What is the ideal power supply

Raspberry Pi: Basics of power supply / power supply

The power supply of the Raspberry Pi is one of the underestimated sources of error. Mini-computers like the Raspberry Pi require a stable voltage AND power supply. With a bad power supply and unfavorable operating conditions, strange effects occur in combination with an unstable system behavior.

The effect called "mystery" in the German-speaking Raspberry Pi forum is usually expressed in the form of broken LAN and WLAN connections and other malfunctions of devices connected to the USB. For example, external WLAN adapters, USB sticks and hard drives.

Overview

  • Problems, nothing but problems!
  • Why why why?
  • Chargers or proper power supply
  • General requirements for the energy supply
  • Interfaces: USB, HDMI, GPIO, Ethernet, CSI
  • Power supply: Raspberry Pi B
  • Power supply: Raspberry Pi B +
  • Power supply: Raspberry Pi 2 B
  • Power supply: Raspberry Pi 3 B
  • Power supply: troubleshooting
  • Back-powering

Problems, nothing but problems!

The resulting effect is referred to as a "mystery", because the malfunction that occurs at first glance has nothing to do with a poor power supply. However, there are an infinite number of problem descriptions in discussion forums. The victims of the mystery report dropouts in the keyboard, mouse, screen display and very often connection problems with LAN and WLAN.
The resulting effects and problems are manifold. It can take hours to troubleshoot without getting closer to the problem. In some severe cases, the error is not always easy to reconstruct.

The problems that arise are often related to the fact that the stability of the voltage regulation leaves something to be desired. A lightning bolt in the upper right corner of the screen indicates that there is imminent danger.

If the plug-in power supply is unsuitable, strange effects can occur during operation.

Why why why?

What does this have to do with the power supply? If you "observe" the power consumption of the Raspberry Pi while using the keyboard, mouse and WLAN adapter with a measuring device, you will notice violent fluctuations. These USB devices are real power guzzlers. A mouse can draw 50 mA or more of current when moved. The same applies to a keyboard or a WLAN adapter. These devices are not exactly designed for low power consumption. That is also quite logical. A conventional notebook or PC has a sufficiently dimensioned power supply. And actually that should also be possible with a Raspberry Pi and any other mini computer. The user only has to take care of it himself. And he takes a conventional plug-in power supply for this. From his smartphone or something else that he still has in his drawer.

Many plug-in power supplies that are typically used to charge smartphones and tablets are not power supplies, but rather chargers for battery-operated devices. These chargers rely on the device being supplied with them not expecting stable voltage. Battery-operated devices are typically charged with electricity, regardless of the stability of the voltage from the charger. Only the charging circuit in the device or battery ensures that the battery is properly charged. This has the advantage that the device manufacturer can install individual batteries and the user can use almost any plug-in power supply.

Not so with a Raspberry Pi or another mini-computer. They are dependent on a stabilized operating voltage of usually around 5 volts. If you operate the Raspberry Pi with any plug-in power supply, it can and will usually work. If, however, the Raspberry Pi and connected USB devices ensure an impulsive current draw, then it can happen that the plug-in power supply is overwhelmed, fails and the voltage then drops. So it falls well below 5 volts.

What happens then? A device that is designed for 5 volts then comes into an unstable operating state. In addition, USB devices expect 5 volts at the USB port. According to the USB specification, a USB device should work between 4.45 and 5.5 volts. There is actually no danger from here.

But now it is the case that the Raspberry Pi also has a say. In the event of undervoltage, the Raspberry Pi takes its USB ports out of operation in order to restore the stability of the power supply by reducing the current consumption. All USB devices go out of operation for a short time at least. And that then leads to the aforementioned malfunctions of individual USB devices or the entire system and explains the dropouts in the USB devices. This applies to the mouse, keyboard, WLAN and LAN connections, as well as USB storage devices.

When the minimum voltage has leveled itself again, the Raspberry Pi puts the USB and the devices connected to it back into operation. While the keyboard and mouse log in again quite quickly and automatically, LAN and WLAN connections may remain deactivated on the software side and have to be activated manually. Unfortunately, in this situation you can no longer access the Raspberry Pi via SSH.

