Basics for Beginners – Easy Ways to Power Your Electronics Project
Every electronics project requires power to operate. Though power systems can get quite complex, many projects are well served with a few basic solutions. Designing a power system is easier when high voltages and currents are not required. Power systems are also simplified when the circuits being driven are not operating at high speeds and when components are not very sensitive to electrical noise. This article discusses power system design for a typical electronics project with low to moderate power needs. The techniques presented here can be applied in more demanding applications, though more analysis and care is required. A basic power system can be considered in three sections: raw power source, power regulation, and power distribution. Safety concerns dominate all three areas. This article stresses the use of off-the-shelf building blocks that reduce risk and design effort.
Power Source
The most common forms of raw power available to most of us are the AC wall outlet and batteries. Solar cells run a close second for some and, for the purposes of this discussion, may be considered as batteries. AC-powered devices must somehow convert AC to DC because integrated circuits (ICs) run on DC. (Refer to the sidebar, “AC versus DC Power” for more information.) The conversion process is called rectification. Batteries, on the other hand, natively supply DC power. AC-powered devices must also step-down the high voltage AC to a more manageable DC voltage. In the US, AC power is nominally 110-120 volts. (AC power is nominally 220-240 VAC in Europe.) When was the last time you saw a logic chip run at 100 volts?! The step-down process generates a lower voltage that is not directly usable by ICs, but that is low enough for a voltage regulator to handle. Batteries offer less complexity here as well, because they are already low-voltage sources. Typical batteries have cell voltages from one to two volts, allowing you to combine as many as necessary to generate higher voltages. Working with AC is a safety headache. High voltages, stepdown transformers, and rectifiers all pose problems that are best avoided. However, this doesn’t mean that you should be restricted to batteries. A familiar “wall-wart” AC-to-DC power module is a great way to use AC power with all the high-voltage safety problems taken care of for you. Wall-warts are available in varying output voltages and currents. Always make sure that the module is certified by Underwriters’ Laboratories (UL) for safety. Whether you use a wall-wart or batteries, the power design task is made easier by working with low-voltage DC. It is easiest to select a wall-wart or battery configuration that provides the lowest practical voltage. If your voltage regulator requires 5.5 V, it is better to use a 6 V module rather than a 12 V module. We’ll talk about a regulator’s voltage requirements later.
Batteries
If you’re using batteries, you have to determine how many are required based on the nominal cell voltage of each battery’s chemistry. Alkaline batteries have a 1.5 V nominal cell voltage.Rechargeable nickel cadmium (NiCd) and nickel metal hydride (NiMH) batteries have a 1.2 V nominal cell voltage. Multiple cells are stacked in series to provide higher voltages. If your regulator requires 5.5 V, that’s four alkaline or five NiCd/NiMH cells. This is why some battery-powered devices cannot accept rechargeable batteries: they were designed for the higher alkaline cell voltage.It is also important to be aware that battery voltage varies with the charge left in the battery. This is in contrast to wall-warts that emit a narrow DC voltage range. A fully charged cell is typically several tenths of a volt higher than the nominal voltage. Likewise, the cell voltage droops as its charge is depleted. Therefore, the combined battery voltage of five 1.2 V cells may range from 7.5 down to 5 V before their charge is depleted. If the voltage regulator’s minimum voltage is above the battery’s minimum useful voltage, the system will cease operation before the batteries are fully depleted. In the case of NiCd and NiMH batteries, some people regard a 1.1 V cell voltage as the point where the cells are effectively discharged. Under this assumption, a fivecell NiCd/NiMH battery pack has a useful minimum voltage of 5.5 V.
Fusing
Fuses are a recommended safety feature. A fuse is placed in series between one of the DC source’s two leads (positive or negative) and your voltage regulator. You select a fuse with a particular current rating. The fuse will allow current to flow up to its rating. If the power system tries to draw more current than the fuse allows, the fuse will blow, or trip, and open the circuit. Fuses can prevent fires and other calamities by preventing damaging high-current discharges that can result from short circuits. If your system requires one amp, you may select a two amp fuse to provide adequate headroom so that the fuse doesn’t trip if you go over your expected power consumption for a few seconds. Wall-wart AC/DC modules should already have overload protection, but adding your own fuse is good practice. Batteries are potentially dangerous because they have no inherent current limiting feature. Low internal resistance is a desirable battery specification because it means that the battery will waste less power when you draw power from it. The battery will try to give you as much current as you ask for. Fusing is highly recommended because even a small battery can generate sparks and catch fire in a short circuit situation.
