Simple electrical question - combining power cables on a magic mirror

Hi guys,
As the title states I think this will be a pretty simple one but as this is my first real project with anything like it I’m not sure on what I need to do.
My problem is I’ve got a magic mirror nearly finished, but currently have two power cables coming out - one for the monitor and one for the Pi. What would be a nice neat way to change that to one power cable into the back, and have a 5v for the Pi and a 19v for the monitor? I considered just having an extension cord running into the back (as I’ll be putting a “lid” on it once everything’s done) with a double adapter, but I’d really like to make it as neat as possible and just knowing that was in there would eat away at me.


welcome to the forum…!!

i think start with the 19 volt rail supply to the screen…look at how much current it puts out or even its wattage .we know its 19 volt i am assuming its putting out approx 20-30 watts approx the pi-4 when under extreme duress will take what i have seen is 15-20 watts i had 2 spinners usb drives one ssd and my k-board and mouse bac last year…this all took 40 watts max you could get away with an 40 wat 19.5 volt supply and a single buck regulator…better known as a sterpdown there quite efficient in their ways/modes of operation there is a nice small 5 amp unit from core quite small good for 30 olts input and adjustabe down from a 19 volt supply…so you need one buck type regulator and the 19 vot 5 amp supply it should be ok because 19x5 is like 100 watts basically its 95 to be exact frommem …plenty of current should you need some extra at some point in time who knows…feed the screen straigt of the black or silver supply and the rpi off the regulator…as well its a small unit the one i have in mind…and its good quality core sell some nice neat and small regulators

this will power the the rpi very well and have plenty of overhead …this is a good thing asit is smal and you can mount it in a vented case ok the circuiti`ll draw it 4 u hang on pls

there are links for the buck regulator on the site under its description …i suggest you have a look…i am shore you can find an 50 t0 70 watt supply on the net or here at the core shop jusy use the help desk on the left during business hours,at , core i just saw this thread so i thought i would hope to point you in ther right direction…better schematic

ok seems i misread the notes on the reg it was thought that the 19 volts would be the pass through…my mistake go with bryce`s advice…soxx but the schematic is correct though…

1 Like

Thanks so much for the detailed reply! So do you mean to use the power brick that came with the monitor, and use it to power both the monitor and the step down? Or does the power brick just power the step down, and the step down has two outputs?

I found a few step downs but nothing from 240, but if the brick is capable there’s lots that step down from 19v - → 5v.

What would be inside the plug for the Pi that means you can plug straight into a wall and get a 5v output? It seems crazy that the Pi only needs that small plug but the monitor needs that big power brick.

If it helps, the brick is labelled as 100-240v 1.3A 50-60hz AC input and 19V 1.31A output.
The Pi is the standard one that comes with it, 100-240v 50-60hz AC input and +5.1v DC 2.5A 12.75W output.

Thanks again for your help!

just to clarify that is the official rpi3 supply is 2.5 amp 5.1 volt yes 12.5 watt …and your screen supply is 19 volt 1.5 amp approx it is not enough to drive both units …so you require am 19 volt 4 amp supply you can find them on the web…just browse for “buy 19 volt 4 amp power supply” etc…etc…

ok back to the brick you could just get an 19 volt 4 amp unit that would put out plenty of wattage approx 80 watts …then feed it into the reg i posted u can draw the adjustable voltage from it and also feed through the 19 volts to your screen if this clears things up 4 you.

Hi Chris,

Ok so as Brian said what you can do is run a DC-DC converter off the output of your power supply to step down the voltage from 19 to 5v for your raspberry pi. Now, you will need to check how many watts your power supply is able to provide, the Pi maxed out will typically use 6.5 watts, call it 7 watts to be on the safe side. If your display requires 15w then you will at least need 22 watts from your power
supply. You may have to purchase a higher wattage power supply the one you have is designed exactly for the power consumption of the monitor.

Something like this would be perfect as you could wire one end to the positive and negative wires (leaving the connector for the monitor in tact) of the power supply and then splice a USB cable onto the output terminals for a constant 5v.

1 Like

Hi Chris,

Most modern power supplies are switched mode supplies. You can read all about the theory of operation here:

If you’re not overclocking your Pi and you’re not running anything from it the Pi 4 will pull about 600mA at idle, a bit over 1.1A flat strap.

As another option to the ones already given above, I’d suggest using this step down / buck converter:

19v x 1.31A is about 25W. So long as your display is using less than about 18W, you’ll probably get away with it. More likely if it’s an OLED screen, as black is the low power state - LCDs on the other hand use the least power when they’re white.

Your other option is to find a more powerful supply. 19v is pretty common for laptop supplies, so there’s a good chance you could find a used one on eBay or similar.

1 Like

i think my supply was the better regulator as it had the input voltage path on the regulator board so a pass through and a bleed off regulator adjustable for what ever board the op is utilizing …would give it more stability i would have thought and also 19 volt black bricks are readable around on the web…5a 4 a 6 a … flooded…on the cheap sellers sites…