Some users feel compelled to pull the micro USB plug on the Raspberry Pi and turn it off hard. However, this should be avoided because "hard shutdown" can damage the file system. If an important system file could not be completely written at the time of switching off, it will not damage the hardware, but the file system. In such a case, an incomplete or faulty state can prevent a successful restart. You may also have to complain about a loss of data.

Plug-in power supplies that are designed as chargers are unsuitable for supplying power to a Raspberry Pi.

Charger or correct power supply?

Many power packs that are used on the Raspberry Pi are not really power packs at all, but rather chargers and therefore only partially suitable as "power packs". But a power supply unit or power supply unit is fundamentally different from a charger.

  • A real power supply unit is a voltage source with an electronically limited maximum current. A power supply tries to keep the voltage stable up to its maximum current, even with briefly high current draws.
  • A simple charger is a power source with an electronically limited maximum voltage. The charger simply tries to deliver the maximum current, regardless of the voltage.

This means that a charger is only suitable as a power supply if the limitation of the maximum voltage works precisely not only upwards but also downwards. Like a power supply is electronically regulated and not just a cheap Zener diode.

Unfortunately, it is now the case that dealers and users are ignoring this. The reason is that neither of them have the appropriate expertise and don't know the difference between a charger and a real power supply.
The dealers are not entirely innocent of this. As a rule, they are not specialists and cannot assess what they receive from the manufacturer or wholesaler and then sell on to the end customer. The plug-in power supplies advertised as "power packs" are mostly just simple chargers.
Users in particular suffer from this because they have to rely on the dealer not selling them any trash. Even if the retailer describes or advertises their plug-in power supplies as Raspberry Pi-capable, that doesn't mean anything.

Just because 5 volts come out of a black plastic part with a long tail and a micro-USB connector doesn't mean that a Raspberry Pi can be operated reliably with it.

General requirements for the energy supply

Basically, the Raspberry Pi is supplied with energy with a voltage of 5.0 volts (V) at the micro USB input. Strictly speaking, it is 5.1 V, which is what most USB plug-in power supplies also deliver. It is 5.1 V because plug connections and cables cause losses. In addition, voltage stabilization is always subject to specimen variance and minor fluctuations in stabilization. A little overvoltage makes sense at this point.

Of course, you can also operate a Raspberry Pi under 5.0 V. This shouldn't be a problem up to a voltage of 4.75 volts. Underneath, you should take great care to ensure that the operating voltage is stabilized, because otherwise considerable instabilities can occur.
Basically you shouldn't try to operate a Raspberry Pi with undervoltage if you are not familiar with it.

The second important value in energy supply is the power consumption, which is specified in amperes (A). Exactly how much power the Raspberry Pi needs depends on what is connected to it. As a rule, you can get by with a 2 A (2,000 mA) power supply unit. But only if you don't connect power-hungry USB devices. Then you need an externally powered USB hub or you should use a power supply with at least 2.5 A (2,500 mA). It is important to know that a Raspberry Pi cannot draw more than 2.5 A. If the overall system needs more power, this is limited by a fuse and can lead to instabilities.

The voltage (volts, V) of a power supply unit is a nominal value that must be adhered to. This means that the power supply voltage and the device operating voltage must match. Both values ​​must not deviate significantly from one another.
The current (amps, A) and the power (watts, W) of a power supply unit, on the other hand, are nominal values ​​that must not be exceeded. It should be noted here that a 10 W or 2 A power supply unit (at 5 V) must not be operated at its nominal value or above it. If you want to pull the 2 A, then the power supply unit is inevitably brought into an unstable state. This means that the voltage collapses and the supplied device fails. If you need 2 A, then you never use a 2 A power supply, but one step above. At least 20% to 30% more. So at least 2.5 A. Better more.

