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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. enter image description here

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enter image description here
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.

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    \$\begingroup\$ Agreed, and it's unlikely that the printer board has that much spare power capacity. \$\endgroup\$ – Transistor Nov 18 '20 at 9:06
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    \$\begingroup\$ This is most likely the reason, though OP is using Pi Zero (possibly W to make network printer); here you will find Pi Zero W power draw: raspberrypi.stackexchange.com/questions/63519/… \$\endgroup\$ – Jack Nov 18 '20 at 9:07
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    \$\begingroup\$ Ah, Don't know why I didn't think about that? I think I will integrate a USB adapter inside the printer. i.e. Mains voltage to the adapter and then adapter powers the Pi. Everything inside the printer. \$\endgroup\$ – Zeeshan Dhillon Nov 18 '20 at 9:21
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    \$\begingroup\$ You need the common connected. \$\endgroup\$ – JonathanG Nov 18 '20 at 9:42
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    \$\begingroup\$ One other thing that might work is looking elsewhere inside the printer for another rail, and possibly using a high efficiency DC/DC converter. I'm sure that there will be rails on there that can provide more (e.g. if there are any DC motors) -- have a search around. That may be safer than altering the mains input. \$\endgroup\$ – Landak Nov 18 '20 at 13:34

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