to about 3 to 5 amps will be encased in grey or black epoxy and can be directly mounted onto a PCB.  The higher current diodes are usually contained inside a metal housing that includes a heat sink or a mounting stud so that the diode can be fixed securely on a heat sink.

 All diodes will have a positive (anode) and a negative terminal (cathode) and the cathode  can commonly be   identified  by a red or black stripe  near one of the leads.  The stripe will correspond with the line in the schematic symbol for the diode.  When following a schematic diagram to build a circuit the diode must be orientated to the line facing the specified way.  If a diode is placed in the circuit in the wrong direction it will not work and it can damage components.

Categories: Components

Capacitors

March 10, 2011 Leave a comment

Capacitors store electrical energy by seperating positive and negative charges.  They store electrons by attracting them to a positive voltage.  When the voltage is reduced or removed the electrons move off aswell.  When the capacitor removes or adds electrons to the circuit it can work to smooth out voltage fluctuations.  The capacitor acts as a delay and can be combined with resistors.  If you increase the resistance it increases the delay in time.  A resistor and capacitor connected together can often be referred to as an RC circuit.  Any circuit I view that has a potential divider and features a capacitor will have something to do with time.

Power supplys that convert AC current to DC current often use capacitors to keep the voltage at a certain level. 

(AC current = alternating current and can run in different directions around the circuit, DC = direct current)

When a capacitor is connected in series with a signal source, such as a microphone, it can block the DC current but pass the AC current.  Most kind of amplifiers would use this function.

Capacitors can be used to make filters that reject AC signals above and below a desired frequency, by adjusting the value of the capacitor it can be possible to change the cut-off frequencies of the filter.

The capacitor itself is actually quite a simple device.  A typical capacitor has two metal plates inside it that are refrained from touching by a dielectric material which acts as an insulator seperating the two plates.  A common dielectric could be a material such as plastic or paper.

 A capacitor is measured by its capacitance and this is valued in Farads.  The higher the value the more electrons the capacitor can store at one time.  A single farad is a large unit of measurement however and so a large amount of capacitors are valued in micro (millionths) of a farad (uF), but there are even smaller measurements such as a nano (nF) and a pico farad (pF) which is a millionth of a millionth!

The working voltage or WV is the highest voltage a capacitor can withstand before the dielectric layers in the component become damaged.  If it is operating at high voltages a spark can develop within the capacitor that perforates the dielectric material and can short the unit.

A typical capacitor designed for a DC circuit rates at no more than 16-35 volts.  A higher voltage is not needed because 3.3 and 12 volts will power these circuits.  A general rule of thumb would be to select a capacitor with a working voltage of at least 10-15 percent more than the voltage in the circuit for safety.

Capacitors come in a variety of shapes.  The aluminium electrolytic and paper capacitors usually come in a cylindrical shape.  The tantalum electrolytic, ceramic, mica and polystyrene capacitors are typically dipped in an epoxy or plastic bath and gives them a more bulbous shape.

  Some capacitors will have their value in farads printed on them and this is quite common with the larger aluminium electrolytic types.  The smaller capacitors such as 0.1 or 0.01 uF mica disk capacitors use a three digit marking system that indicate it capacitance and tolerance.  This system is based on picofarads and a number using this marking system such as 103 will mean 10 followed by three zeros as in 10000 pico farads.  To convert pico to micro farads the decimal point is moved six places to the left, so if it is 10,000pF it would be converted to 0.01 Uf.

The markings on the individual capacitors would be as so:

 nn (a number from 01 to 99) = nn pF

101 = 0.0001 μF

102 =  0.001 μF

103 =  0.01 μF

104 = 0.1 μF

221 = 0.00022 μF

222 = 0.0022 μF

223 = 0.022 μF

224 = 0.22 μF

331 = 0.00033 μF

332 = 0.0033 μF

333 = 0.033 μF

334 = 0.33 μF

471 = 0.00047 μF

472 = 0.0047 μF

473 = 0.047 μF

474 = 0.47 μF

 Another system to look out for which is less common, uses numbers and letters and would be marked like this 4R1.  The R represents the position of the decimal point so it would mean 4.1.  This system does not indicate the units and so could be in micro or pico farads.  To test these capacitors I could use a multimeter or a capacitor meter.  It is worth noting that the capacitor should be plugged directly into the testing instrument as capacitance can increase with long leads making the reading less accurate.

