Index
INTRODUCTION
Circuit Symbols
1N4001 to 1N4007
Silicon Power Rectifiers Electrical Characteristics Specifications
Their value will depend on
the current and the degree of smoothing required. Example
Say for example we want a power
supply to give 9V at 1 A.
VDC= 1.41 XVAC
- 1.41 X9V
=
12.69V
Peak ( 9V at 1A load) Loading and Nominal Voltage One thing to be aware of with this type of power supply circuit is the voltages given by the formulas are nominal only. Because the actual output voltage of a transformer varies according to its load, the DC output of the power supply will also vary. As well as this, there is a voltage drop across the diodes which will vary according to load. If you need a very precise voltage, the best solution is to use one of the regulated power supply circuits shown in the zener Diode and Voltage Regulator sections of this ebook. You will see that most regulated circuits use one of the circuits above to produce unregulated DC, then regulate it to a consistent voltage that is independent of the load.
Page 9 The OA91 is a small signal germanium point contact diode. It is suitable for a wide range of RF detector and small signal rectifying applications.
Specifications Ip Forward current 50mA
VR Reverse
Voltage 90V Vp, @ ip = 10mA 1.05V @lF = 0.1mA 0.1V
The crystal set consists of a tuned circuit which selects the wanted station or frequency, and a detector, which separates the information (music, speech etc.) from the radio transmission. The audio voltage produced is an exact replica of the sound from the radio station. The detector diode rectifies the incoming signal,
leaving a half wave radio signal which varies in amplitude with the
audio signal. The fixed capacitor C2 shorts out or 'bypasses'
the RF signal, leaving only the audio.
The
circuit below is for a Crystal set using a readily available Ferrite rod
and pre-wound aerial coil.
RF Monitor Meter
Page 10 The 1N4148 is a general purpose signal diode suitable for a wide range of switching and low power rectifying purposes. It is equivalent to the 1N914.
Zener Diodes
Unlike conventional
diodes, zener diodes are deliberately intended to be used with the anode
connected to a negative potential (or 0v) and the cathode connected to
the positive potential. When connected in this manner, zener diodes have
a very high resistance below a certain critical voltage (called the
zener voltage). If this voltage is exceeded, the resistance of the zener
drops to a very low level. When used in this region, essentially constant voltage will be maintained across the Zener, despite quite large changes in the applied currents. This is illustrated graphically in the figure below. It can be seen that
beyond the zener voltage, the reverse voltage remains practically
constant despite changes in reverse current. Because of this, Zener
diodes may be used to provide a constant voltage drop, or reference
voltage. The Basic Voltage
regulator circuit is shown below. It uses only one resistor and one
zener diode. This is called a SHUNT REGULATOR. See SERIES REGULATOR
below. The resistor R1 represents an external load. When this load is connected, some of the current flowing through the zener will now pass through the load. The series resistor Rs is selected so that the minimum current passing through the zener is not less than that required for stable regulation. It is also necessary to ensure that the value of Rs is such that the current flow through the zener cannot exceed its specified power rating. This can be calculated by multiplying the zener voltage by the zener current. The design procedure is as follows:- 1) Specify the maximum and minimum load current, say 0mA and 10mA. 2) Specify the maximum and minimum supply voltages (say 12v) but ensure that the minimum supply voltage is always at least 1.5v higher than the zener voltage being used. 3) In the circuit shown the minimum zener current is 100µA. Thus the maximum zener current (which occurs when there is no load connected) is 10ma plus 100µA equals 10.1mA. 4) The series resistor
must conduct 10.1mA at the lowest input supply voltage, so the minimum
voltage drop across Rs will be 1.5v. Thus the value of Rs
will be:- 1.5v / 10.1X10-3
= 148.5 ohms This could be changed to
the nearest preferred value of 150 ohms. the zener current times the series resistor.
This is the maximum (worst case) zener
current. To work out the resulting power dissipation, we multiply this
current by the zener voltage. In this example this works out at:- Any zener over this in
power rating would be suitable in this circuit. Typical zener diodes drift in their voltage at about +0.1%/°C at the higher voltages. At lower voltages this goes negative reaching -0.04%/°C at around 3.5v. This may be made use of in temperature sensing devices. The circuit below shows how a bridge consisting of two similar zener diodes and two resistors can indicate temperature differences when one zener is held at standard temperature and the other is subjected to the conditions to be monitored. If a 10v zener is used, it will have a temperature coefficient of +0.07%/°C giving a change of 7millivolts per degree C.
Non Standard Voltages Non standard voltages can be obtained by connecting zener diodes in series. The diodes need not have the same voltages since this arrangement is self equalizing.
Page 12
Zener Noise Zener diodes generate noise voltages. These may vary between 10µV and 1mV depending on zener voltage and rating. This noise is easily suppressed by placing a 0.01 to 0.1µF capacitor across it. This reduces the noise voltage by a factor of at least 10.
Zener Diode as a Calibration Signal When supplied with alternating current, the zener diode will limit both the negative and positive halves of the AC cycle. The waveform will be asymmetrical, since the zener will limit almost immediately in one direction, but will not limit until its zener voltage in the other direction.
Increased Power Handling Although zeners can be paralleled for higher power operation, it is usually a better idea to use a series transistor with a zener reference. This configuration improves the power handling and also the regulation of the circuit by a factor equal to the current gain of the transistor.
Page 13 Constant Current Regulation This simple circuit maintains a constant current (within approx 10%).
The circuit below uses the zener as a 'fuseblower'. The zener is selected so that under normal operation it is not conducting. If the circuit develops a fault and the power supply voltage rises above the zener voltage, the zener will come 'on' and draw a heavy current, blowing the fuse.
