How to choose a good power supply? - manual

1. Product label

The figure shows an example of a plug-in power supply label. The branding of the product (ESPE) and the details of the company responsible for marketing the product are obvious, as are the rated output voltage (24 V) and the maximum load current (0.5 A). But the information about the input voltage range (100-240 VAC) indicates that the power supply can be used in many countries around the world: Europe, where it is 230 VAC at the socket, the UK, where we have 240 VAC, and the USA, where the mains operates at 110 VAC/60 Hz.
The wide input voltage range makes it easier for the power supply to operate when the mains is not very stable, the voltage fluctuates within wide limits or there are short power outages caused, for example, by switching on large machines with powerful motors in the area. Then, the wide voltage range is able to protect the device from shutdown (e.g. router reset), because even if the voltage drops significantly for a moment (e.g. by half), the power supply will still be able to function.

The symbol    is, in turn, the energy class designation "Energy Star Compliance Level VI", the latest standard for minimum power supply efficiency and no-load power consumption. Level VI means in practice a power supply that is very economical and heats up very little. Level VI efficiency requirements were set in 2016, so a power supply with this symbol is also a modern design.  

The sign   indicates the polarity of the voltage in the output plug. The standard for most devices is the pulse in the centre of the plug. On flat cables, the one with the plus power supply is colour-coded.

  is in turn an important symbol confirming that the power supply is made in the second protection class. In other words, they have reinforced double insulation between the mains and the output. The double insulation gives greater safety of use. Typically, such power supplies do not require earthing, i.e. they are connected to the mains via a two-wire cable, rather than a three-wire cable with a PE protective conductor, as is the case with class '1' equipment, i.e. with weaker insulation.  

2. Output cable

The output cable of a good power supply should be both thick and flexible. These are contradictory requirements, which is why many power supplies have problems with this. A thick cable means high resistance to mechanical damage, such as being run over by an office chair wheel. A thick cable also has larger cross-section copper cables inside, so the voltage drop across it is small. And a low voltage drop means low electricity loss and less heat build-up in the cable, in other words, it ensures a more efficient power supply. Thick and energy-efficient cable is usually rigid, but good power supplies can have thick and flexible cables because they use silicone plastic, which has properties similar to rubber. The difference in cable flexibility is most noticeable at low ambient temperatures, where poor cables lose their flexibility completely.
A good power supply also has cable insulation made of safe materials, i.e. not containing harmful chemical compounds, non-flammable, and not emitting harmful substances at high temperatures.

3. Protection against environmental influences  
A good power supply is one that will operate safely and for a long time in conditions of varying temperatures and humidity. The basis of such protection is an airtight plastic enclosure, preventing the possibility of moisture penetrating inside. Heat exchange with the environment also takes place through the plastic, so it is important that the power supply housing is not very small. A high-power power supply housed in a small enclosure will have difficulty dissipating heat to the environment. Such an enclosure will be hot and the conditions inside may cause rapid ageing of components, such as the drying out of capacitors. If mounting conditions allow, it is worth choosing a power supply in a larger enclosure.  

4. Operating temperature range
The wider the operating temperature range, the better the power supply as a rule, with the maximum temperature being the most important. The higher the maximum temperature, the greater the reliability of the power supply, because this means that it is made of better quality materials and components. The minimum temperature is less important, as it only indicates in practice the range within which the manufacturer guarantees the parameters stated in the data sheets.

5. Energy conversion efficiency
Power conversion efficiency tells you how much of the power taken in by the power supply from the mains is lost during power conversion. These losses are, in other words, the heat given off in the power supply's components and the higher the efficiency, the less heat and energy loss. A high efficiency indicates the class of the unit and defines quite clearly whether the power supply is good or not. For good medium power consumer power supplies, i.e. 15-75 W, the power conversion efficiency approaches 90%. In low-power power supplies, the efficiency tends to be somewhat lower. The higher the efficiency, the lower the heating of the power supply and therefore also the maximum temperature of the case. Many good power supplies operating at room temperature with a full load have only a slightly perceptibly warmer case. Those of poorer quality are not infrequently hot.   

