0
\$\begingroup\$

I'm working on an RPi-Pico (Adafruit Feather RP2040) project that runs off of computer USB power (5V/500mA-1A).

I need to simultaneously move 3 motors that run at 6V-8V/250mA, but only for approximately 1 second.

So far I've put together a Falstad circuit that almost does what I want it to do. I haven't added any boost converters to go up to the 6V-8V/250mA, but (as far as I know) the watts are high enough when the switch is open.

The problem I'm having is the microcontroller I'm using can only output 5V/500mA max, and I can't get the circuit tuned where the Pico is only outputting 500mA. Currently it steady-states at about 2.7A.

Ultimately what I'm trying to do:

Switch Open:

  • 5V/500mA through the RPi
  • 6V-8V/250mA for 1 second through the motor load

Switch Closed:

  • 5V/250mA through the RPi

falstad circuit

\$\endgroup\$
5
  • \$\begingroup\$ I assume you're powering the Raspberry Pi via USB? If you're not married to USB power for some reason it may work better to use a supply that's powerful enough for the motors, power the Pi off of that, and control the motors with the Pi. \$\endgroup\$
    – TimWescott
    Mar 2 at 0:49
  • 1
    \$\begingroup\$ @TimWescott unfortunately the goal is for it to be a computer accessory, so I am married to computer USB power \$\endgroup\$
    – Emmett Janczak
    Mar 2 at 1:39
  • \$\begingroup\$ usb won't give you more than 500ma if you also use it for communication. If you don't use it for communication, then use a generic usb socket instead of the built-in one, giving you a raw 5v, which you can feed into a pin to power the pico and drive the motors aside the pi. \$\endgroup\$
    – dandavis
    Mar 2 at 6:22
  • \$\begingroup\$ @TimWescott I do intend to also be using communication through USB. I'm also limited to 500mA by the pi itself, it has a maximum, momentary output of 500mA. As far as I can tell, I won't be able to escape the 5V/500mA without using another power source (which ideally I wouldn't do) \$\endgroup\$
    – Emmett Janczak
    Mar 2 at 15:01
  • \$\begingroup\$ "Currently it steady states at about 2.7A". This contradicts three motors at 250mA (which should add up to 750mA). I think you need to edit your question with more detail. Do you mean that your motors consume 2.7A forever, or for one second, or what? Why does it consume almost four times more than your motor current figure suggests? If you can't answer that directly, you may want to include a schematic of how you're driving your motors, and perhaps even comment on what the motors are driving (because that affects the current they'll pull). \$\endgroup\$
    – TimWescott
    Mar 2 at 19:04

1 Answer 1

Reset to default
0
\$\begingroup\$

A 6V cap loaded by 250 mA is equivalent to 24 ohms might have a coil Rs of 2~3 ohms that defines the surge start. A supercap, if added to sustain a 1V/s sag would require Ic=CdV/dt or 250 mF.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.