Don't think I've seen it mentioned specifically, but presumably you are using the USB C connector as power input. Does seem a rather high voltage drop to me. Not done a full test (i.e. sufficient to print real numbers), but I think I'm seeing barely any voltage drop on a Pi5 with SSD. What measurement points are you using?1) With an RPi5 8GB that is off and has no periphery connected and no SD card inserted, I measure the voltage on the GPIO 5V pin at 5.05V.
When pressing the "on" button, the voltage on the GPIO 5V pin drops to 4.7V almost immediately.
This happens for two different RPi5 (purchased from different vendors) and also with 2 other non-Raspberry power supplies that are rated at >25W@5.1V.
Sounds perfectly possible. Basically there are two methods of overcurrent protection in power supplies generally:2) When having a buffer capacitor (for short power-out coverage) connected to the 5V GPIO, the RPi5 power supply does not provide any current after getting plugged into the wall socket, while the other non-Raspberry power supplies do (with a current limited to about 5A as expected based on the specification). Is this due to some overcurrent protection in the official power supply that shuts it down completely instead of limiting the current (which just is so high before the capacitor reaches a certain voltage)?
a) Simple current limit, where the supply switches from its normal 'constant voltage' operation to a 'constant current' mode, during which the output voltage drops. Can cause problems because anything connected to the power rail will likely be operating outside its specified limits, so there are no guarantees what will happen.
b) 'Foldback' current limiting - where if the rated current is exceeded for a short time, the output turns off completely. In some cases you have to turn the supply input off then on again for the PSU to reset; in others the output will be turned on at intervals to see if the problem causing the overcurrent has cleared (often called 'hiccup' mode). [I've actually encountered this problem, where the equipment we were powering had a switch mode power supply that had a big inrush current at startup. We solved it by having an adjustable time where the output was forced on durung a retry, coupled with a constant current limit. The sad thing was that the switchmode IC used in the equipment actually had a pin to limit the inrush current, which the designers had chosen not to use.]
Statistics: Posted by stevend — Sat Nov 29, 2025 10:45 pm