Rasp Friend
Posts: 2
Joined: Tue Nov 05, 2019 4:57 pm

Compact Power Supply for Pi and Hat

Tue Nov 05, 2019 5:15 pm

Hello! I am making a robotics project with a Pi 3 and this servo motor hat: https://www.adafruit.com/product/2327
I want to power two servos, like so: https://www.adafruit.com/product/169

I will evidently need two power supplies, one for the Pi and one for the hat. An external powerbar sounds like the most straight forward choice and indeed that would be sufficient. However, I was wondering if an alternative choice exists, such that I could have only one power plug exiting the robots body. I'm thinking of using this: https://www.cableleader.com/14-power-co ... 5-15p.html

My question is that would it be ok to have everything (Pi + Hat + transformers + splitter) plugged-in inside a wooden box?
Is there a reason why this plan wouldn't work, like concerns of overheating?
Is there any other options to consider, like another IC module that can combine power supplies?

dustnbone
Posts: 201
Joined: Tue Nov 05, 2019 2:49 am

Re: Compact Power Supply for Pi and Hat

Tue Nov 05, 2019 10:32 pm

You could just use a single large 5V supply, as that's the operating voltage of both the Pi and the servo controller.

The page you linked to for the servo board shows a 5V 4A supply on the right as a suggested item, that is probably sufficient but if you're planning to run many/large servos and/or other peripherals connected to (and powered by) the Pi you might need something bigger.

Sparkfun sells a 5V 20A Mean Well unit for $25, Mean Well are a well known brand for industrial electronics.

https://www.sparkfun.com/products/14098

I'm guessing they make smaller 5V supplies, you'll probably have an industrial electronics supplier nearby if you live in a city of any size, they will be able to help you get a appropriate supply if you don't want to wait for one in the mail.

As for cooling, dry wood is a pretty good insulator. You're dealing with fairly low power levels, dissipation inside the box will be on the order of 10-25W as a guess, depending on servo activity, the amount of processing, etc the Pi is doing, and the efficiency rating of any power supplies inside.

So I'd expect overheating to be a potential issue inside a small all wooden box, yes. If you can at least have some kind of ventilation in the form of a grate or screen you'd probably be ok. That's way better than zero.

Another way to handle it, if you do want to keep it sealed up, is to have at least some part of the box constructed of a conductive material. A thin sheet of metal (copper is best, aluminium pretty good) in place of a section of wood will drastically increase heat dissipation. Especially if you can mount a few of the warmer bits (like the power supply) to it either directly or with some kind of thermal interface pad between.

Rasp Friend
Posts: 2
Joined: Tue Nov 05, 2019 4:57 pm

Re: Compact Power Supply for Pi and Hat

Wed Nov 06, 2019 10:59 pm

Thanks for the super response. I expect my servos to require a minimal load. Essentially, it will occasionally move a popsicles stick up and down in such a way that looks like a robot is flapping its arms.

For heat, I will add vents in the back. No problem.

I currently have the 5V 4A power supply that you mentioned so I'll try to use that.

If I don't go with the Mean Well unit or similar power supply, how should I connect one supply to two ICs?
I can think of two ways:
1) Connect the motor module to the +5vcc and GND of the PI.
2) I can strip the wires and solder the two plugs into one to make a splitter. Is that what you meant?

Thanks!

dustnbone
Posts: 201
Joined: Tue Nov 05, 2019 2:49 am

Re: Compact Power Supply for Pi and Hat

Wed Nov 06, 2019 11:19 pm

Connecting to the 5V and GND on the expansion header should be fine for your application, if you run into issues with low voltage on the Pi you could do the splice instead. Either one will work fine, just make sure you pay attention to polarity when making your connections.

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