6. Overload protection
A good power supply must not break down due to a short circuit, overload, overvoltage or overheating. Electronic protection is an integral part of any design, but poor ones do not usually have protection against overvoltages occurring in the mains supply, i.e. in practice they do not contain a varistor. Output overload protection is not always the same either. Many power supplies go into current stabilisation mode during an overload and so operate all the time. Others switch off for a while and then try to switch on again after a short time. In the latter case, the power supply may have trouble starting up if the load draws a large inrush current when switched on.

7. Output power
The output power of a power supply is the product of the output voltage and the maximum load current. E.g. 12 V × 3 A = 36 W. The power of the power supply must be greater than the energy demand of the load. If the load draws a higher current at start-up, such as an electric motor, a light bulb, it is worth buying a power supply with a higher wattage so that start-up is trouble-free. A good power supply always has a power reserve for this purpose. This is typically 25-35%.

8. Output voltage
The rated output voltage of the power supply is given on the label, but the actual voltage may differ from it. The no-load voltage is also a tad higher than under load. In a good power supply, voltage fluctuations as a function of load changes are not large, usually no more than 3%, and the output voltage of the power supply without load is very close to the rated value or a little (e.g. 2%) higher. A multimeter is useful for checking this.

9. Safety in use
In addition to supplying energy to the load, the task of the power supply is to galvanically separate (isolate) the output of the power supply from the mains. This means that the output cable must not be electrically connected to the input, as this is the only way to ensure safe operation. The separation takes place in the transformer, where the windings are separated by insulating tape. The quality of this tape, its temperature resistance, its thickness and its ability to withstand high voltages is crucial to ensure the safe use of the power supply. This insulation must be able to withstand surges occurring in the mains, for example during a storm. The strength of the insulation is defined by standards, but good power supplies have better insulation than is required. Good power supplies can withstand test voltages of 3000-4000 VAC.
 
10. Buzz and production quality
The quality of a power supply is the quality of the materials used in its manufacture and assembly. A casing with non-smooth edges, buzzing or squealing coming from the interior are symptoms that the manufacturer does not pay attention to the quality of production.

11. Compliance with standards
Many issues related to power supplies are defined by IEC/EN standards. These include issues related to safety, i.e. the quality of electrical insulation, the resistance of the power supply to electromagnetic disturbances and the levels of emission of such disturbances. The standards additionally prohibit the use of harmful substances during production, such as lead, and certain additives in plastics that impede flammability. A good power supply meets such standards in reality and often by a margin.

12. Manufacturer's brand and warranty
Not everything can be seen, and not everyone is knowledgeable about power supplies, so often the brand of power supply is equated with quality. Good power supplies also come with a longer than standard 2-year warranty.

14. Data sheet
A lot of information about the parameters of a power supply can be found in the data sheet. In practice, every power supply has such a card available, but as was the case in life, a card is unequal to a card. A good power supply has a technical card in Polish. It contains a table with individual parameters describing data about the power supply input, output, environmental conditions, safety, compliance with standards, up to such trifles as dimensions, weight and packaging. A good data sheet also has a picture of the product and a technical drawing of the case. A poor power supply will usually have a data sheet in English, the parameter list will be short and only deal with the most 'unobtrusive' data. The more parameters in the data sheet, the risk that the customer will complain that their power supply does not meet an item. Hence, usually the cheap power supplies have very modest technical data sheets, the data of the good ones are unlikely to fit on one page of A4.

15. Confidence in the seller
When looking for good quality power supplies on the market, avoid suppliers with a short market history and rather choose those who have been in the market for years. This is because those with a long history have more market experience, better sources of supply and, by virtue of operating on a larger scale, are able to offer more excellent power supplies at a comparable price. Reputable suppliers are able to take more responsibility for the product and provide reassurance that they will not disappear from the market a moment after the sale leaving the user with a problem. These are universal and trite statements, but they need to be repeated so that we do not forget them.