Starting Point (Part 6)
Synpac Battery Eliminator
As it is hoped this series will show, it is within the capabilities of practically anyone to gain a good basic understanding of electronics. Each part of the series is accompanied by a simple constructional project which demonstrates the practical application of the theory that has been covered, as well as being a useful and worthwhile piece of equipment in its own right.
So far we have only considered voltages and currents that are of fixed polarity and constant amplitude (or change in amplitude relatively slowly). Signals of this type are known as 'direct currents' or DC if the accepted abbreviation is used. There is another and equally important type of signal known as an 'alternating current' or just AC in its abbreviated form. An alternating current starts at zero and increases to its peak positive value, after which it then falls back to zero again. It then increases in amplitude again, but with the opposite polarity to the first part of the signal. As before, it climbs to a peak value and then falls back to zero again. One transition from zero to peak amplitude and back to zero again is a 'half cycle', and two half cycles of opposite polarity together give one complete 'cycle'.
An alternating signal can be represented graphically, as shown in Figure 1(a). Here the horizontal axis represents time and the vertical axis is representative of voltage. Alternatively, if the signal is fed to a resistive load the vertical axis could represent the current flowing through the load since the voltage and current would be proportional to one another. The waveshape shown in Figure 1(a) is known as a 'sinewave' and is important in that it is comprised of just one 'frequency'. Frequency is simply defined as the number of cycles in a given length of time and the basic unit of frequency is the 'Hertz'. One Hertz is equal to one cycle each second. A signal having (say) 300 cycles in a one second period has a frequency of 300 Hertz, or 300Hz in its abbreviated form.
In electronics it is quite normal to encounter quite high frequencies, and in order to avoid the use of excessively high numbers it is normal to use the following prefixes to the word Hertz:
kilo (kHz), equal to 1000 Hertz
Mega (MHz), equal to 1000000 Hertz
Giga (GHz), equal to 1000000000 Hertz
Thus a signal having 200000 cycles per one second period could be said to have a frequency of 200000 Hertz, but would normally be said to have a frequency of 200 kHz.
As stated earlier, a sinewave has just one frequency component, but any other waveshape will actually contain at least two frequencies, and could comprise any number of frequencies. When dealing with other waveforms it is quite normal simply to state the fundamental frequency of the signal and ignore the fact that other frequencies are present, but it is nevertheless important to bear in mind that these other frequencies are present as it can sometimes be of considerable practical importance.
We have all used AC signals as the ordinary UK mains supply is just one of many common examples of AC signals. The amplitude of AC signals can be expressed in several ways and the figure of 240 volts normally quoted for the UK mains supply is 240 volts RMS (Root Mean Square). In simple terms, an AC power, voltage, or current figure given in RMS is an equivalent quantity to the same figure given as a DC quantity. For example, if 18 volts RMS are applied to a resistor, the power dissipated in the resistor and the heat generated within it will be the same as if 18 volts DC were to be applied to the resistor.
AC quantities are sometimes given in terms of peak value, or peak-to-peak value. The peak value is simply what it says: the highest value achieved during one cycle. For a sinewave signal this is more than the RMS value; about 1.414 times larger in fact. The peak-to-peak value is simply the peak positive value added to the peak negative value. For a sinewave signal this is obviously double the peak value and 2.828 times the RMS value.
It is often necessary to change an AC signal to a DC one, and a simple example of this would be if it was necessary to power low voltage DC equipment from the AC mains supply. The first step in converting the high voltage AC input to a low voltage DC output is to reduce the input voltage to the required level. This could be done using a potential divider circuit but there are two drawbacks to this method. Firstly, the potential divider circuit would waste a great deal of power. For example, if we required an output voltage of 24 volts from a 240 volt AC input, there would be 216 volts across the dropper resistor and 24 volts across the output. The current flowing through the dropper resistor would be the same as that flowing at the output, and the power developed across the dropper resistor and across the output would be proportional to the voltages across these. Thus just 10% of the input power would appear at the output with the other 90% being wasted in the dropper resistor as heat. Lower output voltages would give even lower efficiency. The second drawback is that a potential divider circuit would give no isolation from the mains supply, and anyone touching the wiring at the output of the dropper circuit would be in danger of receiving a severe electric shock.
A much better way of obtaining the reduction in voltage is to use a component called a 'transformer'. This consists of two coils in close proximity to one another, and relies on the fact that if an AC signal is applied to a coil of wire (or even just a straight piece of wire) a varying magnetic field is produced around the coil (or piece of wire). If a coil is placed within a varying magnetic field, an AC signal is induced into the coil. Thus, by placing two coils close together and feeding an AC signal into one, an AC output signal can be taken from the other.
What makes a transformer so useful is that by having more turns on the input coil than on the output coil it is possible to obtain a step-down in voltage. By having more turns on the output coil than on the input winding it is possible to have a voltage step-up through the component. The input winding is called the 'primary' and the output winding is called the 'secondary'.
