Hey guy,
Octopi is running on a RasPi 3B+ and a 3.5" Display is attachet.
First I was using a 2A PS and got a message "low power" and this flashing flash 
After that I swaped to a 3.5A PS ... now it is not flashing, but still there ...
My Amp-Meter shows 0.75A ...
What's going on there?
Cappy
I find this hard to believe that you've found a microUSB-based 5V 3.5A power supply that actually delivers 5V @ 3.5A to the Pi.
For example, look at this ridiculous product description on Amazon.
"Pwr Replacement Charger 5V 2A 2.5A 3A 3.5A for Raspberry Pi 2 3 B+ Power Supply Fast Micro-USB Rapid Charger Compatible with Raspberry-Pi Power Adapter Extra Long Cord USA UL Listed 2Y Warranty"
It's got both "power supply", "power charger" as well as "2A", "2.5A", "3A" and "3.5A".
At least they were honest when they showed all their marketing/sales people who laugh when you don't realize that they're lying.
What's crazy to me is that some engineering type is giving this the thumb's up, suggesting that he's laboriously tested it. One of the paragraphs though suggests:
"For those of you who don't have engineering degrees, the gist is this: The supply delivers 3.5A at just over 4.65V, which is more than enough to power an overclocked Raspberry Pi 3B+ plus attached USB devices and fan, without any of the dreaded "lightning bolts" appearing. To be honest, 3.5A is really pushing the limits of the microUSB connector. That's your bottleneck, as the contacts are so small they add appreciable resistance, which lowers your voltage as the current increases."
Bullshit. 4.65V is 0.01V above the minimum threshold for any of the Pi computers. It needs to deliver 5.0V. Period.
Are you using a standard microUSB cable from the charger/ps to the pi? If so, it's the cable. That's why the official pi power supplies have their own dedicated cable. A standard microUSB cable has very fine wires that struggle to carry even one amp
Hi,
yes I am using a standard cable ...
I will check it.
But ... I we have a loss off voltage at the cable ... the current must go up, right?
Is it possible to connect the power somewhere at the port directly?
Cappy
If you're familiar with that character Tim Allen of Home Improvement, he was fond of super-sizing all sorts of things. This sounds a bit like that, of course.
The standard "wisdom" with respect to bypassing the microUSB power connector on a Raspberry Pi 3B computer would be to bring the power directly to either the GPIO pins or by soldering it directly to the motherboard. I will note that the gauge of wire to bring in 5V @ 3.5A is usually bigger than the average consumer products that you'll find.
If this were me and I wanted to rig something (perhaps temporarily), I might then have two red wires (BOARD pins 2 & 4) and two black wires (BOARD pins 6 & 9). I'd do this since I have a stach of F/M and F/F jumper cables which fit these. By using two pair, you'd be bringing in the power over more copper.
If you're attempting to solder 5V/ground to the motherboard, it isn't the friendly-looking RUN pair of holes next to the Type A USB connectors, that's for a reset switch. Your safest ground to solder to would be any of the eight larger mount holes for the two pair of Type A USB connectors; they are all grounded themselves. Your best bet for 5V on the motherboard is marked PP7 and it's near the microUSB connector.
18AWG is probably the required wire size in your situation.
