I have a solid PSU putting out 5.4VDC capable of 60amps/300watts. I have 5.4VDC going into the Pi3, but get the lightning bolt/undervoltage condition. On Pins 2 & 6 on the GPIO I'm only getting ~4.5/4.6 VDC. I am measuring with a Fluke 289, not some $5 meter. Is there some diode between the USB port and the GPIO causing the voltage drop? (I'm going to make a cable to rule that out.)

Edit: Yup was cable. (I hate making cables). Anyone got a line on micro USB power only cables with a heavier gauge? Say 22 or 24? Right angle would be killer.

  • Glad to hear you've sussed the cause of your problem! If you would, please "tick" the check mark in the answer - this will help others. And btw, "shopping requests" are not allowed here - questions are typically closed by the admins if they contain shopping requests. But... I recall reading that some USB cables will have a wire gauge as large as 20AWG, but I've never been able to find one. I think your best bet is to go with a "name brand" manufacturer as they have spec sheets that will disclose the AWG ratings of the power wiring. – Seamus Dec 1 '20 at 20:30
  • See Raspberry Pi Power Limitations and DO NOT exceed 5.25V – Milliways Dec 1 '20 at 23:40
  • @Milliways I'm not sure where this 5±0.25V advice comes from, but I can tell from experience that 4.75V is too low. – Dmitry Grigoryev Dec 2 '20 at 10:43
  • Does this answer your question? Raspberry Pi Power Limitations – Dougie Dec 2 '20 at 21:02

There is a voltage drop in the wires and contact points of the connector. Measure the voltage between PSU ground and Pi ground, and you will likely see they are off by about half a volt: that's 1/2 of the voltage you lose. Another 1/2 is lost between PSU +5V and Pi +5V.

Your cable seems to be quite bad. Find a shorter cable with thicker wires. And lower the PSU voltage if you can: 5.4V is really on the limit of what you can feed to the Pi without risking to damage it.

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