Technical -> DC Rectification

	DC Rectification

Transformers by their nature can only operate with the application of a continuously varying input voltage, one that swings from positive to negative and back again, the AC supply.

The requirement for some electrical applications of a DC, or positive only, supply requires a method of converting the AC output of the transformer into a DC waveform. This conversion process is known as “Rectification”. A diode is a semiconductor device that will only permit a current to flow in one direction, in simple terms it is a device with a zero resistance when a positive voltage is applied across it and an infinite resistance when a negative voltage is applied across it.

Single Phase Systems

Taking an AC voltage waveform from the output of a transformer to the load via a diode will result in the following DC waveform

This is a very basic form of rectification known as “Half Wave Rectification”

By using 4 diodes in the configuration shown it is possible to achieve the DC waveform shown in Fig . This is known as “Full Wave Rectification” and the diode configuration is referred to as a “Bridge Rectifier”. This is the most common rectifier format.

An alternative method of achieving full wave rectification involves a 2 diode system with a center tap on the transformer as shown below

This is known as a “Bi-phase Rectifier”.

Each of these systems is providing a DC output for a load but it is not a steady voltage DC. Quite often it will be adequate to supply the load e.g. the coil in a contactor or a DC motor but some loads will require a steady DC voltage, similar to that of a battery. This can be achieved by the use of “Smoothing” components, Capacitors and Inductors. The varying DC supply circuits shown above are therefore referred to as “Un-smoothed”.

Capacitor Smoothing

The fig shows the effects of placing a capacitor onto the output of the un-smoothed circuits the effect now is to produce a reasonably stable DC voltage level with a small ripple voltage the size of which is determined by the load current and the size of the capacitor.

Further levels of smoothing can be achieved by using inductors and capacitors to produce an output filter. The use of this type of filter is generally governed by the size and cost of capacitors required to achieve the level of smoothing required.

When the DC is smoothed using a capacitor the current waveform changes quite significantly. Given a linear load the current waveform on an unsmoothed DC supply will follow the voltage waveform as indicated by the blue curve in Fig X however the introduction of the smoothing capacitor will alter the current waveform to that of the pink curve. This will require the transformer to be designed to compensate for the larger voltage drop associated with its regulation to maintain a specific DC voltage level

For safety reasons Carroll & Meynell DC supplies using Capacitors in the design are fitted with “Discharge Resistors” to drain the capacitor of its charge when the unit is switched off without a load connected

3 Phase Systems

The three phase rectifier operating on the output of a transformer as shown in fig will automatically, as a direct result of the overlapping of the various phases, produce a smoothed DC output with a 4.5% ripple.

This type of 3 phase rectification is known as “6 Pulse Rectification” for obvious reasons
The advantage of three phase rectification over single phase is the ease of achieving a reasonable level of smoothing for larger power loads without the requirement for expensive smoothing components. At low power the single phase systems often have the advantage of cost.

Other methods of 3 phase rectification involve the use of separate Star and Delta outputs to produce “12 Pulse Rectification”, each secondary is rectified with its own 3 phase bridge and the rectifier outputs may be connected either in parallel or in series. This will reduce the ripple down to 2.5%, this system also reduces the effects of current harmonics on the supply. Generally for low power applications this will not be required.

12 Pulse Parallel Connected	12 Pulse Series Connected

If required we can also design for larger number pulse systems (18 and 24) using interconnected star configurations

Variable DC supplies

Some applications require the DC voltage to be variable e.g. the speed control for a DC motor. This can be achieved by the addition of a variac to the circuits described above. The variac will adjust the AC input to the transformer and the DC output voltage will follow the AC variation.

Regulated DC supplies

Do not confuse regulated DC supplies with Smoothed DC supplies.
A smoothed DC supply will offer a DC voltage level with a ripple voltage where the average DC voltage level will rise and fall with the normal fluctuations in the AC supply voltage.
A regulated DC voltage will offer a stable DC output voltage with extremely low ripple that does not vary as the input supply fluctuates. Regulated DC supplies require much more complex circuits than those described above

Rectifier Design

Diode devices are rated for a specific current at 25°C however during operation they generate heat increasing their temperature. As the temperature of the device increases the maximum diode current has to be de-rated. Heatsink components are usually required to limit the temperature the diodes can reach in operation by dissipating the most of heat they generate.

Carroll & Meynell adopt a conservative policy when designing our own rectifier units for DC load currents rated up to 30A to ensure the reliability of the units.
Above this rating we use rectifier units built to order by a reputable manufacturer with specialist expertise in this area.