Bredhurst Receiving and Transmitting Society

Syllabus Sections:-

Mains Power Supplies

3p.1 Recall the circuit diagrams and characteristics of different types of rectifier and smoothing circuits (i.e. half wave, full wave and bridge).

When considering a "basic" power supply there are several sections into which it breaks down.

• Transformer

• Rectification and

• Smoothing

The transformer must be able to

• supply the correct AC output voltage to allow for rectification and smoothing

• be able to supply sufficient current to the output

The rectification must be able to

• change the AC into DC

• have sufficient current carrying capability to give the output required

The smoothing capacitor must be able to

Supply as steady voltage with little or no ripple (variation in the output voltage) when a load is applied. If the capacitor is too small there will be ripple on the output.

Note: from a bridge rectifier there are 100 pulses per second charging the capacitor because it has pulses of DC from both sides of the AC wave form.

As for this part of the course as you are not actually having to build a power supply some of the details will be left out.

Unless you are powering your equipment from a battery then you will need a method to convert the AC supply from the mains at 230V (approx) to a suitable stabilised DC supply.

The following will explain the various parts of a basic power supply but in no way must this be used to construct a unit as it is only in a very basic form.

There are dangers of death when using mains supply.

TRANSFORMER

The first part that a typical 13.8V power supply needs is a transformer. This is used to convert the mains AC voltage of about 230V on the primary coils to about 18V AC on the secondary coil.

TRANSFORMERS

RECTIFICATION

BRIDGE RECTIFICATION

Then we need a means to convert the AC output from the secondary coil to DC. For this we use what is called a BRIDGE RECTIFIER. This is simply 4 diodes set in a certain formation so that from two input connections at AC there are then two outputs at DC.

A diode only conducts in one direction, and conventionally this is a flow of electrons in the direction of the Arrow head on the circuit diagram. By the clever use of 4 diodes we can arrange them so there is a positive terminal and a negative terminal.

The animation shows that no matter which way the AC arrives there is always a flow from positive to negative on the DC side.

watch the RED dot first and follow its track from the AC input to the DC output then watch the Blue dot as it returned from the circuit to the AC.

You should recall the sine wave diagram which represents the AC flow in the primary.

At certain times the voltage drops to zero (crosses the centre line) and thus the power supply needs some way to maintain an output when there is no AC input.

The 18V AC RMS actually has a peak voltage of about 25V. After rectification this gives a peak voltage of 25V DC but it is anything but smooth meaning if we looked at the DC voltage on an oscilloscope it would still have a waveform.

To try and maintain in the first instance a constant 25V DC output we needs the component to fill in the gaps in the wave form or to smooth the output voltage.

Smoothing

Such a component is called a CAPACITOR which can be charged up to the max voltage (25V) and then when the voltage in the DC waveform is dropping off, the capacitor then "fills" in the gap by discharging, and then is charged up again as the voltage rises above the level of discharge. If there is no load on the output the capacitor will stay charged between peaks of the DC waveform but if there is a load on the output it will drop down between peaks depending upon the size of the value of the capacitor. This slight dropping back is called the "Ripple Effect". The bigger the value of the capacitor the smaller the ripple.

Also the size of the capacitor will be related to whether the rectification is half wave, full wave or bridge. The capacitor has less work to do in the bridge and full wave as will be seen from the diagram below as there are more peaks of the waves form to charge up the capacitor.

So long as the output voltage does not drop below the voltage required when on load then this simple power supply is starting to take shape BUT is it not good enough yet as the output voltage is :-

• with low load much too high and with high loads only a little above what is required, and with considerable ripple.

Such a power supply would not be suitable for a typical transceiver.

To rectify UK mains AC 50Hz (or change it to DC) we use diodes. Diodes can be simply described as one way valves, they pass current in one direction (forward biased) but not in the other direction (reversed biased). So carefully connected diodes can provide a rough DC output.

Various forms of rectification

Half wave rectification

Using one diode as a rectifier will pass current only when the sine wave is in the positive or forward half of the cycle, provided that is how it is connected. With the result that the current output, although only flowing in one diode is dc but not smooth dc, which makes it unusable for most electronic circuits. The positive current pulses can be smoothed out by the insertion of a capacitor which we can call the RESERVOIR CAPACITOR. As it name suggests, the capacitor stores some of the charge whilst the diode is conducting, then releases the charge to provide a current flow when the diode is not conducting. The diode needs a voltage to make it conduct, typically 0.6 volt for silicon diodes and 0.2 volt for germanium diodes, so there is always a voltage drop across a diode. The frequency of the ripple is 50hz or mains ripple.

Half wave rectification will give a DC output, but although the voltage off load would be the same, there will be considerable ripple and poor regulation compared to a full wave circuit. If we use 2 diodes with a centre tapped secondary winding on the transformer we can have full wave rectification.

Full wave rectification (using 2 diodes)

If we connect the centre tap of the transformer to the common of the circuit the centre tap will be at zero volts because the voltages at each end of the secondary winding will be equal and opposite, ie when one end is positive the other will be negative, so the centre will always be zero volts.

Note: No smoothing capacitor shown

By connecting a diode to each end of the transformer secondary winding, when either end becomes positive then the respective diode will conduct. By commoning the ends together, both halves of the wave in turn are connected to the load. The frequency of the ripple current will be twice or 100hz.

