We discussed how to flash OS on SD card from different operating system like Windows, Linux or Ubuntu and MAC. Now we want to insert SD card on Raspberry Pi and give proper power supply for Raspberry Pi. This is discussed in this session.
The Raspberry Pi owner needs to give the power supply extra respect. Unlike many AVR class boards, where the raw input voltage is followed by an onboard 5 V regulator, the Pi expects its power to be regulated at the input. The Pi does include onboard regulators, but these regulate to lower voltages (3.3 V and lower).
Figure shown below illustrates the rather fragile Micro-USB power input connector. There is a large round capacitor directly behind the connector that people often grab for leverage. It is a mistake to grab it, however, as many have reported “popping it off” by accident.
The 3.5 W represents a minimum requirement, so we should overprovision this by an additional 50% since peripherals like monitor, keyboard, mouse etc. want additional power:
Allow 50% extra capacity for your power supply.A power supply gone bad may cause damage or many other problems. One common power related problem for the Pi is loss of data on the SD card.
• They may get hot and be a potential fire risk.
In general, look for a power supply that says it can supply 700mA or more. If it specifies a number of watts (W) rather than mA, divide the number of watts by 5 to get the mA figure. So, a 5V 10W power supply can supply 2A (2000mA).
Follow these steps to perform a voltage test:
1. Plug the Raspberry Pi’s micro-USB port into the power adapter’s USB port.
😰Caution Be very careful with your multimeter probes around the pins of P1. Be especially careful not to short the +5 V to the +3.3 V pin, even for a fraction of a second. Doing so will zap your Pi! If you feel nervous or shaky about this, leave it alone. You may end up doing more harm than good. As a precaution, put a piece of wire insulation (or spaghetti) over the +3.3 V pin.
The left side of figure below shows the DMM probes testing for +5 V on header strip P1. Again, be very careful not to touch more than one pin at a time when performing these measurements. Be particularly careful not to short between 5 V and 3.3 V. To avoid a short-circuit, use a piece of wire insulation, heat shrink tubing, or even a spaghetti noodle over the other pin.
The right side of above figure shows the positive DMM probe moved to P1-01 to measure the +3.3 V pin.
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| Inserting SD card on Raspberry Pi |
One of the most frequently neglected parts of a system tends to be the power supply—at least when everything is working. Only when things get weird does the power supply begin to get some scrutiny.
The Raspberry Pi owner needs to give the power supply extra respect. Unlike many AVR class boards, where the raw input voltage is followed by an onboard 5 V regulator, the Pi expects its power to be regulated at the input. The Pi does include onboard regulators, but these regulate to lower voltages (3.3 V and lower).
Figure shown below illustrates the rather fragile Micro-USB power input connector. There is a large round capacitor directly behind the connector that people often grab for leverage. It is a mistake to grab it, however, as many have reported “popping it off” by accident.
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| Inserting a micro USB for power. Micro USB power input and capacitor shown in red circle. |
Calculating Power
Sometimes power supplies are specified in terms of voltage, and power handling capability in watts. The Pi’s input voltage of 5 V must support a minimum of 700 mA (Model B). Let’s compute a power supply figure in watts (this does not include any added peripherals):
As per Ohm's law,
P = V × I
=5×700mA
= 5 × 0.7 ;since 700mA=0.7A
= 3.5 W
The 3.5 W represents a minimum requirement, so we should overprovision this by an additional 50% since peripherals like monitor, keyboard, mouse etc. want additional power:
P = 3.5 × 1.50
= 5.25 W
The additional 50% yields a power requirement of 5.25 W.
Allow 50% extra capacity for your power supply.A power supply gone bad may cause damage or many other problems. One common power related problem for the Pi is loss of data on the SD card.
