Every electronic or electric project needs power, but how to select what kind of power supply you project needs.

There are so many types out there, what are the things to keep in mind when browsing your local store or digging in a box of recycled parts?

In this instructable I hope to give you a clear idea of what the differences and caveats are.

Note: I only cover mains powered projects here.

Step 1: Specifications

First you need to find out what specifications you are looking for, and they are all interlinked.

Output voltage(s)/power
Cost
Input voltage
Efficiency
Ripple/noise
Size
Durability

You can spend days looking trough all sorts of folders, catalogues, websites and so on for your ideal power supply.
Most likely: it does not exist and you will have to settle for less.

Step 2: Output Voltage/power

For most people this is the only consideration when selecting a power supply.
Of course this is the most important, your project needs the voltages and power, you can't change that.

You can however change the way you supply it.

Example:
If you need 5V for an Arduino and 12V for your motors, a buck converter would be a good idea instead of a PC power supply or 2 wall warts.

Step 3: Cost

Cost seems simple, your junkbin is free and the store has simple price labels.

But it is not so clear cut, efficiency plays a big role.

Example:
We need 5 Volt 0.5 Ampere for an Arduino project with a screen that is going to be on permanently.
In your junkbin you find an old 12V wall wart, your Arduino accepts 7-12V on the input so you are done.

These old wall warts are usually only about 45% efficient and have a high consumption.
The Arduino uses an linear power supply to bring the 12V down to 5V.

You use 5V 0.5A -> 5*0.5 = 2.5 Watt.
The power supply in the Arduino converts 12V into 5V by turning the 7V difference into heat.
This means that at 12V it is still 0.5A, so 2.5W becomes 6W.
The wall wart is 45% efficient, so 6W output means about 13W input (45% of 13W is 6W).

So of the 13W that is used from the mains only 2.5W is actually used.
10.5W wasted in heat, over the course of a year is about 92kWh wasted in heat.
The rough average kWh price in Europe is about 20cent, in the USA about 12cent. (dollars or euro's don't really matter in these estimates)
That means you are wasting 11-18 dollar/euro per year on the wasted power in your free power supply.

A simple USB charger is about 70% efficient and outputs 5v directly which reduces the wasted power from 10.5W to 1.1W.
This is only 8 kWh /year, or a saving of more that 10 euro/dollar per year, plenty to buy a USB power supply.

Step 4: Input Voltage

In mains powered projects the input is normally mains power, but mains is something different around the world.

The mains power supply around the world varies from 110V to 250V and everything in between and lots of 'mains' powered project are actually powered by a output of some other equipment such as the USB port of a server.

In general, power supplies with greater possible input ranges are less efficient.
Power supplies with a switch (such as the picture) don't have that problem.

Higher input voltages also lower efficiency (too bad I'm in a 240V country).

Newer power supplies get better every generation. This is mostly because the internal frequency in power supplies gets higher and higher which makes power transfer more efficient.

Step 5: Efficiency

Efficiency is the amount of power that goes into your power supply compared to the amount of power that comes out.

Here are some efficiency figures for power supplies you are likely to find.

Old heavy wall wart 40-50%
Modern switchmode wall wart (e.g. usb chargers): 70-80%
Laptop adapter: 80-90%
Computer power supply: 60-90% mostly dependant on age: older=worse.
Normal transformer: 60%
Toroid transformer: 70%
Open-frame supply: 80-90%

Of course these are rough estimates, in general these rules apply:

The newer the design, the better the efficiency.
More powerful power supplies are more efficient (For example the transformers used by energy companies are up to 98% efficient even though they are normal transformers)

The efficiency also varies wildly over the amount of power consumed.
You will normally find the highest efficiency with the power supply at about 75% load.

Don't be temped to use a high power supply at lower loads, efficiency drops like a brick at low loads.

Step 6: Ripple/noise

Do you do any audio/analog in your project, you'll really care about the noise and ripple.

In digital designs it usually doesn't really matter.

Ripple: How flat is the output voltage under load.
Ripple always has the frequency of the internal frequency of the power supply.
Transformers will always be at the mains frequency (50/60Hz).

Switch mode (newer adapters, computer supplies, open frame etc) will usually have a frequency in the 5-150kHz range, which often changes if the power supply is under load.
This can be a show-stopper for a audio system.

Noise: How clean is the output signal.
This is mostly based on the quality of the components used in the power supply and the operation principle of the power supply.

Linear power supplies have very low noise because there are no switching components in the power supply, this makes the design automatically absorb the noise that is in the system.
Switching power supplies effectively work by transforming small amounts of energy to different voltage levels, although this is very efficient, every switch action gives a lot of noise.

Ripple and noise can be filtered, using low-pass filters.

If efficiency is important but ripple/noise are also important (for example in an audio amp) you could look at both linear and switching at the same time.

For example if you need 12V, use a 15V switching power supply and connect a linear supply to go from 15V to 12V.
That should greatly improve the noise levels.

Step 7: Size

The size of a power supply depends mostly on output power and efficiency.

If the power is greater you need a bigger power supply.

If the efficiency is greater the power supply will be smaller for the same output power.

Power supplies are always limited by the size of the transformer, but the higher the frequency the smaller and lighter the transformer can be.
The smaller the parts are, the more power fits into the same box.
A 1000W transformer is about 20kg, and the size of a small table.
A 1000W modern PC power supply is less than 5kg and easily fits in a shoebox.

Step 8: Durability

How durable/reliable should your project be?

The simpler the power supply the more reliable it becomes.
The more industrial it is the more reliable it becomes.
The hotter a power supply becomes the faster it breaks.

Sorted by reliability:

A bare transformer, almost indestructible.
Old wall wart, simple in design, not much to break.
Open-frame power supply, usually build to last decades continuously running.
Modern wall wart, low power, low heat. Just don't pick a cheap one.
Laptop adaptor, completely solid, but get hot.
Computer power supplies, often don't like strange loads and cheap fans.

The single component that is usually the first to break is the electrolytic capacitors in the filtering.
These work on a water-based liquid electrolyte inside of the cans, which leaks out over time.
The hotter they get, the quicker they fail.

Step 9: Summary

So many things to look out for and I've only scratched the surface.

A short summary.

When powering something continuously, pay close attention to the efficiency, or it could get expensive.
A big old power supply is usually not a good choice unless you are looking for high reliability.
When doing analog, audio or otherwise critical filtering is important, but selecting a good power supply helps more that you would think.

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