Improving temperature stability If better temperature stability is required than can be obtained with a single zener, a good trick is to use an ordinary forward biased silicon diode. This makes use of the fact that the forward voltage temperature coefficient of a silicon diode is approximately -2mV/°C. The temperature coefficient of the silicon diode and the zener diode cancel out, giving an almost temperature independent voltage reference. The use of the forward biased diode also allows 'trimming' of zeners to voltages other than the preferred value available. A silicon diode when forward biased will have a voltage drop of 0.7v. When put in series with a zener it will increase the reference by this much. Thus a 6.2v zener plus a silicon diode will give a voltage of 6.9v.
The circuit below uses zener diodes to give a split or dual power supply which is ideal for running ICs such as op-amps. The power input only needs to be an unregulated single rail DC source. When selecting Rs it should be remembered that the zener is the sum of the voltage of the two zeners.
These two circuits show typical use of zeners in power supply circuits. The circuit below is designed to give increased current capacity. It will supply up to 1A with suitable heatsinking of the transistors.
Page 14
These two circuits show typical use of zeners in power supply circuits. The circuit below is designed to give increased current capacity. It will supply up to 1A with suitable heatsinking of the transistors.
Semiconductor
Devices
There are two types of transistor, NPN and PNP. The names relate to the 'sandwich' structure of the two types of transistor. They are shown below. For practical purposes, the important difference between the two types of transistor is that in NPN transistors the current flows from emitter to collector. In PNP transistors the electrons flow from collector to emitter.
Bipolar Transistors Bipolar Transistors are current amplifying devices. When a small signal current is applied at the input terminal (the base) of the bipolar transistor, an amplified reproduction of this signal appears at the output terminals (the collector).
There are 3 useful way of
connecting the input signal for amplification. Common Base Mode In this mode, the signal is introduced into the emitter-base circuit (Thus the base element is common to both the input and output circuits. In this mode, the input impedance is low (i.e. it puts a heavy load on the signal source). The output impedance is fairly high. This type of circuit gives voltage gain and slightly less than unity current gain.
Page 16
Common Emitter Mode In this configuration, the signal is introduced into the base-emitter circuit. This arrangement has moderate input and output impedance. It gives both current and voltage gain. Current gain is measured by comparing the base current and the collector current and so is equivalent to HFE A very small change in base current produces a relatively large change in collector current. Depending on the type of transistor this will vary from 5-600.
This is the most commonly used circuit, very often found in audio amplifiers. For an explanation of hfE see definition below.
In this configuration, the signal is introduced into the base/collector circuit and is 'extracted' from the emitter/collector circuit. The input impedance of this arrangement is high and the output impedance is low. The voltage gain is less than unity while the current gain is high. This configuration is used as an impedance matching device. Commonly called an emitter follower, it is also often used as a current amplifier in power supplies.
Darlington Pair
This arrangement is sensitive to temperature and varying gains of transistors. A better arrangement is shown above (b). This stabilizes the operating point of the transistor because an increase in collector current drops the collector voltage and thus decreases the base bias.
Page 18 Alpha (a) Gain In the common base mode,
the emitter is the input electrode and the collector is the output
electrode. The alpha is the ratio of the collector current lc
to the emitter current IE. It is always less than 1. In the common emitter
mode, the base is the input terminal and the collector is the output
terminal. The beta is the ratio of the collector current lc
to the base current IB. This is the frequency at which the alpha or beta (according to the type of circuit) drops to 0.707 times its 1 kHz value.
Transition Frequency (fT) The frequency at which the small-signal forward current transfer ratio (common-emitter) falls to unity.
Breakdown voltage This defines the voltage between two electrodes at which the current rises rapidly. The breakdown voltage may be specified with the third electrode open, shorted or biased to another electrode.
Secondary Breakdown High voltages and
currents passing through a transistor cause current to be concentrated
or focused on a very small area of the transistor chip causing localized
overheating. This is important in power transistors which are often
designed to minimize this effect. Saturation Voltage (Vcesat) For a given base current, the collector-emitter saturation voltage is the potential across this junction while the transistor is in conduction. A further increase in the bias does not increase the collector current. Saturation voltage is very important in switching and power transistors. It is usually in the order of 0.1v to 1.0v
Safe-operating-area Power transistors are often required to work at high currents and high voltages simultaneously. This ability is shown in a safe operating area curve.
Ptot Vcbo The dc voltage between the collector terminal and the base terminal when the emitter terminal is open-circuited.
vceo The dc voltage between
the collector terminal and the emitter terminal when the base terminal
is open-circuited.
Page 19
All have a maximum power dissipation of
500mW. They have essentially similar specifications and can generally be
substituted for one another (within the PNP and NPN groups of three
each). All devices are housed in standard TO-92 plastic packages.
Page 20
A Simple Amplifier
Relay driver This simple circuit increases the sensitivity of a relay so that it will trigger at 700mV at 40uA. Any relay with an operating current of less than 60mA and operating voltage of less than 12v is suitable. The circuit's supply rail should be at least 3v higher than the operating voltage of the relay.
The circuit will work with any relay with a coil resistance higher than 180 ohms and a pull in voltage of less than 12v.
FM transmitter This circuit, is about as simple as a transmitter can get. The coil is etched onto the printed circuit board, but can be easily substituted by 6 turns on a 4mm diameter former.
Page 21 This circuit is an astable multivibrator or square-wave generator. The circuit is suitable as a morse code generator. The frequency of operation can be raised by making the value of the capacitors smaller. The speaker can be any general purpose 8 ohm type.
BD139/140 Driver
Transistors Features • High gain (hFE40-250) • High fT (250MHz for BD139, 75MHz for BD140)
to pages: 22 to 41
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