Supplying power for electronic circuits

Electronic circuits require dc voltage and current to operate. Some circuits use a "single-sided" power supply, meaning that the reference voltage is 0 V and the operating voltage is some positive value, such as 5, 12 or 15 Vdc. The value of the voltage depends on the type of circuit or equipment to be powered.

08/01/2005


Electronic circuits require dc voltage and current to operate. Some circuits use a "single-sided" power supply, meaning that the reference voltage is 0 V and the operating voltage is some positive value, such as 5, 12 or 15 Vdc. The value of the voltage depends on the type of circuit or equipment to be powered.

Other circuits require a "bipolar" supply, meaning that voltages are both positive and negative with respect to the reference voltage of 0 V (or circuit ground). Audio amplifiers, instrumentation amplifiers and many types of signal conditioning use bipolar supplies such as

Power supplies can be a linear or switching mode. A linear power supply is the easiest type to design, build, troubleshoot and understand. However, a switching mode power supply is much more efficient. For the purpose of this article, we will discuss low-voltage linear supplies only.

A linear power supply has a transformer that steps down (or steps up) the line voltage. A rectifier and a filter capacitor transform low voltage ac into moderately filtered dc, which still has some ripple. Finally, a regulating circuit removes the remaining fluctuations and the excess voltage, providing the correct amount of dc voltage at the output.

Stepping down the voltage

Transformers convert ac from supply voltage level (typically 120 Vac) to either a higher or lower voltage level. Many types of electronic equipment use dc voltages that are much lower than line voltage, which requires a step-down transformer. However, some electronic circuits, such as parts of computer or workstation monitors, require higher voltages, which require step-up transformers.

A transformer requires ac to operate. It is constructed by wrapping two coils of wire around a single piece of iron. When an alternating current passes through one of the coils (transformer primary), the iron becomes magnetized. The magnitude and polarity of this magnetism varies according to the applied current and voltage. While the current through the primary of the transformer creates the magnetism, the secondary of the transformer responds by generating electricity from the constantly expanding and collapsing magnetic field.

In addition to the ability to step voltages up or down, transformers offer electrical isolation. Even when load requirements do not necessitate changing the voltage level, transformers are frequently used because the primary and secondary are connected magnetically, but they are not connected electrically.

Making dc from ac

Rectification is the conversion of ac to dc. A diode is a device that allows current to flow in one direction only (Fig. 1). Diode behavior is similar to the behavior of a hydraulic check valve, which allows fluid to flow through it in only one direction.

Applying an ac sine wave to a diode enables current to flow when the polarity of the diode matches the corresponding half of the ac waveform and blocks current from flowing during the other half of the cycle. A half-wave rectifier demonstrates the concept of rectification (Fig 2).

The advantages of a half-wave rectifier are its simplicity and its economics. This circuit uses only one diode to convert ac to pulsating dc. Therefore, this type of circuit requires fewer parts. But diodes are relatively inexpensive, so saving money on parts is typically not an issue.

The half-wave rectifier has many disadvantages. The obvious drawbacks are difficulty of filtration and inefficiency. Perhaps the only time it would be feasible to consider using the half-wave rectifier is for very low dc power levels of about

A rectifier becomes more efficient by adding another diode to conduct during the other half of the ac waveform, making it a full-wave rectifier. The full-wave version is more efficient and more practical because it conducts during both halves of the ac sine wave.

There are two types of full-wave rectifiers: full-wave center-tapped and full-wave bridge. A full-wave center tapped rectifier circuit requires that the step-down power transformer have a center tap (Fig. 3). However, the full-wave bridge rectifier circuit provides full-wave rectification without the necessity of a center-tapped transformer. The input to the circuit is applied to the diagonally opposite corners of the network, and the output is taken from the remaining two corners (Fig. 4). The bridge rectifier is a very practical method of full-wave rectification in applications where a center-tapped transformer is unavailable or not feasible. The bridge rectifier has the best transformer utilization but requires the use of four diodes.

Smoothing out the ripples

The rectifier converts ac voltage to dc. The voltage still fluctuates, although the direction of the current is no longer changing.

Typically, a power supply filter circuit uses capacitors to improve the purity of the dc voltage after it is rectified. Capacitors store electricity. The amount of electricity they can store, and for how long, depends on the size of the capacitor. Assuming that an adequate capacitor value was selected during design, the capacitor will continue to supply power when the incoming voltage level drops below its maximum.

The purity of the resulting dc voltage depends on several factors. The most important consideration is the amount of current the powered equipment requires. If the capacitor must provide only a small amount of current, its charge depletion between cycles is minimal. Increasing the value of the capacitor allows the power supply to provide more current between rectifier pulses.

Regardless of the size of the capacitor, some amount of fluctuation still occurs. For circuits that require a very stable voltage, such as computers, process control instruments and automated controls, the only real solution is to employ voltage regulation.

Regulation

Generally, a regulated power supply uses a sensing circuit to constantly monitor its own output. When the powered circuit or equipment demands more current, the sensor sends a signal to the voltage regulator, which adjusts its output accordingly. A voltage regulator is a fairly simple circuit that operates in a closed-loop mode to regulate the power supply output.

An active filter/regulator circuit can be complex, depending on the purity requirements of the dc voltage. Many modern power supplies use regulator ICs and sophisticated circuit designs. However, a simplified explanation involves using a resistor to sense power demand. A typical sensing circuit uses a high wattage resistor with a very low, but very precise resistance in series with the power supply output as a sensing device. Typical resistance is 0.1 Ohm, and the wattage depends on the size of the power supply. The resistor's precision is important because the current flowing through it determines the voltage drop across it. As this voltage drop varies, the voltage difference is applied to an amplifier circuit, the output of which is inversely proportional to its input (inverting amplifier).

Regulation counteracts any deviation from a preset voltage and current level (Fig. 5). Even though the ripple from a filter circuit alone may be small, many circuits require almost none, and the active filter/regulator circuit is extremely effective in minimizing it.





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