 Capacitors in general can sometimes be less than exact and the value printed on the capacitor can differ from its actual value,  this is caused by manufacturing variations.  Like resistors, capacitors are rated by tolerance and this will come as a percentage.  On most capacitors a single code will indicate its tolerance.  This can either be found on its own on the capacitor itself or placed after the three number mark such as 103Z

With the three digit mark, such as 103Z.  The letter Z means a tolerance and in this case it would be of +80% to -20%.  These are the common code letters which will indicate the capacitors tolerance:

 B + 0.1 pF

D + 0.5 pF

F + 1%

G + 2%

J + 5%

K + 10%

M + 20%

Z + 80%, -20%C + 0.25 pF

 The value of a capacitor will change with temperature and this is known as temperature coefficient.  Again this should be indicated on the capacitor usually as a three digit code such as NP0 which indicates negative/positive zero.  A capacitor with this marking would indicate a high temperature tolerance.  Capacitor manufacturers have begun to use the EIA marking system which indicates temperature tolerance.  The three characters in each mark indicate its temperature tolerance and the maximum variation within the stated temperature range

 The table is as follows:

EIA CAPACITANCE CODE VS MICRO-PICO-NANOFARAD Go to:   Surface Mount Capacitor Kits
EIA CODE MF PF NF EIA CODE MF PF NF EIA CODE MF PF NF EIA CODE MF PF NF
0R5   0.5   300   30   621   620   104 0.1 100000 100
1R0   1   330   33   681   680   124 0.12   120
1R2   1.2   360   36   751   750   154 0.15   150
1R5   1.5   390   39   821   820   184 0.18   180
1R8   1.8   430   43   911   910   224 0.22   220
2R0   2   470   47   102 0.001 1000 1 474 0.47   470
2R2   2.2   510   51   112 0.0011 1100 1.1 105 1   1000
2R7   2.7   560   56   122 0.0012 1200 1.2        
3R0   3   620   62   132 0.0013 1300 1.3        
3R3   3.3   680   68   152 0.0015 1500 1.5        
EIA CODE MF PF NF EIA CODE MF PF NF EIA CODE MF PF NF EIA CODE MF PF NF
3R9   3.9   750   75   162 0.0016 1600 1.6        
4R0   4   820   82   182 0.0018 1800 1.8        
4R7   4.7   910   91   202 0.002 2000 2        
5R0   5   101   100   222 0.0022 2200 2.2        
5R6   5.6   111   110   242 0.0024 2400 2.4        
6R0   6   121   120   272 0.0027 2700 2.7        
6R8   6.8   131   130   332 0.0033 3300 3.3        
7R0   7   151   150   392 0.0039 3900 3.9        
8R0   8   161   160   472 0.0047 4700 4.7        
8R2   8.2   181   180   562 0.0056 5600 5.6        
9R0   9   201   200   682 0.0068 6800 6.8        
100   10   221   220   822 0.0082 8200 8.2        
EIA CODE MF PF NF EIA CODE MF PF NF EIA CODE MF PF NF EIA CODE MF PF NF
110   11   241   240   103 0.01 10000 10        
120   12   271   270   153 0.015 15000 15        
130   13   301   300   183 0.018 18000 18        
150   15   331   330   223 0.022 22000 22        
160   16   361   360   273 0.027 27000 27        
180   18   391   390   333 0.033 33000 33        
200   20   431   430   393 0.039 39000 39        
220   22   471   470   473 0.047 47000 47        
240   24   511   510   563 0.056 56000 56        
270   27   561   560   683 0.068 68000 68        

The final mark that can be found on a capacitor especially tantalum and aluminium electrolytic types is a polarity symbol.  Most capacitors will use the minus (-) for the negative terminal but the (+) will not be shown for the positive.  Only the larger value capacitors such as 1uF and upward will be polarized and therefore will feature a marking.  If the capacitor is polarized it is even more vital to ensure that it is in the circuit the right way round to avoid damaging it or the other components.

