Power Supplies

Question File Number 21



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The Typical Power Supply

The purpose of a power supply is to take electrical energy in one form and convert it into another. The usual example is to take supply from 230V AC mains and convert it into smooth DC.

Power Supply

This DC may be at 200 volt to provide (say) 200 mA as the high tension source for valve operation, or 5 volt at (say) 1 Amp to feed transistors and other solid-state devices.

The above diagram shows the separate stages in this conversion. Each will be considered in turn.

Protection

There should always be a fuse in the phase or active AC mains lead for protection if a fault develops in the equipment. The fuse should be of the correct rating for the task.

See Safety Keep some spare fuses handy.

The Transformer

When two inductors (or more) are mounted together so their electromagnetic fields interact, we have a transformer. A power supply almost invariably, contains a transformer.

Transformer

A transformer generally comprises two (or more) sets of coils (or windings) on a single core, designed so that maximum interaction and magnetic coupling takes place. The windings are insulated from each other and insulated from the core. The windings may be wound on top of each other.

At low frequencies the core may be made up from thin laminated soft-iron plates forming closed loops and designed to reduce eddy current losses. At higher frequencies the core may be dust-iron, ceramic ferrite, or air-cored (as for RF coils).

The winding used to generate the magnetic flux is called the primary (connected to the AC supply). The winding in which current is induced is the secondary (or secondaries).

The input supply must be an alternating current. The input current sets up a changing magnetic field around the input or primary winding. That field sweeps the secondary and induces a current in that secondary winding. See AC

The Turns Ratio

The number of turns on each winding determines the output voltage from the transformer. The output voltage from the secondary is proportional to the ratio of the turns on the windings.

For example, if the secondary has half as many turns as there are on the primary, and 100V AC is applied to the primary, the output will be 50V.

Transformers can be step-up or step-down (in voltage). With twice as many turns on the secondary as there are on the primary and 100 V applied, the output would be 200V.

A function of the transformer is to provide an AC supply at a voltage suitable for rectifying to produce a stated DC output.

The power output from the secondary cannot exceed the power fed into the primary. Ignoring losses, a step-down in voltage means that an increase in current from that lower-voltage winding is possible. Similarly, a step-up in voltage means a decrease in the current output. So the gauge of wire used for the secondary winding may be different to the wire used for the primary. (The term gauge of wire refers to its cross-sectional area.)

There will be some energy losses in a transformer, usually appearing as heat.

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Rectifiers

There are three basic rectifier configurations, half-wave, full-wave and bridge. We will look at each in turn. We will use semiconductor rectifiers only.

The Half-wave Rectifier

Here is a very basic power supply, a transformer feeding a resistor as its load with a rectifier inserted in the circuit.

Halfwave

Without the rectifier, the load would have the full secondary alternating voltage appearing across it.

The rectifier will conduct each time its anode is positive with respect to its cathode. See Devices

So when the end of the secondary winding shown + is positive, the diode acts as a short-circuit and the + appears across the load. Current flows around the secondary circuit for the time that the diode is conducting. The voltage drop across the diode can be regarded as negligible - about 0.6 volt for a silicon device.

The waveform appearing across the load is shown in red on the graph. One-half cycle of the AC from the transformer is conducted by the rectifier, one half cycle is stopped. This is pulsating DC - half-wave rectified AC. Later we will put this through a filter to smooth it.

The Full-wave Rectifier

This is two half-wave rectifiers combined - it uses a center-tapped secondary winding and one additional diode.

Fullwave

Each side of the centre-tap has the same number of turns as our previous example - and each works for half the cycle as our half-wave rectifier did.

The top half of the secondary works with one diode like the half-wave circuit we have just considered.

When the polarity of the secondary changes, the upper diode shuts off and the lower diode conducts.

The result is that the lower diode fills in another half-cycle in the waveform when the upper diode is not conducting.

The Bridge Rectifier

This uses one single winding as the secondary and four diodes - two are conducting at any one time.

Bridge

Note the configuration of the diodes:

Diodes on parallel sides point in the same directions.

The AC signal is fed to the points where a cathode and anode join.

The positive output is taken from the junction of two cathodes.

The other end of the load goes to the junction of two anodes.

The operation is simple: Parallel-side diodes conduct at the same time. Note that the two + points are connected by a diode - same as in the two previous cases. The other end of the load returns to the transformer via the other parallel diode. When the polarity changes, the other two diodes conduct.

The output waveform is the same as the full-wave rectifier example shown before.

The main advantage? A simpler transformer - no centre-tap and no extra winding. Diodes can be small and cheap. A bridge rectifier can be purchased as a block with four wire connections.

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Smoothing The Output - The Filter

Each of the three circuits studied above produces an output that is DC, but it is DC with a waveform showing a large ripple. The ripple is the waveform shown in red in the three examples. DC from a power supply should be smooth and non-varying in amplitude.

The half-wave circuit produced a ripple of the same frequency as the input signal, 50 Hz for input from a mains supply.

The other two examples produced a ripple that is twice the frequency of the mains supply - i.e. 100 Hz.

How can we remove the ripple? By using a filter circuit comprising filter capacitors and often a choke.

A capacitor wired across the load will charge up when the diode conducts and will discharge after the diode has stopped conducting. This reduces the size of the ripple. The blue lines in this diagram illustrate this.

Filcap

The choice of capacitor is important. Electrolytic capacitors are generally used because a very large value capacity can be obtained in a small and cheap package.

The capacitor value chosen depends on the purpose for the supply. Capacities of the order of thousands of microfarads are common for low-voltage supplies. For supplies of 100V and upwards, the capacity is more likely to be 50 microfarad or so. It depends on other factors too. The voltage rating of the capacitor and its wiring polarity must be observed (electrolytic capacitors have + and - connections).

When a diode conducts, it must supply current to the load as well as charge up the capacitor. So the peak current passing through the diode can be very high. The diode only conducts when its anode is more positive than its cathode. You can see from the diagram how the addition of the capacitor has shortened this time.

The switch-on current through a power supply diode must also be considered. Charging a large capacitor from complete discharge will mean a high initial current.

Filter

A choke and an additional capacitor are often used to filter the output from a rectifier, as shown in this diagram.

The choke is an iron-cored inductor made for the purpose and it must be able to carry a rated DC current without its core saturating.

Internal Resistance

All power supplies exhibit internal resistance. A torch light will dim as its battery ages. The internal resistance of its battery increases with age. On open circuit, without the bulb connected, i.e. with no load current being drawn, the battery may show its normal voltage reading. When the load is applied and current flows, the internal resistance becomes apparent and the output voltage droops or sags.

The effects of internal resistance can be reduced substantially by using a regulator. This added electronic circuitry winds up the voltage as the output load current increases to keep the output voltage constant. It keeps the voltage constant as the load current widely varies. See Regulated Supplies

Choice of Supply

A power supply (also a battery) must have sufficient reserve energy capacity to provide adequate energy to the device it is working with. For example, pen-light dry cells are not a substitute for a vehicle battery.

Similarly, a power supply for an amateur radio transceiver, (to substitute for a vehicle battery), must be chosen with care to ensure that the maximum load current can be supplied at the correct voltage rating without the voltage sagging when the load is applied.



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Compiled Sun Nov 28 2010 at 8:41:44pm


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