The case for buying
Making things yourself is much of the appeal of hobby electronics, and low-voltage circuits are a great place to learn. Power supplies that connect to household mains are different. They sit between a dangerous voltage and everything else in the project, and mistakes can mean electric shock or fire. For most makers, a ready-made, certified supply is the safer, cheaper and often better-performing route.
This is general guidance. Rules for electrical products differ between countries and regions, and the manufacturer’s instructions and local regulations apply. Mains wiring in a building is a job for a qualified electrician, which is covered in the article on why mains voltage projects call for professionals.
What certification gives you
A certified supply is one that has been tested against recognised safety standards by an appropriate body or has been declared compliant by the manufacturer under the rules of the market where it is sold. Testing typically examines things such as insulation between the mains side and the output, behaviour under fault conditions, heating, and resistance to fire. A home-built supply is very unlikely to have been tested for any of these, and the failures that matter most, such as insulation breaking down or a part failing short, are exactly the ones that are hard to spot on the bench.
Professionally made supplies also benefit from design effort that is difficult to reproduce at home: carefully chosen spacing between conductors, appropriate components, quality control and protective circuitry.
Markings to look for
Marking schemes vary by region, so check which apply where you live and be wary of markings you cannot verify. Some general points:
- A recognised safety mark for your market, applied by the manufacturer or a testing body. Some marks only declare compliance, while others indicate independent testing. A product with no marks at all, or with vague logos, deserves suspicion.
- Manufacturer and model identification, so you can look up the specification and find support.
- Input rating: the mains voltage and frequency range the unit is designed for, which must suit your region.
- Output rating: the voltage, and the maximum current or power it can deliver.
- Class or insulation symbols, such as the double-insulation symbol, where relevant, which tell you about the construction and whether an earth connection is expected.
- Compliance with other rules, such as restrictions on hazardous substances or energy-efficiency requirements, where they apply.
Counterfeit marks exist. A very low price, missing detail, poor print quality, a flimsy case or a suspiciously light weight can be warning signs, though none is proof. Buying from reputable sellers reduces the risk, and if a unit seems doubtful, do not use it.
Choosing the right output
The supply needs to match the build.
Voltage. Choose a supply whose output matches what the circuit expects. If the circuit needs a lower voltage on a rail, a regulator can step it down; the trade-offs are covered in the comparison of voltage regulators. Many projects use a common voltage such as 5 V or 12 V, but follow the requirements of your parts.
Current and power. Add up the loads and choose a supply rated comfortably above the maximum, as in the planning approach for hobby power planning. A supply running at its limit tends to run hotter and shorter-lived, and a supply that is too small may sag or shut down under load. Remember peak currents from motors and radios.
Regulated or unregulated. A regulated supply holds its output voltage steady across the load range. An unregulated one may deliver a higher voltage when lightly loaded. For electronics, regulated is generally the safer choice unless the documentation says otherwise.
Polarity and connector. Barrel connectors differ in size and in which contact is positive. Check the polarity marking before connecting, since reversed polarity can destroy parts. This is a case where reading datasheets and documentation saves trouble.
Protective features worth seeking
Look at the specifications for what protections the supply claims, and treat claims on unbranded products with caution:
- Short-circuit protection, so a fault on the output does not cause the unit to overheat.
- Over-current protection, limiting the current to a safe level.
- Over-voltage protection, preventing the output from rising dangerously if the supply fails.
- Over-temperature protection, which reduces output or shuts down if the unit overheats.
- Isolation between input and output, which is part of what keeps the low-voltage side safe to touch.
- A sensible enclosure, with no exposed live parts and suitable ventilation.
These features reduce risk but do not remove it. They also do not remove the need for fusing and correct wire sizing on the output side, as described in the discussion of fuses, wire gauge and current limits.
Using a supply well
- Use it within its stated ratings and in the environment it was designed for. Indoor adapters do not belong outside or in damp places.
- Give it airflow. Do not cover it with cloth or leave it buried under items.
- Check cables and plugs for damage. Replace a unit with a cracked case, frayed cable or burnt smell instead of repairing it.
- Do not open a mains supply to modify it. Capacitors inside can hold a charge after unplugging, and the internal parts may carry dangerous voltages.
- Check the efficiency and standby claims with some scepticism, as with other efficiency claims, and if you want to know what a device really uses, energy monitoring with a plug-in meter can show it.
When a bench supply or battery is the better choice
Adjustable bench supplies are valuable for testing, with their adjustable current limits protecting experiments. Choose certified ones too. For portable projects, a battery is the natural choice, with its own considerations covered in the articles on battery chemistry and charging. A pre-built battery pack with protection and a certified charger is generally preferable to assembling your own.
The bottom line
For anything that connects to mains, buy a certified, ready-made supply from a reputable source, match its ratings to your load with margin, and keep the project on the low-voltage side. Your time is better spent on the circuit you actually want to build, and outcomes are far more predictable when the power source has been properly tested.