Raspberry Pi Power Supply

A Raspberry Pi power supply is usually a mains adapter, which provides a DC current of 1-ampere and 5.0 volts. The current requirement is different for each version of the Raspberry Pi. Model A has a requirement of 0.3-amps, model B has a requirement of 0.7-amps, and model B+ requires 0.6-amps. However, they all require 5-volts through a Micro USB B plug and socket arrangement.

Voltage regulation, noise, and ripple are critical parameters in digital electronic circuits, and therefore a good power supply is vital. A poor quality power supply can cause all sorts of havoc such as corrupted memory cards, keyboard and mouse not recognised, intermittent lock-ups, and spontaneous hardware resets.

USB Problems / Issues

Powering a Pi from a desktop or laptop USB socket usually does not work, and if it does then it is very unreliable. The model B requires 700 mA current, whilst the USB can supply only a maximum of 500 mA at best, and typically around 100 mA without enumeration. The USB B socket does not use the data connection; therefore, the computer does not sense anything connected to the port, which results in a default 100 mA supplied through it.

No Keyboard or Mouse

The Raspberry Pi Dead article shows what to look for if your Pi shows absolutely no signs of life. If it does not detect the USB keyboard and mouse then the Raspberry Pi USB Not Working article might shed some light and provide some solutions.

People often end up disappointed when their new gadget does not work as expected, however the Pi is a great product and if a good quality power supply is used, there should be no problems.

Power Supply via GPIO Socket P1

If you are not experienced with measuring voltages correctly, then do not use the P1 socket to power the Raspberry Pi. The +5 V power line does not have fuse protection, and therefore it is extremely dangerous if your board has a short circuit. If you would like your Raspberry Pi to last, you should avoid using the five-volt pin.

Input Circuit

One of the first things I checked was the circuit diagram of the input circuit of the Raspberry Pi and this is what I found. The Micro USB B socket uses only the power pins, and the data pins D+ and D- are not used, therefore this socket is for the purposes of providing a power supply and not for the transfer of data. Without the data pins the operating system cannot enumerate the device and therefore the USB port runs on the default power level of 100 mA.

Polymeric PTC Fuse

On the revision 2 board F3 is a miniature surface-mount device (SMD) fuse rated at 1.1 A / 6 V. It is a polymeric positive temperature coefficient (PPTC) fuse marked "T075" on the component. It begins increasing its resistance when the current level is 0.75 A, and becomes fully open at 1.1 A. Therefore, be mindful of the current you are drawing when you connect all the USB devices into the port.

A polymeric fuse exhibits an increase in resistance during an over-current situation. This has the effect of choking the current flow. When this happens, the device is "tripping" or the fuse has tripped. Once tripped, the fuse requires some time to reset itself. Using a polymeric fuse is a good design choice by the designers, especially since "older" children may make shorts on the board. For more information regarding the fuse and its replacement please read Raspberry Pi Dead article.

Voltage

The fuse has a 6-volt rating; however, the input voltage should be no more than 5.1-volts. A good power supply should maintain the voltage level even when other USB devices are connected. If you find that the voltage drops to 4.6-volts or 4.7-volts, then you have a poor quality adapter that is not regulating the supply.

An adapter that provides 5.1-volts or 5.2-volts tends to work better because many USB devices do not comply precisely with the electrical requirements, and therefore load down the voltage. Hence, having a slightly higher voltage results in the voltage level coming down to 5.0-volts when all the USB devices are connected.

Input Circuit Design

From the circuit diagram, you can see that D17 is an SMBJ series Transient Voltage Suppression (TVS) diode. Its purpose is to protect the sensitive electronic circuits of the Raspberry PI from sharp voltage spikes called transient voltage events. This is a good idea as there are bound to be voltage spikes to the power input and this component provides a small amount of robustness to the device.

C6 is an electrolytic capacitor that provides a very small amount of smoothing to the input power. This is obviously not enough but the circuit expects the external power source to provide the additional smoothing.

C2 and C3 are decoupling capacitors, which provide a small amount of filtering of any digital noise that may end up on the power planes. Digital circuits usually generate noise on the voltage lines as they switch very fast.

From this, one can see that there is a small amount of protection incorporated into the Raspberry Pi input circuit.

Homemade DIY Power Supply

If you want the best power supply, with generous line regulation and filtering, then you are going to have to make it yourself. However, you should only do so if you are experienced and confident in electronics, otherwise the new Pi will have a very short life span! It is extremely dangerous to play with un-isolated mains transformers, as they can be lethal.

From studying the input circuit, I deduced that a high-end power supply would be one with a 5-volt regulator, which would keep the voltage rail fixed at precisely 5.1 V no matter the load connected to the USB ports. A common garden-variety 7805 linear regulator will work fine. In addition, a current smoothing capacitor of at least 1000 µF would be pure indulgence and luxury, as the current would have absolutely no ripple. Reset and power switches would be pure indulgence as well and worth considering. Such a circuit does not exist though; therefore, one would have to make it.

It just so happens that I already had a simple linear power supply circuit which I made earlier, shown in the 5 V PSU for USB Devices and Raspberry Pi article. If you have a conventional adapter providing more than 5 V, then this circuit will regulate the voltage to 5.1 V and maintain it. An advantage of this circuit is that it eliminates exposure to mains voltages and transformers.

Quality Adapters

Many sellers on Amazon, and eBay sell adapters, however they vary in design, construction, and purpose. The cheapest ones tend to be phone chargers for the purposes of charging a battery. Consequently, those ones may not have the necessary line regulation or provide the correct current filtering. Many sellers also offer cheap phone chargers, relabelled as a "Raspberry Pi Adapter" with a hiked up price. Those ones usually do not last very long or provide a quality performance.

An adapter specifically designed for powering the Raspberry Pi, tends to perform well, and it will have the required line regulation and smoothing to provide an adequate performance. For example, the ones sold at RS, Farnell, and Maplin are specifically for powering the Pi and cost as little as three pounds. Therefore, you would do well to get something like the RPI-PSU-UK-MK1 model. By purchasing the official adapter, it also means that you are supporting the Raspberry Pi foundation.

Current

The flow of current expressed in amps, is an important parameter to consider when buying an adapter. Many sellers offer adapters capable of providing two amps (2 A) and those are fine, and work well. However, the Raspberry Pi model B has a 0.7-amps fuse, which will trip when the current flowing through it is more than this amount. Therefore, the additional current capacity of the adapter is never going to be used.

In the following sections of this article, you will see example adapters, their markings, and meanings.

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