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Disclaimer: I have no formal electronics training.

I am reading this book: Raspberry Pi Zero Cookbook [2017]

Here's the paragraph from that book that's got me confused:

The Raspberry Pi documentation allows up to 2.5 A for the Pi's power supply, in case you don't want to use a powered USB hub. For this cookbook, we will start with a powered USB hub but work toward low-power and low-profile solutions.

I was under the impression that, from an abundant supply of amps, a device will draw only the amps it needs. But the first sentence above implies that you should not connect a power source to the Zero board that is more than 2.5 amps. Can you please set me straight?

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marked as duplicate by Dave Tweed May 30 '17 at 20:30

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    \$\begingroup\$ You impression is correct - a device will draw the current it requires. \$\endgroup\$ – JIm Dearden May 30 '17 at 19:56
  • \$\begingroup\$ @JImDearden Thanks. So the book's wording is misleading, correct? There should be no upper limit for how much amps a power supply provides. \$\endgroup\$ – Sabuncu May 30 '17 at 19:58
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    \$\begingroup\$ It only draws as much as it needs when operating normally. If you give it a power supply capable of 50A, you can turn an annoying short circuit into a catastrophic incident with sparks and burning electronics and flying shards of plastic. \$\endgroup\$ – JRE May 30 '17 at 20:02
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    \$\begingroup\$ The documentation refers to how big of a supply is needed if the Pi was to draw maximum current. This says that at absolute maximum the Pi will draw is 2.5A and at first to provision a supply that can handle this. Later the documentation will show how to get away with a smaller supply. \$\endgroup\$ – vini_i May 30 '17 at 20:15
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    \$\begingroup\$ It depends on the load. Ordinarily, a load is a voltage device preferring a constant voltage (or even a variety) and self-regulating to draw only the current that it needs, such as that Raspberry Pi. In that case a power supply rated for higher current is fine. However, there are current devices - notably LED emitters and moreso fluorescent, neon, mercury vapor, metal halide, sodium etc. bulbs, which want a specific current and need the power supply to deliver and limit that current. You need a very special constant-current power supply for those. \$\endgroup\$ – Harper May 30 '17 at 20:57
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You are correct. The load normally dictates the current. I say normally, since a fault condition can take whatever you are plugged into can supply.

Cost and size usually dictate what kind of power supply we want to use for a particular application. You want it big enough to have enough margin for tolerances, but not so large that you are paying extra or requiring a larger enclosure.

Sometimes, for simpler circuits, we can calculate the required currents. However it is also common for larger boards to build and test circuits with larger supplies, eg, bench supplies so we know exactly the kind of currents we need. Then design, or chose, the appropriate power supply to drive it. I think that is what your quote is alluding to. That is..The Raspberry Pi Zero board will draw a maximum of 2.5 A.

Another factor you need to consider is that regulator circuits, especially switchers, do not have a flat efficiency curve. Too large a supply can run at horrendous efficiency numbers with a lot of ripple.

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  • \$\begingroup\$ Thanks. Then, would it have been better (and correct) had the book had specified "The Raspberry Pi documentation allows a minimum of 2.5 A for the Pi's power supply..."? \$\endgroup\$ – Sabuncu May 30 '17 at 20:12
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    \$\begingroup\$ @Sabuncu NO.. it means the most your circuit will ever take is 2.5A, so you need to start out with one that big. Once you have your design knocked out you may be able to cut that back to 1A or less.. SO it's kind of a max and a min... if you see what I mean. \$\endgroup\$ – Trevor_G May 30 '17 at 20:13
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    \$\begingroup\$ OK, I understand the book's wording now, but only after you explained it. The book should have said: "The Raspberry Pi Zero board will draw a maximum of 2.5 A." The way the book currently reads is poorly worded. It incorrectly implies that a power supply used for the board may only supply up to 2.5 A. \$\endgroup\$ – Sabuncu May 30 '17 at 20:18

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