This is a summary of some load tests I did on the ATXRaspi. People often ask: How much load can the ATXRaspi support?
I used the following hardware, and I wanted to see how much current the official DSI display will draw in addition to the Pi.
- RaspberryPi3, Model B v1.2 (Raspbian Jessie, no overclocking)
- small 0.2A fan that is a popular accessory and often used with the Pi
- the official Pi Foundation DSI Display
I threw in the 0.2A fan just to put an extra strain on the power supplies. Here are the 3 supplies I tried, and while all could run this setup without instability issues, the Under Voltage Warning (UVW) would come up at times, especially on the Apple supply which would show it constantly and the 5V GPIO rail would stay at ~4.8V. The best was the CanaKit 5V/2.5A, with this supply the UVW would only appear intermittently during boot, and would settle at ~5.05V after boot. So the CanaKit supply was used for the following tests.
The trusty Fluke 87V was used to measure the LCD amps and the GPIO 5V rail during boot to a console. Here is the setup and average mAmps used by the LCD at boot time:
Here are the peak recorded mAmps used by the LCD, note that this MAX was registered during (re)booting. The Pi LCD settles to ~450mA after booting is complete.
And here are the measured Volts with the CanaKit 5v/2.5A supply, again note that the 4.93V was recording during booting, after which it settles to ~5.0V:
A basic scope roll measurement was done to look at any MIN/MAX peaks and overall 5V rail behavior, note how the voltage starts at ~5.2V then the LCD starts up and voltage goes to a stable ~5V where it settles during/after boot, otherwise no odd dips or spikes:
Here is the Under Voltage Warning that might show up. With the CanaKit supply this showed intermittently only during boot. With the other supplies it was more apparent and with the Apple supply it was constantly showing all the time. Without the LCD this warning would never show with the CanaKit supply, but would show with the others.
Even so, the Pi behaved normally with all these supplies and there was no observed instability.
According to official Pi release notes, the UVW shows when the 5V voltage rail drops below 4.65V. In the tests above this condition could not be recorded with any of the supplies tested, either with an oscilloscope (roll 128ksps) or with the Fluke87V in Voltage Peak Detect Mode (1ksps+). Even with the worst performer (Apple supply) the MIN peak was around 4.7V and settled around 4.85V after boot.
The conclusion is that ATXRaspi supports quite a hefty load, but to avoid the Under Voltage Warning you really need a very good supply, at least 2.5A.
This content was also posted under the official ATXRaspi Guide.