Current Requirement
Since the power supply being sought produces one output voltage (5 V), you’ll likely see adapters with advertised current ratings instead of power. In this case, you can simply
factor a 50% additional current instead:
Isupply = IPi × 1.50
= 0.700 × 1.50
= 1.05 A
To double-check our work, let’s see whether this agrees with the power rating we computed earlier:
P = V × I
= 5 × 1.05
= 5.25 W
The result does agree. You can conclude this section knowing that you minimally need a 5 V supply that produces one of the following:
• 5.25 W or more
• 1.05 A or more (ignoring peripherals)
Supplies that can meet either requirement, should be sufficient. However, you should be aware that not all advertised ratings are what they seem. Cheap supplies often
fail to meet their own claims, so an additional margin must always be factored in.
To double-check our work, let’s see whether this agrees with the power rating we computed earlier:
P = V × I
= 5 × 1.05
= 5.25 W
The result does agree. You can conclude this section knowing that you minimally need a 5 V supply that produces one of the following:
• 5.25 W or more
• 1.05 A or more (ignoring peripherals)
Supplies that can meet either requirement, should be sufficient. However, you should be aware that not all advertised ratings are what they seem. Cheap supplies often
fail to meet their own claims, so an additional margin must always be factored in.
Peripheral Power
Each additional circuit that draws power, especially USB peripherals, must be considered in a power budget. Depending on its type, a given USB peripheral plugged into a USB 2 port can expect up to 500 mA of current, assuming it can obtain it. (Pre Rev 2.0 USB ports were limited to 140 mA by polyfuses.)
Wireless adapters are known to be power hungry. Don’t forget about the keyboard and mouse when used, since they also add to the power consumption. If you’ve attached an RS-232 level shifter circuit (perhaps using MAX232CPE), you should budget for that small amount also in the 3 V supply budget. This will indirectly add to your +5 V budget, since the 3 V regulator is powered from it. (The USB ports use the +5 V supply.) Anything that draws power from your Raspberry Pi should be tallied.
Model B Input Power
The Raspberry Pi’s input voltage is fixed at exactly 5 V (±0.25 V). Looking at the schematic in figure below, you can see how the power enters the micro-USB port on the pin marked VBUS. Notice that the power flows through fuse F3, which is rated at 6 V, 1.1 A. If after an accidental short, you find that you can’t get the unit to power up, check that fuse with an ohmmeter.
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| Raspberry Pi Model B Rev 2.0 input power |
If you bring the input +5 V power into the Pi through header P1, P5, or TP1, for example, you will lose the safety of the fuse F3. So if you bypass the micro-USB port to bring in power, you may want to include a safety fuse in the supplying circuit.
Figure shown below is the 3.3 V regulator for the Pi. Everything at the 3.3 V level is supplied by this regulator, and the current is limited by it.
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| 3.3V power |
Model A Input Power
Like the Model B, the Model A receives its power from the micro USB port. The Model A power requirement is 300 mA, which is easily supported by a powered USB hub or desktop USB 2 port. A USB 2 port is typically able to supply a maximum of 500 mA unless the power is divided among neighboring ports. You may find in practice, however, that not all USB ports will deliver 500 mA.
As with the Model B, factor the power required by your USB peripherals. If your total nears or exceeds 500 mA, you may need to power your Model A from a separate power source. Don’t try to run a wireless USB adapter from the Model A’s USB port if the Pi is powered by a USB port itself. The total current needed by the Pi and wireless adapter will likely exceed 500 mA. Supply the wireless adapter power from a USB hub, or power the Pi from a 1.2 A or better power source. Also be aware that not all USB hubs function correctly under Linux, so check compatibility if you are buying one for that purpose.
3.3 Volt Power
Since the 3.3 V supply appears at P1-01, P1-17, and P5-02, it is useful to examine figure shown above(shown previously) to note its source. This supply is indirectly derived from the input 5 V supply, passing through regulator RG2. The maximum excess current that can be drawn from it is 50 mA; the Raspberry Pi uses up the remaining capacity of this regulator.
