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DC Power Supply Basic Unregulated Circuits Essentials Component Values

OddMix.com - Technical Note - TN091208 - by Karl Nagy

Fig. 1. Block Diagram of an Unregulated and Regulated DC Power Supply [2 KB]
Fig 1. Block Diagram of an Unregulated and Regulated DC Power Supply

Power supplies are required whenever an electrical circuit is used. For some of the circuits that require very small power or for portability, batteries may be quite sufficient. For all other circuits, some form of DC (Direct Current) power supply may be necessary. The DC in the name of these power supplies refers to the type of output they provide. DC power supplies draw their operating current from the readily available household power that is supplied in the form of AC (Alternating Current).

In one of its simplest form DC power supplies are made up of three main parts Figure 1. The power transformer is the component that is connected to the incoming AC line. Its purpose as the name suggests is to transform the usually high voltage to the necessary lower (or higher) value. The next important element is the rectifier. The original rectifier diode is the discovery of Thomas A. Edison who started the vacuum tube era. In today's circuits rectifiers are solid state devices that are made of silicon, germanium, gallium arsenide or other more exotic materials.

Fig. 2. Half-Wave Rectifier Unregulated Power Supply Circuit [2 KB]
Fig 2. Half-Wave Rectifier Unregulated Power Supply Circuit

Fig. 3. Half-Wave Rectifier Unregulated Power Supply Circuit Waveforms [3 KB]
Fig 3. Half-Wave Rectifier Unregulated Power Supply Circuit Waveforms

The purpose of the diode rectifier is that it allows current to flow in one direction only. That way the diode rectifies the alternating current cycles. Usually in any voltage regulator circuit the last stage is filtering. The filter circuit takes up energy during the peak section of the active current cycle and releasing it slowly to provide energy from its reserves as needed. Its action fills out the spaces between the half waves and to help to smooth out the output voltage to provide an unchanging voltage much like a battery. When more exact DC voltage is desired a regulator section and additional filtering stages can also be added.

The way the diodes are connected within the power supply to use one or both half waves of the available AC, their resultant connection is either a half or full wave rectifier. Half wave rectifiers - Figure 2 - produce pulses made of top or bottom half waves of the power line depending on the diode's orientation in the circuit. When line power is available, these recurrent pulse trains repeat endlessly at the frequency of the power line at 60 times per second. When a full wave rectifier circuit is used, - Figure 4. - the half waves appearing twice as often and their combined frequency is 120 cycles/second or Hz - Figure 5. Filtering is easier with higher frequencies and it is advantages to select full wave circuit whenever possible.

Fig. 2. Full-Wave Rectifier Unregulated Power Supply Circuit [2 KB]
Fig 4. Full-Wave Rectifier Unregulated Power Supply Circuit

Fig. 3. Full-Wave Rectifier Unregulated Power Supply Circuit Waveforms [5 KB]
Fig 3. Full-Wave Rectifier Unregulated Power Supply Circuit Waveforms

The first part of the block diagram is usually the transformer or voltage converter stage. In older and simpler power supply it is a transformer. The rectifier stage is required for all DC power supply circuits. A filter circuit with more or less capacitors is designed according to the load requirements. For more exact load requirement a regulator circuit may be used with additional filtering. The regulator circuit can provide regulation against line or load current and/or voltage variations. Short circuit and current limiting is also accomplished in that stage. Laboratory type precision power supplies contain all five stages as a minimum. For really serious tinkering and circuit development activity, several good voltage regulators and a good circuit simulator are a great help.

On Figure 3 the top trace shows the power supply circuit of Figure 2 where TR1 is a 12 VAC transformer, D1 is a 1N4002 diode. The power supply output is loaded with a 12 Ohm resistor resulting in a 1 Ampere load current and C1 = 1 uF (micro-Farad) filter capacitor. On the second trace shows the same load and output current with C1 = 5,000 uF filter capacitor. And the third trace shows the same load and output current with C1 = 10,000 uF filter capacitor used.

On Figure 5 the top trace shows the power supply circuit of Figure 4 where TR1 is a 12 VAC transformer, D1 and D2 are a 1N4002 diodes. The power supply output is loaded with a 12 Ohm resistor resulting in a 1 Ampere load current and C1 = 100 uF (micro-Farad) filter capacitor. On the second trace shows the same load and output current with C1 = 1,000 uF filter capacitor. And the third trace shows the same load and output current with C1 = 5,000 uF filter capacitor used. Both on Figure 3 and Figure 5 the horizontal scale is 5 mS/cm and the vertical scale is 5 Volts/cm.


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