To calculate capacitance there are a number of different formulas that are basically the inverse of the formulas for resistors.

To calculate the value of numerous capacitors laid in parallel I simply have to add them up C1+C2+C3=total capacitance.

To calculate the capacitance of two capacitors wired in series would be: total capacitance =C1 x C2 divided by C1 + C2.

To calculate three or more capacitors wired in series it is a little more complicated:

total capacitance=                                 1

                                                                                                                                                                                   

              1  divided by C1        +             1  divided by C2              +      1 divided by C3

 

 

Categories: Components

Potentiometer

March 10, 2011 Leave a comment

A potentiometer is a manually adjustable resistor.  One terminal of the potentiometer is connected to the power source whilst another goes to ground.  The third terminal runs across a strip of resistive material, this strip tends to have a low resistance at one end and its resistance gradually increases to a maximum resistance at the other end, the third terminal serves as the connection between the power source and the ground and is usually interfaced to the user with some form of control in the form of a lever for example.  The user adjusts the position of the third terminal along the resistive strip in order to manually increse or decrease resistance.  By controlling resistance, the potentiometer determins how much current runs through the circuit.  If it is used to regulate current then the potentiometer is limited by the maximum resistivity of the strip.

1.  Chassis mounting volume on/off control

Volume control with a logarithmic track and double pole mains switch that switches both live and neutral power lines to completely isolate the equipment when switched off.

2.  Dual potentiometer with two pole on/off switch

Two independent potentiometers operated by concentric spindles.  Used as volume and tone controls in old mains radios.  The rear (volume) control has a logarithmic track and the front (tone) control, a linear track. The volume control also operates a double pole mains switch at the rear.

3.  High power wirewound preset

Insulated preset with a wirewound track for high voltages (hundreds of volts) and substantial currents.  The connection pins on this potentiometer are designed for soldering directly into a PCB.

4.  High voltage insulated pre−set

Using a carbon track for smoother operation than 3 and insulated with p.t.f.e. to withstand hundreds of volts, but at lower current than 3. For use in cathode ray tube display equipment.

5.  Single, chassis mounting potentiometer

For general user control use.  Note the long insulated spindle that may be cut to the required length.   Available in a range or resistance values with linear or logarithmic carbon track.

6.  Dual ganged potentiometer

Two potentiometers sharing a single spindle are referred to as being “ganged” (What one does, the other does.) Intended for applications such as stereo audio equipment so both channels may be adjusted simultaneously.

7.  Multi−turn pre−set

Two views of a precision slider preset, the wiper is made to slide slowly along the track by means of a screw thread turned by a small plastic gear wheel at the end.  Provides a simple way of producing an accurately adjustable voltage.

8.  PCB mounted potentiometer

Standard user potentiometer for mounting on the edge of a printed circuit board (PCB). Note the small square hole in the case designed to allow cleaning fluid to be sprayed inside the control from an aerosol can. Wiper contacts tend to tarnish over time and carbon tracks become rough with wear, leading to “noisy” (e.g. crackling sound when a volume control is adjusted) operation. Spraying a mixture of alcohol and lubricant inside gives some relief and extends the life of the control.

9.  Sub−miniature skeleton preset

Skeleton presets refer to controls without an enclosing case. A basic track and wiper that can be adjusted using a small insulated adjusting tool, NOT a screwdriver! Intended for general setting up purposes and only occasional use.