USB

In general, every USB device without its own power supply unit must be supplied with power from the Raspberry Pi. The current consumption can be very different depending on the device. For example, the power consumption of a keyboard can be between 0.1 A and 0.5 A (100 mA and 500 mA), just as an example. To make matters worse, USB devices can certainly draw current impulsively. That can throw the Raspberry Pi or the power supply out of step.
For safe continuous operation you have to check the power consumption of the USB devices individually. If you notice instabilities on the part of the Raspberry Pi, an externally powered USB hub is a solution. It is better to use USB devices whose power consumption is not that high.

GPIO

What does the GPIO have to do with the power supply of a Raspberry Pi? If you connect the GPIO header, then quite a lot.
A single GPIO port can deliver a maximum of 16 mA without being damaged. All GPIO pins together no more than 50 mA. Assuming 40 pins, of which only 26 can be wired as GPIO, then the current that can be drawn from a GPIO port when all of them are used as outputs at the same time is just 2 mA.
Since you will not use all pins as outputs at the same time, the current consumption per pin should not be higher than 3 to 5 mA. The fewer, the more pins you can use as outputs and the more stable the Raspberry Pi runs.

HDMI

What does the HDMI have to do with the power supply of a Raspberry Pi? If you want to connect a screen to the HDMI port, then quite a lot. On average, the HDMI connection consumes up to 50 mA of current (measured).
From the Raspberry Pi B + onwards, up to 400 mA can be drawn. This is important for HDMI devices without their own power supply. HDMI to VGA converters get enough power from the HDMI port.

Ethernet port (RJ45)

What does the Ethernet port have to do with the power supply of a Raspberry Pi? If you want to connect the Raspberry Pi to the network via this, then quite a lot.
If you have connected a remote station to the Ethernet port with a cable, the current consumption is around 60 mA (measured).

CSI - Camera Connector Interface

A camera module for the Camera Connector (CSI) requires around 0.25 A (250 mA). Unfortunately, it was not possible to find out whether this was true, because the data sheets of the relevant providers do not provide any information on the power consumption of your camera modules.

Summary: power consumption (official)

Raspberry PiPower consumption
Board (typical)USB peripheralsMaximum
Model A200 mA500 mA700 mA
Model B.500 mA500 mA1.2 A
Model A +180 mA500 mA700 mA
Model B +330 mA600 mA / 1.2 A (can be activated)1.8 A
Model 2 B330 mA600 mA / 1.2 A (can be activated)1.8 A
Model 3 B330 mA600 mA / 1.2 A (can be activated)1.8 A

Power supply: Raspberry Pi model A and B

The requirements of the Raspberry Pi model A and B are 5 V and 0.7 A (700 mA), which corresponds to 3.5 W. The Raspberry Pi only reaches 0.7 A if you also operate an HDMI monitor, Ethernet network and USB devices on it. In addition, there is the power consumption of these devices when they are powered by the Raspberry Pi.

  • Raspberry Pi: approx. 700 mA
  • HDMI: approx. 50 mA
  • Ethernet: approx. 50 mA
  • USB: approx. 200 mA

Total power consumption is 1 A (1,000 mA), protected by a fuse. We recommend a power supply with 1.2 A. The reason is that you should never operate a power supply too close to its rated load. So if you calculate with 1 A, then you take 10 to 20% more. This is the only way to ensure that the power supply is stable. If more current is drawn even for a short time than the power supply can deliver, then the voltage collapses, which acts like switching off the voltage and the Raspberry Pi goes out.
Unless the Raspberry Pi is fully exhausted with all its possibilities, you can also get by with a power supply of 1.0 A or less. But only in exceptional cases.

There are often reports that the Raspberry Pi Model B is not running particularly stable and occasionally gets out and then has to be restarted. The reason: The power supply of model B is a bit shaky, which is why a real power supply, not a charger, with at least 1.2 A and more is recommended. If you use a power supply with less than 1.2 A, you have to expect instabilities. Especially when you still operate devices on HDMI and USB that do not have their own power supply and therefore pull them out of the interface.
If in doubt, it is always advisable to operate USB devices on an externally powered USB hub.

Power supply: Raspberry Pi model B +

A major advantage of the B + model is the better stability of the power supply overall. Not only can 4 USB devices be connected to the B + model, but also without an externally powered USB hub.
In total, the B + may draw 2.0 A (2,000 mA) from its power supply unit. Provided that the power supply delivers that too.

