Power supply buying guide

Power supply buying guide

John, DIYer & IT developer, Brighton

Guide written by:

John, DIYer & IT developer, Brighton

71 guides

A standard, switching, laboratory or electric panel power supply converts the network's power to a lower voltage. It will take a 220 V  current and convert it to 12 V, 24 V and so on. The power, the voltage, the intensity but also the type of plug and output are the main factors to consider.

Important features

  • Power
  • Voltage
  • Intensity
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What is a power supply?


A power supply unit converts the electric network's voltage from 220 V to a lower DC voltage of no more than a few dozens of volts, better suited for the electronic devices used in our homes. Different technologies are put in practice, but the basic principle is always the same: to obtain a power supply that is stable, despite any network fluctuations, and adapt the voltage for consumer devices that require power.

The output voltage and the maximum current are two essential characteristics of a power supply.

A good power supply is able to maintain the output voltage's nominal value for a period of time less than or equal to the maximum power supply, with the highest possible efficiency. A high efficiency means that a maximum of power flows to the power supply's output, which in turn may heat up little. It’s good for both the environment and your wallet!

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Power supply unit

What are the different types of power supply?

Standard power supply


It is by far the oldest technology and is also often referred to as a linear regulator. A transformer lowers AC voltage from 220 V or 50 Hertz to a lower voltage, around a few cozen volts, but still with a 50 Hz frequency.

A so-called "capacitor" level will convert this voltage, sometimes positive, sometimes negative, into always positive voltage. The current is then smoothed, with the resulting tension being greater than or equal to a certain threshold. A final electronic step, referred to as the control levelmaintains a constant output voltage, by clipping the last level.

These power supplies are rather heavy, expensive and with an average efficiency ranging from 25% to 50%. The consumed power, that has therefore has to be paid for, can range from double to quadruple the amount that the device needs. This type of power supply is ideal for small electrical appliances that consume very little, as you can afford to a bit of energy loss, especially since these power supplyunits are very compact. Standard power supply units are also ideal for devices that are highly sensitive to electromagnetic interference.


Switching power supply

A switching power supply unit, which has been around since the 1970s, is compact, inexpensive and efficient, with a yield ranging from 75 to 95% . Their qualities certainly explain their great success!

Their basic operating principle is relatively simple. The network current is converted into a high DC voltage generally around 300 V. A transistor then clips this voltage by opening and closing several thousand times per second. This pulse train is then re-converted into DC voltage, the type we need. The length of time which the capacitor is active determines the output voltage level and can be adjusted. This voltage is of course regulated electronically to be as stable as possible. These switched-mode power supply units tend to create electromagnetic disturbances on the power grid, which can interfere with other devices. If that is the case, a high-end power suppy unit will be required. Equipped with more efficient filters, it protects the rest of the network from any disturbances that may result from the clipping, as well as protecting the device itself at the same time. 


Laboratory power supply

As its name suggests, this type of power supply is used in laboratories to power experimental electronic circuits and to test a range of voltages in varying intensities. A laboratory power supply makes it possible to propose one or more outputs on which a voltage ranging from 0 V to a few dozens of volts can be set. The maximum current can also be adjusted. You can also find different  stable voltages to power transistor technologies (2.7 V for TTL technology, 5.5V for CMOS transistors ... if this isn't chinese for you!).A laboratory power supply has measuring instruments for real time monitoring of the voltage and possibly the current output. The laboratory power supply unit is protected against any possibility of a short circuit. Even in case of incorrect assembly, there is no risk of seeing your power supply unit go up in smoke.

Power supply for electrical panel


Many home automation devices and circuits require a low voltage power supply, which typically does not exceed 20 V. If these devices are connected to the electrical panel you must choose a power supply that can be directly integrated into it.Although this solution is more expensive, it provides a clean and perfectly integrated solution. Take a look through your supplier's catalog, you will certainly find a specific transformer for an intercom or your painting's communication box.

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Laboratory power

How to size a power supply?



The power (P) delivered by the power supply, expressed in watts, is simply calculated by multiplying the product of the voltage (U), expressed in volts, by the intensity I, in amperes. So P = U x I.

This is the theoretical maximum power that the power supply unit can provide. It is better to choose a higher power, and leave yourself a 20%margin of safety. Operating at maximum power, your supply unit will heat up and wear out prematurely. It is not uncommon for low-end model's to be very optimisticly judged by their manufacturers, often overestimating their product's performance. A comfortable margin of safety is therefore crucial. Be careful, however, not to oversize the power. Nothing is stopping you from buying a power supply that is ten times more powerful, but you will pay the price for a power supply unit that is hardly performing at all.


The adapter must provide the exact expected voltage required by the powered device. If the supplied voltage is too high, the appliance can burn out. It's a risk that you do not want to take, and we completely understand! Conversely, a low voltage will result in the device possibly not working. If it is slightly lower, you can always try to use it. It really depends on the case! The cylindrical plugs are connected at two points on the device: one inside, and the other outside. In this case, you will need to ensure that the proper polarity has been respected. If the polarity is reversed, that is to say the - in place of the +, or vice versa, the connected appliance is again liable to heat and toast. In short, be sure that the voltage and polarity of the device and the power supply are the same .


As we have just seen, the voltage is dtermined by the device and the power is obtained with the formula P = U x I. The power unit's power, including your margin of safety, is based directly on the power supply's maximum intensity value. It must, of course, be greater than the maximum intensity expected by the device.

Choosing the right power supply unit comes down to:

  • Determining itsvoltage;
  • Calculating its power, depending on the maximum intensity consumed, with a safety margin of about 20%.

Without forgetting that :

  • For low power, like for battery-operated type, the linear power supply is suitable;

  • For high power, a switching power supply is required due to its superior performance;

  • A good quality power supply, although more expensive, maintains the load better, and produces less disturbances on the network;

  • If you need a laboratory power supply unit, consider the amount and type of outputs you need. Otherwise, determine the type of plug that connects the appliance to its power supply, as well as the polarity of the plug if it's a round plugs;

  • If the power supply unit is just to tide you over, be sure to correctly estimate your needs! Do you require use of different voltages or different output plugs?

More Information

Whether you're renovating or have a fancy new toy to power up, follow the links below for power-supply related accessories, advice from our editors and more helpful guides:

  • How to choose your electrical panel?
  • How to choose your circuit breaker?
  • How to choose your communication box?
  • How to choose your intercom?
  • How to choose your electrical outlets?
  • How to choose your programmable outlets?
  • How to choose your electrical connections?
  • How to choose your electrical extension cords?
  • How to choose your electric radiator?
  • How to choose your battery for electric tools?
  • How to choose your multi-meter?
  • How to choose your energy manager?
  • How to choose your energy consumption indicator?

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Guide written by:

John, DIYer & IT developer, Brighton, 71 guides

John, DIYer & IT developer, Brighton

Since I was a child, I was always interested in manual and technical works. Always fascinated by woodworking, I took advantage of my first flat as a playground. On the cards: electricity (of course, safety first!) and some partition walls; but also decorating with the help of the missus, made-to-measure furniture and little tricks to optimise the space, all the while remaining as original as possible. When the little one arrived, I started building bits and pieces for him! Lacking space, I have not got a permanent workshop and certain tools I dream about but are not part of my collection. Not to worry, I already know a lot about DIY and I have a high-tech profile that I hope will guide you in your decisions!

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