10.  Miniature skeleton preset

A larger version of 9. Both of these controls are designed for PCB mounting. Upright and flat mounting versions are available. Modern types are usually fully enclosed but this example shows construction and operation more clearly.  Small presets may have either carbon or “cermet” (a mixture of ceramic and metal) tracks.

My next task will be to design some basic digital circuits using potentiometers.

Categories: Components

Resistors

March 3, 2011 Leave a comment

Resistors limit the amount of current that reaches a component such as an LED.

In some circuits different voltages need to be supplied to different parts of a circuit which can be done with resistors.  If two resistors are joined it forms a voltage divider, if the two resistors are of equal value the voltage in between the two resistors is half that of the rest of the circuit.

A resistor can control the voltage/current going into a component, so for instance placing a resistor at the input of a transistor controls how much the transistor amplifies a signal.

A resistor can protect the input of sensitive components.  If a resistor is placed at the input of a sensitive component the resistor limits the amount of current travelling to it protecting it from damage.  A 470 resistor is usually a sufficient one to protect LED’s.

A fixed resistor supplies a specific resistance depending on its value.  This is determined by a colour coding system that starts at the edge of the resistor and is combined of 4, 5, or sometimes 6 bands of different colours to give the resistor its value.

A resistors resistance is measured in ohms the bands of colour indicate what resistance it will provide.  The colour coding is standardized across the world and the amount of bands featured on the resistor dictates whether it is a standard precision or high precision resistor.

On a standard resistor bands one, two and three indicate its value, for example a resistor with a yellow, orange, red band would be 4,3 x 1000 so it would be 43000 ohms or 43k.  The fourth band represents its tolerance level which is usually between 5 and 10% of the resistors tolerance.

A resistors tolerance takes into account any variations that have taken place during manufacture.  If a resistor had a 2k marking for example the actual value may be slightly higher or slightly lower and this potential variation in the value is known as its tolerance and will be shown as a percentage so a +5 percent tolerance means that the value of the resistor may vary between 5 % above or below the stated value.  By knowing the tolerance I can decide on whether the resistor is going to be suitable for the circuit or whether I may have to change it for usually a higher value.

A high precision resistor may have the value printed on it or they may have five bands.  These will have a tighter tolerance than standard resistors where bands one through four represent the value and the fifth band represents its tolerance usually +1%.  If a circuit needs a specific value such as in timing than a high precision resistor would be used.

Resistors can also be measured by their power, this is measured in watts, the higher the watts the more heat there is generated by electrons travelling through the circuit.  Components can only stand so much heat before becoming damaged and the power rating tells you how many watts can safely pass through the resistor.  Watts are calculated by

P=VxI

P is the power measured in watts, I is the current measured in amps and V is the voltage measured across the resistor, so if the voltage is 5 volts, and 25 milliamps of current go through the resistor the watts are calculated by multiplying 5 by 0.25 to get 0.125.

To calculate the resistance of a single resistor in a circuit is easy using the table featured previously, but resistance will change if you add resistors in parallel or in series together.  To calculate resistors placed in series I just add the values of the resistors together.

For resistors laid in parallel it is a little more complex.  It is important to know the formulas though as resistors will only come in a limited number of common values, yet some circuits need a specific value that can only be created by inserting two or more resistors in series or parallel.  Also resistors are not the only components that have resistance, a motor for example may have some kind of resistance, and for some applications I might need to calculate the combined effect of having these various resistances in a single circuit.

To calculate the resistance of two resistors in parallel the formula is:  total resitance = R1 x R2 divided by R1 + R2

So if the values of the resistors were 1.2k (1200 ohms) and 2.2k (22o0 ohms) the total resistance would be:

1200 x 2200 =2640000

1200 + 2200 = 3400                         2640000 divided by 3400 = 776.47 total resistance

To calculate the total resistance of three resistors in parallel the formula is as follows

total resistance=                          1

                        

1 divided by R1    +    1 divided by R2      +  1 divided by R3………………

 
 

 

 
 
 
 

 

Categories: Components