Hardware Reference
In-Depth Information
How well could we have done if we used the LM7805 regulator? The following is
a best case estimate, since I don't have an actual current reading for that scenario. But
we do know that at least as much current that flows out of the regulator must flow into it
(likely more). So what is the absolute best that the LM7805 regulator could theoretically
do? Let's apply the same current draw of 410 mA for the Raspberry Pi at 5.10 V, as shown
in Table
2-3
. (This was operating without HDMI output in use.)
Table 2-3.
Hypothetical LM7805 power use
Parameter
Input
Output
Units
Voltage
7.1
5.10
Volts
Current
0.410
0.410
Amps
Power
2.91
2.09
Watts
The power efficiency for this best case scenario amounts to this:
209
291
.
.
=
0 718
.
The absolute best case efficiency for the LM7805 regulator is 71.8%. But this is
achieved at its
optimal
input voltage. Increasing the input voltage to 12 V causes the
power dissipation to rise considerably, resulting in a 42.5% efficiency (this calculation is
left to the reader as an exercise). Attempting to operate the LM7805 regulator at 15.13 V,
as we did with the buck converter, would cause the efficiency to drop to less than 33.7%.
Clearly, the buck converter is much more efficient at converting power from a higher
voltage source.
Signs of Insufficient Power
In the forums, it has been reported that ping sometimes doesn't work from the desktop
(with HDMI), yet works OK in console mode.
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Additionally, I have seen that desktop
windows can freeze if you move them (HDMI). As you start to move the terminal window,
for example, the motion would freeze part way through, as if the mouse stopped working.
These are signs of the Raspberry Pi being power starved. The GPU consumes more
power when it is active, performing accelerated graphics. Either the desktop freezes (GPU
starvation) or the network interface fails (ping). There may be other symptoms related to
HDMI activity.
Another problem that has been reported is resetting of the Raspberry Pi shortly after
starting to boot. The board starts to consume more power as the kernel boots up, which
can result in the Pi being starved.
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If you lose your Ethernet connection when you plug in a USB device, this too may be
a sign of insufficient power.
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