Bridge rectification (also full wave but uses 4 diodes)

Finally the most widely used form of rectification is the full wave bridge rectifier. No centre tapped transformer is needed because the arrangement of the diodes is such that the end of the winding which at any one moment is positive is always connected through the diodes to the same end of the load.

Note: No smoothing capacitor shown

Value of the smoothing capacitor

When a capacitor is used across the output terminals + and - of the Half wave rectification there are more missing parts to the wave form than for the Full wave rectification . Thus there is more work to be done by the smoothing capacitor in Half wave rectification and a larger value capacitor is required.

Calculating the actual value of the capacitor needed is not part of the course but it must be of such a size the the wave form ON LOAD is reduced as far as possible at all times to a single straight line of DC voltage. The amount of the deviation from this ideal straight line is called "ripple" and it must be kept to a minimum.

3p.2 Understand the need for rectifier diodes to have a sufficient peak inverse voltage (PIV) rating and be able to calculate the PIV in diode/capacitor circuits.

PEAK INVERSE VOLTAGE

• PEAK INVERSE VOLTAGE (PIV) IS THE MAXIMUM REVERSE VOLTAGE THAT A DIODE CAN STAND AND REVERENCE TO THIS MUST BE STRICTLY ADHERED TO,

• REMEMBER the PIV on half wave rectification and full wave using two diode ( called bi phase), is twice the peak voltage whereas full wave bridge rectification the PIV is the peak voltage. All diodes have to be sized for PIV and FORWARD CURRENT capability. A safety margin must also be added, usually two times. With Diodes so cheap go for as large as you like!!

• Also bridge rectifiers are cheap there are advantages in using a single, non-tapped secondary, as the regulation of the transformer will be better. Remember that a bridge circuit will need an 18V winding, whereas the bi-phase (centre tap, two diodes) uses a 36V winding, centre taped, and this has to use thinner wire for a given transformer size. The wire is in fact overloaded on the half cycle it conducts, but gets a rest on the other half cycle - hence it copes.

• Similarly the ripple current is twice the load current for half wave rectification, 1.414 times for full wave (2 diodes) and the same as the load current for full wave bridge rectification. Diodes often have capacitors fitted across them to bypass any high voltage spikes that may damage them. Similarly where diodes are connected in series to increase their overall PIV rating, then balancing resistors are used to even out the reverse voltage across them.

• At zero current you will get a little more than 18v AC but at full load it will be less. It would be obviously desirable to have 18V AC whatever the load but that does not happen. Transformers with good regulation have a little over 18V off load and a little under at full load.

• So you can see some of the advantages of using one method of rectification and disadvantages of using others.

RMS

3p.3 Understand the function of stabilising circuits and identify different types of stabilising circuits (i.e. Zener diode/pass transistor and IC)

Note: questions on the characteristics of individual components are covered earlier in this syllabus, e.g. 3n.2. This sub-section is on complete circuits.

The Regulation must be able :-

• for amateur radio use supply a fixed voltage output usually 13.8V

• output must be able to detect some failure whether it is:-

  • over voltage

  • over current

Stabilising circuits / REGULATION

The other part that is needed is some form of REGULATOR to keep the output voltage at say 13.8V all the time without any regard as to the level of current being drawn.

In its simplest form, for low current use, a regulator could be as simple as a Zener diode and a resistor in series across the DC supply. The value of the zener diode determines what the output voltage is.

The next stage would be to add a pass transistor as shown below. Here again the zener determines the output voltage.

The next stage in complexity would have a regulator which consists of a few more components as shown in the diagram below as it develops. In this case it is the variable resistor as part of the potential divider chain of resistors that determines the output and thus can be "set" to the output required.

The transistor at the top is called a PASS TRANSISTOR as it is passing all the current going to the output terminal. The lower transistor is part of the regulator circuit. Please note a ZENER diode has been used at a voltage of 5.6V. Such a rated diode perform very well and can control the voltage close to 5.6V without change due to heating.

With the power turned on the preceding part of the circuit will supply 25V to the collector of the pass transistor. R1 will then supply a small current to the base of the pass transistor and current will then flow through the transistor to the output terminal where the diagram is marked 13.8V.

The 13.8V is controlled by the potential divider chain of R2 and VR1 which controls the current flowing into the base of the second transistor. If the 13.8 goes higher then more current flows into the base turning TR2 on harder. This results in there being less current available to the base of the pass transistor, hence the output falls to a point where there is equilibrium in the circuit. The zener is always conducting, providing a reference voltage against which the output voltage is compared. If the 13.8V goes lower then less current flows into the base and TR2 begins to turn off. This results in there being more current available to the base of the pass transistor, turning it on more, and the output voltage rises to the point where again there is equilibrium in the circuit.

This action continues and thus regulates the output voltage and stabilizes it to 13.8V +/- a very little.

VR1 is used to set the output level at 13.8V initially.

The use of an IC

It is possible to carry out many of the above functions with an IC such as LM723 which is a voltage controlling IC. Discussion in the course is not required other than to be aware that IC's can control voltage circuits of a power a supply when used in conjunction with pass transistors and other components.

REGULATION