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I have Raspberry Pi 3 B+,and I'm considering upgrading to a Raspberry Pi 400. I understand that it is recommended to use a 5.1V 3A power supply, but can I use the 5V 2.5A supply I already have?

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  • Not sure that this is an upgrade (a Pi4 is as upgrade). One thing you need to consider is the Pi4 (and I assume Pi400 use a USB-C power connector, so you would need an adapter.
    – Milliways
    Commented Dec 28, 2021 at 1:00

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Yes - you can power your Pi 400 with a 2.5A supply, but you will be limited on how much current your USB devices can draw. There's a table in the "official" documentation that lists the Typical Power Requirements for all RPi models.

As you can see, 3.0 amps are recommended for a power supply to support the RPi 400, and this figure includes 1.2A for USB peripherals, and 0.8A for bare board. Since the RPi 3B+ supply is rated at only 2.5A, you should plan on reducing the USB peripheral load on your 400 by 0.5A (to 0.7A) to maintain the same "margin."

Be sure to read the manufacturer's notes beneath the table; in particular the final sentence If you’re not sure [of the power rating of the devices you plan to connect to the Raspberry Pi], we would advise you to buy a powered USB hub. In this respect, your situation is no different than if you had a 3.0 Amp power supply: This table, and the accompanying notes, provide guidelines to help you manage your power budget; the only difference really is that your power budget with a 2.5A supply is a bit smaller. You can overload a 3.0 Amp supply just as you can a 2.5 Amp supply.

You should understand one likely consequence of drawing too much power from your supply is that its output voltage will drop below the low voltage threshold of 4.63V. If you're concerned about breaching this threshold, you should keep a watch for the "lightning bolt" display on the monitor, or some of the other low voltage indicators.

If you want to go a bit further, you may also wish to read the documentation on vcgencmd; in particular the get_throttled command which will provide a bit pattern (binary) value indicating the occurrence (past or present) of low voltage conditions. And finally, if you're interested in how the RPi detects a low voltage condition, you can read the analysis in this answer.

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  • This is fallacious. The USB current is limited by a current limiter, not the SOC power budget.
    – Milliways
    Commented Dec 28, 2021 at 3:43
  • @Milliways: I don't agree with your comment, but I can certainly be convinced with sound logic and references. All of the power must come from the Supply. If some of that power is not going to the USB, then it is available for the bare board. That has nothing to do with current limiters on USB peripherals. I maintain the 400 can be powered with a 2.5A supply, but the budget for peripherals must be reduced.
    – Seamus
    Commented Dec 28, 2021 at 4:03
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Current consumption by a Pi 4 with a 7" screen and a keyboard is about 1.25 A.

This is not the same as a Pi 400, but should give you a general idea that the device will run. However, USB current draw from devices that you plug in will be limited.

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