In a theoretically perfect transformer the step-up or step-down ratio of the transformer is equal to the number of primary turns divided by the number of secondary turns. Therefore, in order to step-down the 240 volt mains supply to a level of 24 volts there would need to be ten times as many turns on the primary as on the secondary in order to give the required ten to one input to output ratio. Also in a theoretically perfect transformer there are no losses and if (say) 24 volts at 1 ampere is taken from the secondary, 240 volts at 100 milliamps would be needed at the input to the primary. In other words the input power matches the power taken from the secondary, and both are 24 watts in our example above.
In a practical transformer there are substantial losses despite steps being taken to give high efficiency, such as the windings having a special laminated core. However, even if losses of around 40% occur through a transformer, this is likely to be far less than would be obtained using a potential divider circuit. The reduction in output voltage produced by the losses is simply overcome by increasing the number of secondary turns in order to restore the secondary voltage to the required figure.
As there is no direct connection between the primary and secondary windings of a transformer, when used in a mains power supply the transformer used to give the voltage step-down also isolates the output from the potentially dangerous mains supply.
Having obtained a low voltage AC signal it is now necessary to convert this to a DC signal. The most simple wave of achieving this is shown in Figure 2(a), and this simply consists of passing the output signal through a rectifier. When the transformer applies a positive-going signal to the diode it conducts and the signal passes through to the output. When the transformer supplies a negative going signal to the diode it does not conduct and blocks the signal from the transformer. This gives an output waveform of the type shown in Figure 1(b). Obviously this does not give a constant output voltage suitable for powering equipment such as musical effects units and transistor radios, and there is actually no output at all for about 50% of the time! As we shall see shortly, it is not difficult to 'smooth' out these fluctuations in the output voltage.
This type of rectification is known as half-wave rectification because only half of the input waveform appears at the output. It has two main drawbacks which result in it being little used in practical circuits. The main one is that since half of each output half-cycle from the mains transformer is unused, rather inefficient use of the transformer results. The second drawback is simply that the fairly long time between signal peaks makes the signal comparatively difficult to smooth to a reasonably steady DC output.
Much better efficiency and easier smoothing can be achieved using a full-wave rectifier circuit. Circuits of this type give an output waveform of the type shown in Figure 1(c), and here the part of the input waveform that was simply removed in the halfwave circuit has been inverted so that it is of the required polarity and partially fills the gaps between signal peaks.
There are two common types of full-wave rectifiers and Figure 2(b) shows one of these. This is the push-pull type which requires just two rectifiers, and needs a mains transformer having a centre tapped secondary winding. When the upper secondary connection is positive relative to the centre tap, the lower connection will be negative relative to the tapping point. On opposite half cycles when the upper secondary connection becomes negative, the lower one becomes positive in relation to the centre tapping.
When the upper connection is positive-going, D1 conducts and supplies a signal at the output. D2, on the other hand, blocks the negative-going signal from the lower secondary connection of the transformer. On the opposite set of half cycles D2 conducts and supplies a positive signal to the output, while D1 blocks the negative signal that it receives. Thus D1 and D2 alternately supply an output half cycle and give full-wave rectification.
The other type of full-wave rectifier needs only a single non-tapped secondary winding, but uses four rectifiers. The latter are connected in a bridge circuit and this will not be considered in detail here as this type of circuit has been covered in a previous part of this series (Starting Point Part 4, E&MM June 1981).
Smoothing of the raw output of a rectifier circuit can be achieved simply by adding a capacitor across the output, as shown in Figure 3(a). The capacitor charges up during signal peaks when one or other of the diodes becomes forward biased. At other parts of the output waveform of the rectifier circuit, the voltage across the capacitor is greater than the output voltage of the transformer and the diodes are reverse biased. The capacitor therefore has to discharge into the load connected across the output of the supply during the gaps between signal peaks, to maintain a reasonably steady output voltage.
In order to do this the capacitor needs to have quite a high value unless only a very low output current is drawn. In practice this means that an electrolytic component must be used. However large the smoothing capacitor is made in value, if an output current is drawn from the unit there will be some drop in output voltage between output peaks from the rectifier circuit. This gives an output waveform of the type shown in Figure 3(b) (the broken line represents the output waveform of the rectifier circuit with no smoothing used).
If a fairly high output current is needed together with a very well smoothed supply, the value of the smoothing capacitor becomes impractically high. There are ways of overcoming this and the most common these days is to use a voltage regulator to give additional smoothing, with a smoothing capacitor of only fairly modest value being used. There can be quite a high level of 'ripple' on the input to the regulator, but provided the input voltage does not fall to a level which is too low for the regulator to maintain its output voltage, a virtually ripple-free output will be obtained. The only problem with this system is that the transformer must give a slightly higher output voltage than would otherwise be required, in order to compensate for the voltage drop through the voltage regulator circuit. However, this is only a minor drawback and using a regulator to provide electronic smoothing is preferable to using a very high value smoothing capacitor.
