I want to convert my simple printer into a network printer with CUPS on Raspberry Pi. I tested everything with Raspberry Pi 4 and it worked perfectly. Now, I want to fit my Raspberry Pi zero inside the printer. My plan is to connect D+ and D- of the printer's USB directly to the D+ and D- of Raspberry Pi's USB port and find some 5V pad on the printer's PCB to power the Raspberry Pi from that. After testing some pads with the multimeter, I found both 3.3V and 5V on the printer's PCB. First I connected 5V and GND pads to the Raspberry but it the Pi didn't boot. When I checked the voltage across Pi's power line, it was not stable and kept changing from 2.5V to 4.2V, I disconnected the 5V and GND from Raspberry Pi and measured it again, but it was stable 5V this time. As soon as I connect it to the 5V pin of Raspberry Pi, it starts changing. Then I connected 3.3V pad on the printer's PCB to the 5V pin of Raspberry Pi. A similar thing happened again. The 3.3V pad on printer's PCB was stable before connecting it to the Pi, but as soon as I connected it to the Pi, it dropped to 1.9V, Please let me know what what's the issue. Thanks.
All the current for every Pi
My guess is that the 5V and 3,3V from the printer PCB don't meet the minimum requirement of supplied current, and thus the voltage drops. The bare board alone uses 150mA typical. If you use the non W/WH model it is still 100mA.