  • Raspberry Pi: approx. 500 mA
  • HDMI: approx. 50 mA
  • Ethernet: approx. 50 mA
  • USB: approx. 1,400 mA

The power consumption of the B + is specified as 0.6 A (600 mA). If you work with a monitor, keyboard, mouse and Ethernet, then that corresponds to measurable values. So it needs less than the model B and should run more stable overall with the same power supply.

According to the specification, all USB 2.0 ports on the B + together deliver 0.6 A (600 mA). That should be sufficient for a 2.5-inch USB hard drive. But not always. A total of around 1.5 A (1,500 mA) remains for the USB devices. Unless you also use HDMI and Ethernet. Then it's a little less. But certainly enough for most USB devices.
In order to be able to draw the maximum of 1.2 A (1,200 mA) from a USB port, you have to do this in the file "/boot/config.txt" with the line "safe_mode_gpio = 4" (old firmware) or " Enable max_usb_current = 1 "(new firmware) to cancel the maximum current consumption of 600 mA. This limit is there so that the Raspberry Pi does not become unstable and go out with a power-hungry USB device.

Power supply: Raspberry Pi 2 B

A power supply unit with 5.1 V and 2.5 A is recommended for the Raspberry Pi 2 B. It is certainly also below 2.0 A. But if you want to operate other devices besides a mouse and keyboard on the USB ports, then you should use a power supply with a high current if you want to ensure a stable power supply.

Power supply: Raspberry Pi 3 B

Especially with the Raspberry Pi 3 you should know that it has a design flaw. Due to a new circuit board design and savings in the use of inferior components, up to 0.6 volts drop on the way between the micro USB socket and the chip. This means that the Raspberry Pi 3 is operated with a conventional USB power supply unit with 5.1 V at the limit of the undervoltage. The problem is solved by the fact that the official power supply for the Raspberry Pi 3 has a voltage of 5.2 volts.

Note: Due to tolerances in the components and in production, not every Raspberry Pi 3 is affected.

Power supply: Raspberry Pi 4 B

In contrast to the previous models, the Raspberry Pi 4 B needs a USB power supply unit with 5 volts and 3 amps with USB Type-C (plug connection).
You have to note that the USB-C port on the Raspberry Pi 4 B is incorrectly implemented and active USB-C cables (with E-Mark chip) recognize the Raspberry Pi 4 B as an audio adapter and therefore do not output any power .
A simple USB-C cable without the E-Mark chip can help. Such cables are usually included as charging cables for smartphones. The usual cables for notebooks, Thunderbold etc. are not suitable for operating the Raspberry Pi 4 B.
The Raspberry Pi Foundation has announced a revision, which is why there will be newer Raspberry Pi 4 B that do not have this problem.

Power supply: troubleshooting

A red flashing LED on the Raspberry Pi indicates a problem with the power supply. You should take care of it urgently, because then the operation of the Raspberry Pi can no longer be ensured and functional failures can occur. As a rule, the problem can be found in connection with power-hungry USB devices or a low-performance power supply unit.

Back-powering

The Raspberry Pi (1) Model A and Model B have a nice feature. The entire power supply and the USB ports are connected to one line. You can operate back-powering via the USB interface.
You just have to make sure that all protective mechanisms are bypassed. There are also indications that data can be lost on the SD card.
In particular, the operation of active USB hubs on models A and B can inadvertently lead to back-powering, because here the hub feeds the Raspberry Pi through the USB port, although the Raspberry Pi is already fed by a USB power supply unit.
The risks of unintentional back-powering should not be underestimated. In principle, only a USB hub that has been specially designed for operation on a Raspberry Pi can help here.

The right power supply

In order to avoid the many problems with an unstable energy supply, a "real" power supply unit with a stabilized voltage should be used. Buying power supplies for the Raspberry Pi is therefore always a game of chance. To avoid a bad buy, power supplies should only be bought where they are explicitly available as accessories for the Raspberry Pi.

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