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This is my first attempt at working with electronics so sorry for the potentially stupid question!

For my A-Level Product Design project I am making a digital photo frame with built in bluetooth speakers. I am using a Raspberry Pi as the controller for the photo frame and plan to use a bluetooth amp for the speakers.

I need my product to use a mains power supply, however I don't know how to split a single mains cable to 2 separate devices on the inside of my product.

From research I have found that the Raspberry Pi needs 2.5A "Recommended PSU current capacity" and 5.1V Power supply (I wish I knew what this meant). Since I have no idea what i'm doing, I am yet to order the bluetooth amp until I know for sure which will work.

My question is; How do I split a mains power between two devices, and do I need to order a bluetooth amp with the same statistics as the Raspberry Pi?

Any help is much appreciated, Ollie

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    \$\begingroup\$ Hey Ollie, welcome to EE.SE. I'm sorry but if you do not know what a 5.1V PSU means, you should stay away from spliting mains cabling, because it can be potentially deadly, either by shock or fire. I suggest you use standard USB supplies instead, just find one that has 2.5A or more output capacity. (In my experience, the RPi will work with much less, the extra current is needed if you're gonna plug other things into its USB ports). \$\endgroup\$
    – Wesley Lee
    Nov 28, 2016 at 10:12
  • \$\begingroup\$ I have spoken to the tech support people in Maplins and they have suggested the same thing. I've picked up a 4.8A 4*USB port plug, splitting 2.4A between each device (more than enough for both). My new issue is finding a bluetooth amp that will run at 5V (the maximum a USB will transfer) \$\endgroup\$
    – Ollie Smith
    Dec 5, 2016 at 15:12
  • \$\begingroup\$ Pretty much all DFP use an external AC/DC adapter instead of an internal one. That said, splitting an AC input inside a case is just a matter of extra wires or traces on a pcb. Since you are not anywhere near ready for Mains transformer designs, avoid this. \$\endgroup\$
    – Passerby
    Dec 29, 2016 at 3:09

2 Answers 2

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It's best to avoid working in mains power at all: Safety; ease of getting UL/CSA listing; and ease of changing power supplies if you change countries.

You may have noticed that almost every small machine uses a wall wart power supply. This is why. They can focus on their low-voltage gadget and not worry about also designing a mains voltage power supply and getting that listed. The wall-warts are commodity items, and are already UL listed.

Lately the trend is USB power, but that only works if your load is less than 5-12 watts. Yours is more.

So first... Choose a speaker module that runs on the same voltage (5.1) as the Raspberry Pi. This won't be hard; 5V is a popular voltage.

Then total the current requirements for both Pi and speaker.

Then look for a wall-wart that outputs the correct voltage and at least the total current, preferably a bit more. Too much current is good, too much voltage is a problem.

Then search for a connector socket which fits the plug on the wall-wart, and fit that into the back of your chassis. Split power from there to both devices.

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Are you using splitting the mains power to go to the RPi and speakers?

If so, what I would recommend you do is find out how much voltage and current is required for the bluetooth speakers.

If the bluetooth speakers need just 5 Volts DC, then it becomes easy in the sense that you just need to invest in an (off the shelf) AC-DC PSU (Mains to 5Vdc with 2.5A current = 12.5W).

So a Mains to 5V DC PSU with a power rating between 13 and 15W should be fine to powerup the entire unit as this will give 2.5A to the RPi and the extra current for the Bluetooth speakers.

Hope this helps. Labmat

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