Raspberry Pi Power Adapter

A Raspberry Pi power adapter is a device, which converts the mains power, which is 240 V AC in the UK, to five volts DC with a 1-ampere current. An adapter such as this drops the voltage first, and then rectifies the current to DC. In the US the mains power is 110 V AC (60 Hz) through a type A plug (NEMA 1-15 P), or type B plug (NEMA 5-15 P) in some states. If you are U.S. based, then you will require an adapter specifically for your country.

The latest three versions of Raspberry Pi require 5-volts DC through a Micro USB B socket. An adapter capable of providing a maximum of one-ampere current usually works fine for all three models, as their current requirement is significantly less than this amount.

If you are considering buying a Raspberry Pi, then I suggest you buy the correct official adapter with it from the same place, as it will save you much time and money in the end. The official adapters sold by the foundation are less than a fiver and very good value.

Phone Chargers

Mobile phones draw their power only from the battery when operating. Even when the battery is charging, it must have a minimum charge before the phone powers up. This is because the purpose of a charger is to charge the battery, and it does not have sufficient line regulation or filtering required for driving a digital circuit. Therefore, many phone chargers will not work with the Raspberry Pi.

No Line Regulation

Many chargers have the absolute minimum of components to cut costs, as it does not require much line regulation to charge a battery. Usually there is a half wave rectifier consisting of a pair of diodes and a 470 µF capacitor for smoothing, which is woefully inadequate to power a computer system such as the Pi.

Many adapters such as this state 5 V DC on the label yet when you measure the voltage you could find voltages as high as 18 V DC. This is because these adapters do not undergo any testing at the factory, and since the phone has a built-in regulator it does not affect its operation. These adapters might be great for charging batteries however; they are not ideally suited for powering a computer. If your Raspberry PI is locking up and crashing then it might be a good idea to measure the DC output voltage using a digital meter.

Switched Mode Power Supply

The newer varieties of switched mode power supply unit (SMPSU) adapters are now very popular amongst mobile phone and portable device manufacturers. They are small and light, and approximately the same size as a British mains plug. Many of them are multi-standard and can operate in different countries. This is of course very useful, as they work excellently as battery chargers, which is their primary purpose. The mobile device usually draws power only from the battery and not the charger because an SMPSU can be extremely noisy producing RF noise.

The design of a SMPSU is very different from a conventional linear power supply. There is usually a high-frequency oscillator driving a MOSFET, which in turn drives the primary of a chopper transformer. The high frequency switching of the inductive load can produce RF noise on the power planes of the Raspberry Pi. The performance of these types of adapters depends upon their RFI filtering components, and due to space restrictions, RFI filtering is often inadequate.

Linear Power Supply

A linear power supply usually consists of a large heavy transformer to drop down the voltage. The transformer usually consists of iron formers and copper coil windings. Back in the early days, they were the size of bricks and weighed just as much. The Sinclair Spectrum Adapter is an example of such an adapter. Back then, all the early home computers had one of these.

Although these types of adapters were a nightmare for the travelling user, they had one good thing going for them, and that was that they produced extremely clean current.

Orphaned Adapters

The first thing people normally do is start looking at all the orphaned adapters that they may have around the house. It might be tempting to try different adapters, however if they do not have the "Micro USB B" plug attached, then people usually chop the plug off and connect the wires directly to the power pins on the P1 socket. The +5 V power pin on the P1 socket is not afforded a fuse, therefore it could spell disaster if your circuit had a short. In addition, if the adapter were to produce high voltages, then that would damage the Pi.

Even if the adapter has the correct plug, you should still be careful and check the voltage as there are bound to be some manufacturers that may not have followed the specification.

The adapters I had were the old-fashioned linear types providing 9 V and 12 V. One good thing about those ones is that a simple voltage regulator such as the 7805 can bring the voltage down to 5.0 V.

Nominal Mains voltage

Always make sure that the adapter you are using is for your country. in the UK, the nominal voltage for domestic appliances is 240 V. Many adapters also show a voltage range such as this one, which shows 100 V to 240 V.

DC Current Symbol

The output of this adapter is DC designated by the solid line with dashes underneath. Feeding AC current will damage the Pi beyond repair.

Expressing Current

On this adapter, mA indicates milliamps and expresses the current, hence, 500 mA is the same as 0.5 A.

Safety Marks

Notice all the safety logos and marks on the adapter. Usually there are at least three to four different marks. The most important ones are the British Safety (BS) kite mark, and BEAB approval. The CE mark appears to be very popular these days; however, this is a self-certification scheme and not based on independent testing.

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