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I need to power an external device which has two pins of 1.2v and two of 3.3v. Is it possible to power them with the PI?

Power Specifications

Source                      Min         Typ     Max         Comments
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3V3 voltage                 3.135 V     3.3 V   3.4651 V |  3.3V ±5%
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3V3 active supply current               850 mA  1100 mA  |  Design for 1.5 A peak
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3V3 supply ripple                               50 mV    |  Peak to peak
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1V2 voltage                 1.14 V      1.2 V   1.26 V   |  1.2 V ±5%
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1V2 active supply current               165 mA  200 mA   |  1V2 supply ripple
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                                                25 mV    |  Peak to peak
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The user with no hat
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1 Answers1

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RPi has 5V and 3V3 power outputs exposed on the GPIO. Since your board needs to be ready for 1.5A max current on the 3V3 input, you cannot use the RPi 3V3 power output as it has a low current rating. You'll end up having to do some transforming: 5V to 3V3 (the RPi 5V supply can handly 1.5A provided you have a robust power supply rated 3A or more) and 3V3 to 1V2 (either from the 3V3 source you get from 5V, or separately from the RPi's 3V3 output). Then make sure you wire it up according to the described implementation.

Phil B.
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    Are you sure there's a point in using a supply bigger than 2 or 2.5A? I thought the total 5V supply was regulated to that point by the main polyfuse: http://raspberrypi.stackexchange.com/a/51616/5538 Also there will probably be a not-insignificant voltage drop from the 5V rail if you pull 1.5A from it: https://raspberrypise.tumblr.com/post/144555785379/exploring-the-33v-power-rail (see figure 2). I suppose there's no harm in testing all this though, and you can take any drop into account. – goldilocks Sep 17 '16 at 13:23
  • Hmmm good point - I thought the polyfuse was regulating only the supply to the RPi itself (including USB), not the GPIO 5V pins. Must have been wrong then. Still nothing stopping OP from tapping into the same power supply for his extension board instead of using the RPi 5V pin :) – Phil B. Sep 17 '16 at 15:17
  • I believe you can bypass that fuse if you power the pi itself from one of those pins -- but of course then it doesn't make much sense to daisy chain the other board. – goldilocks Sep 17 '16 at 15:21