Note that the smoothing capacitor charges to the peak output potential of the transformer, less any voltage drop through the regulator circuit. It does not charge to the RMS output voltage, which is the figure normally quoted in transformer specifications. It is also worth noting that in any of the rectifier circuits shown here the polarity of the output signal can be altered simply by reversing the polarity of the rectifier or rectifiers employed in the circuit.
We covered simple voltage regulators using discrete components in the previous part of Starting Point. Modern circuits often use integrated circuit regulators which enable highly efficient stabilisation to be achieved using very few components. The simplest type of IC regulator is the three terminal type which is primarily intended for use in fixed output voltage supplies, and is available with various output voltages.
This regulator is used in the manner shown in Figure 4(a), and as can be seen, requires only two discrete components. These are both decoupling capacitors which are needed to ensure that the regulator IC does not become unstable and in practice these should be connected physically close to the regulator device.
One problem with three terminal regulators is simply that there may not be a type available which has the output voltage you require. In such instances one solution to the problem is to use a four terminal IC regulator as this enables the output voltage to be set at virtually any desired level.
Figure 4(b) shows the way in which a four terminal regulator is used and this is very similar to using a three terminal device. The only difference is that the four terminal type has its additional leadout connected to the output of the circuit via a potential divider (R1 and R2). By a feedback action, the regulator is actually stabilising the additional terminal (the control or cont. terminal) at a certain potential, rather than stabilising the output at one particular voltage. The control input is often stabilised at 5 volts, although some devices use a different voltage. If we assume a figure of 5 volts is used and the control terminal is connected direct to the output, obviously the output will be stabilised at 5 volts. If the potential divider is introduced into the circuit there is obviously a voltage drop from the output to the control terminal, and the output will stabilise at somewhat more than 5 volts in order to maintain 5 volts at the control terminal. If we take a simple example, making the two arms of the potential divider the same value would give half the output voltage at the control terminal, and the output would stabilise at 10 volts in order to give 5 volts at the control input. The output voltage is equal to R1 plus R2, divided by R2, and multiplied by 5 (or whatever voltage is needed at the control terminal).
Incidentally, both three and four terminal regulators incorporate current limiting circuitry which protects them against damage due to short circuits across the output.
This month's project is a battery eliminator which gives a well smoothed and regulated output of nominally 9 volts and can supply currents of up to 500mA. It is primarily intended to be used with musical effects units and it has four output sockets so that up to four units can be powered from the eliminator. How- ever, it can be used to power any 9 volt equipment that does not consume more than 500mA, and can power any number of units provided the total current drain does not exceed 500mA.
The circuit diagram of the Battery Eliminator is shown in Figure 5. T1 is the isolation and step-down transformer and the mains supply is coupled to its primary winding via on/off switch S1. LP1 is a neon on/off indicator and is a type having a built-in series resistor for 240 volt mains use (do not use a type which does not have this resistor). There are two identical 6 volt secondary windings on T1, which are wired in series to effectively produce a single 12 volt winding. Note that the 0 volt terminal of one winding must connect to the 6 volt terminal of the other (not 0 volt to 0 volt or 6 volt to 6 volt), otherwise the output from one secondary will cancel out the signal from the other winding and there will be no output whatever!
The output of T1 is coupled to a bridge rectifier by way of fuse FS1. As the circuit has output current limiting, FS1 is included as a protection against faults in the circuit rather than output overloads. C1 is the smoothing capacitor.
The regulator circuitry uses exactly the same configuration as the one shown in Figure 4(b) and described earlier. The specified regulator is a type which stabilises with 5 volts at the control terminal, and R1 and R2 therefore give a nominal output potential of fractionally over 9 volts (which is comparable to a slightly used 9 volt battery). R1 and R2 are precision (1%) components so that the output voltage is set with good accuracy. The output sockets are simply wired in parallel across the output of the unit, and as few or as many sockets as desired can be used here.
An instrument case having approximate outside dimensions of 114 x 152 x 44mm. makes a good housing for the Battery Eliminator, but it is not essential to use a case of this type. For reasons of safety though, the case should be a metal type and earthed to the mains earth lead. It should also be a type having a screw-on lid, rather than one having a lid which simply clips in place.
Reference to the photographs shows the general layout of the unit. A solder tag is fitted on one of the mounting bolts of T1 in order to provide a chassis connection point for the mains earth lead.
Figure 6 shows the component layout of the Veroboard panel and the wiring of the unit. The board has 24 holes by 25 holes and there are no breaks in the copper strips. IC1 is fitted with a small finned heatsink which is simply bolted on to the heat-tab of the component. FS1 is fitted in a chassis mounting fuseholder which in actual fact is bolted on to the component panel. When the board and all the point-to-point wiring has been completed, mount the board on the base panel of the case using ½in. spacers to keep the underside of the board well clear of the metal case.
Thoroughly check all the wiring, especially the mains wiring, and fit the lid of the case in position before connecting the unit to the mains and testing it. If possible, check that the output voltage is correct before connecting the unitto any equipment and be careful to connect the output of the unit with the correct polarity (the tip of the plug is positive and the barrel is negative).
Feature by Robert Penfold
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