# Voltage drop at 5V pin when drawing current with DC motor

I encounter a power behavior for my Raspberry Pi 4B that I struggle to explain. I try to power a (geared) DC motor using the 5V pin. DC motor is rated for 3-12V (preferred 6-8V) and is claimed to draw <150mA. From what I have read (Raspberry Pi Power Limitations), the 5V pin is directly linked to the power supply (here an official Raspberry Pi power supply). I would therefore expect a steady 5V.

When connecting the motor to the 5V / GND pins (through a breadboard), the voltage measured on the motor drops to ~3.9V and the motor starts. I tried to boost the motor using a MT3608 DC-DC boost, knowing that even targeting 10V on the motor and assuming a max current drawn of 200mA, I would be far from the theoretical minimum of ~550mA (How many current can I put in the 5V pin?). To make it work, I need to boost the output voltage quite high (~9V) to erratically get it started (I blame the breadboard bad connections for this). Then the output voltage of the MT3608 drops down to ~4.5V and its input voltage (which should be 5V) drops down to ~3.8V. The motor is still more sluggish than when directly connected to a simple 9V battery.

What did I miss with the Raspberry 5V pin / power supply, and how can I properly power the DC motor? Is it related to the intrinsic torque/current/voltage property of the DC motors?

• Why not power the motor with an external power supply? You have not mentioned your motor driver. Some can drop a volt or more between input and output so 5V to 3.9V is not that unlikely. I do not see this as a Pi specific question.
– joan
Commented Jul 4, 2022 at 19:45
• For the convenience of using only one power supply/plug if it's possible (the place where I intend to use it has only one wall socket, and the math seems to add up for powering it with the Pi). Regarding the driver, I plan to use a DRV8833, but for debug purpose I tried first ditectly on the 5V pin. If the 5V comes directly from the USB 5V, then I guess my issue is with the power supply. Commented Jul 4, 2022 at 20:48
• You can use a power strip to get more plugs. In addition, it is a very bad idea to power a motor directly from a pi, because it can destroy your pi. I learned this several years ago when I was into robotics. Commented Jul 4, 2022 at 21:42
• A power supply the Raspberry Pi is NOT!
– Gil
Commented Jul 5, 2022 at 0:46