Powered USB Hubs
If your power budget is stretched by USB peripherals, you may want to consider the use of a powered USB hub. In this way, the hub rather than your Raspberry Pi provides the necessary power to the downstream peripherals. The hub is especially attractive for the Model A because it provides additional ports.
Again, take into account that not all USB hubs work with (Raspbian) Linux. The kernel needs to cooperate with connected USB hubs, so software support is critical.
Power Adapters
This section pertains mostly to the Model B because the Model A is easily supported by a USB 2 port. We’ll first look at an unsuitable source of power and consider the factors for finding suitable units. During selecting Power adapters you should kept following things on your mind.
The basic electrical specification for a power supply suitable for a Raspberry Pi is that it supplies a regulated 5V DC (direct current).
The amount of current that the power supply must be capable of providing depends both on the model of Raspberry Pi and the peripherals attached to it. It is worth getting a power supply that can easily cope with the Raspberry Pi and you should consider 700mA to be a minimum.
If you buy your power supply from the same place that you buy the Raspberry Pi, then the seller should be able to tell you if it will work with the Raspberry Pi. If you are going to be using a WiFi dongle or other USB peripherals that use significant amounts of power, then I would get a power supply capable of 1.5A or even 2A. Also beware of very low-cost power supplies that may not provide an accurate or reliable 5V.
The power supply and connector are actually the same as those found in many smart phone chargers. If they are terminated in a micro USB plug, then they are almost certainly 5V (but check). The only question, then, is if they can supply enough current.
If they can’t, then a few bad things can happen:
• They may get hot and be a potential fire risk.
• They may just fail.
• At times of high load (say, when the Pi is using a WiFi dongle), the voltage may dip and the Raspberry Pi may reset itself.
In general, look for a power supply that says it can supply 700mA or more. If it specifies a number of watts (W) rather than mA, divide the number of watts by 5 to get the mA figure. So, a 5V 10W power supply can supply 2A (2000mA).
Using a power supply with, say, a maximum current of 2A will not use any more electricity than a 700mA power supply. The Raspberry Pi will just take as much current as it needs.
In figure shown below, I measure the current taken by a Raspberry Pi model B and compare it with a Raspberry Pi 2 model B.
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| Raspberry pi current consumption during booting |
The newer Raspberry Pis (the A+, B+, or Raspberry Pi 2) are far more power-efficient than the older models, but when the processor is fully occupied and has a lot of peripherals attached, they can still reach similar current requirements.
In above figure, you can see that the current rarely gets above 500mA. However, the processor isn’t really doing very much here. Were you to start playing HD video, the current would increase considerably. When it comes to power supplies, it’s usually better to have something in reserve.
Voltage Test
If you have a DMM or other suitable voltmeter, it is worthwhile to perform a test after powering up the Pi. This is probably the very first thing you should do, if you are experiencing problems.
Follow these steps to perform a voltage test:
1. Plug the Raspberry Pi’s micro-USB port into the power adapter’s USB port.
2. Plug in the power adapter.
3. Measure the voltage between P1-02 (+5 V) and P1-25 (Ground): expect +4.75 to +5.25 V.
4. Measure the voltage between P1-01 (+3.3 V) and P1-25 (Ground): expect +3.135 to +3.465 V.
😰Caution Be very careful with your multimeter probes around the pins of P1. Be especially careful not to short the +5 V to the +3.3 V pin, even for a fraction of a second. Doing so will zap your Pi! If you feel nervous or shaky about this, leave it alone. You may end up doing more harm than good. As a precaution, put a piece of wire insulation (or spaghetti) over the +3.3 V pin.
The left side of figure below shows the DMM probes testing for +5 V on header strip P1. Again, be very careful not to touch more than one pin at a time when performing these measurements. Be particularly careful not to short between 5 V and 3.3 V. To avoid a short-circuit, use a piece of wire insulation, heat shrink tubing, or even a spaghetti noodle over the other pin.
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| Measuring Voltages |
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