Regulation
Wall-wart and battery voltages vary with time and load. Yet, electronic circuits generally demand tightly regulated voltage rails, often with tolerances of±5%. A voltage regulator converts a varying input voltage into a relatively static output. There are numerous regulator circuits and components. The easiest and, arguably, the most reliable solution is to use an off-the-shelf integrated linear regulator. Linear regulators convert a higher voltage to a lower voltage by dissipating the difference as heat. This is not the most efficient method, but it works well for situations of low to moderate power consumption. Linear regulators work without hassle because they do not require many external components or careful tweaking and they do not produce noise as do switching regulators. They are a mature technology that has been around for decades and parts are made by numerous manufacturers. Figure 3 shows how a typical fixed linear regulator is connected. The regulator has three terminals: input, output, and ground. (Some regulators are adjustable and contain an adjustment terminal in place of ground. More information on fixed and adjustable regulators is available in my book Complete Digital Design.) Capacitors on the input and output are per the manufacturer’s recommendations. An optional discharge diode prevents the output voltage from significantly exceeding the input voltage when power is turned off. This protection may be required by certain regulators. A long-popular linear regulator family is the 7800, with the 7805 being a well-used 5 V device. These and newer regulators are manufactured by companies including Fairchild Semiconductor, Linear Technology, Maxim Integrated Circuits, and Texas Instruments.
Regulator Specifications
The major attributes to look for in selecting a linear regulator are power rating and drop-out voltage. Power rating is simply how much current and voltage a regulator can handle. Power ratings are dependent upon cooling capability, such as a heatsink, that you may provide. For low power levels of perhaps one watt or less, cooling may not be a concern. As you reach towards several watts, you should investigate thermal analysis and cooling to ensure a safe, working system. Drop-out voltage is the minimum difference between input (unregulated) and output (regulated) voltage that the device can handle. If the input falls below the drop-out voltage differential, the regulator cannot maintain the output within its specifications. Regulators are available with drop-out voltages from around 2 V to near 0 V. This is where a system’s minimum required input voltage comes from. If you need a 5 V regulated supply and your regulator has a drop-out voltage of 0.5 V, the raw DC supply must not fall below 5.5 V. Low drop-out regulators potentially save power because they work with a smaller input/output differential. Remember that a linear regulator dissipates the input/output differential as heat. A smaller differential means less heat and a more reliable system. Of course, you must provide a low enough input voltage to take advantage of low drop-out power savings.
Power Distribution
Power distribution schemes vary based on the circuit assembly technology at your disposal. The best solution is to distribute power on a printed circuit board (PCB) with continuous copper planes for each voltage rail. This is the lowest inductance and resistance method. Power connections between regulators and planes should be with multiple vias to keep inductance and resistance low. If you are wiring a circuit by hand on a breadboard, try to minimize the distance that power must travel from the regulators to the other components. Longer wires increase inductance, which increases the circuit’s susceptibility to higher frequency problems. Whether using a PCB or breadboard, adequate power supply decoupling is essential to minimize disruptive electrical transients that result from switching digital circuitry. A minimum decoupling provision is to place a high-frequency ceramic capacitor at the power leads of each integrated circuit (IC) on your board. A small capacitance — such as 0.1 μF — should be used so that the capacitor can be effective at the high frequencies generated by switching logic. It is equally important to minimize the wire lengths between each capacitor and its associated IC to reduce the inductance of the capacitor circuit. Longer wire lengths can nullify the beneficial effects of decoupling capacitors. Figure 4 shows a basic decoupling scheme for a small system. Each IC has a 0.1 μF decoupling capacitor. There are also a couple of higher-value “bulk” capacitors (100 μF) that provide lowerfrequency decoupling. Voltage regulators have a finite response time to changes in current demand. The bulk capacitors help with these lowerfrequency transient events. Exact values for the bulk capacitors are usually imprecise, especially given wide tolerances in components. Recommendations may be available from regulator data sheets.
Safety
Safety considerations cannot be over-emphasized when dealing with power circuits. Proper fusing, insulation, cooling, spacing of components, and component selection should not be neglected in the haste to build your creation. It is always wise to conservatively de-rate components based on their power specifications. For example, if a component is to dissipate one watt, select a component that is rated for two watts. Building a safe, reliable power circuit is not difficult for small systems and will give you confidence in your project.
- By Ryan James | AssociatedContent
- Loulith Galenzoga
