Something to Split Power / Volatage

Thread Starter

Thomas Robinson 1

Joined Jun 7, 2018
6
Hey guys... I'm an old guy who is new to electronics as a hobby, my knowledge is VERY basic for this stuff.

I had an idea the other day to build a project using a RaspberryPi and an old laptop LCD. I see on eBay there are a lot of controllers for old laptop screens and my sisters work gave me all their old laptops so, I have a lot here and hopefully can find a controller for it. I also have a RaspberryPi here loaded up with arcade games. You know where this is headed.

I was planning on a full cabinet... in which case I'd have a power bar inside, a computer monitor, the RaspPi AC adapter etc, loots of room. I decided I wanted to build something super compact, briefcase sized.

Is there some sort of power splitter I can get/modify that allows me to use the 19v laptop AC adapter, and basically split it's power to a 5v 2.5A outlet for the RaspPi, and 12v 3A for the LCD controller?

Ideally I want a since AC adapter powering the whole project, instead of a since power cord with 5lbs of adapters and plugs inside.

Thanks
 

Papabravo

Joined Feb 24, 2006
16,830
Hey guys... I'm an old guy who is new to electronics as a hobby, my knowledge is VERY basic for this stuff.

I had an idea the other day to build a project using a RaspberryPi and an old laptop LCD. I see on eBay there are a lot of controllers for old laptop screens and my sisters work gave me all their old laptops so, I have a lot here and hopefully can find a controller for it. I also have a RaspberryPi here loaded up with arcade games. You know where this is headed.

I was planning on a full cabinet... in which case I'd have a power bar inside, a computer monitor, the RaspPi AC adapter etc, loots of room. I decided I wanted to build something super compact, briefcase sized.

Is there some sort of power splitter I can get/modify that allows me to use the 19v laptop AC adapter, and basically split it's power to a 5v 2.5A outlet for the RaspPi, and 12v 3A for the LCD controller?

Ideally I want a since AC adapter powering the whole project, instead of a since power cord with 5lbs of adapters and plugs inside.

Thanks
The generic answer is yes, provided you do not violate the immutable rule of DC-DC power conversion. The immutable rule is this:

Power out will ALWYS be less than power in. Sometimes it will be much less.

So your power out requirements are:

5V @ 2.5 A = 12.5 Watts, and​
12V @ 3A = 36 Watts​
For a total of 48.5 Watts​
For a back of the envelope calculation, I normally estimate the efficiency of a conversion scheme at 80%. It might be better, but if it is worse an alternative should be considered. So that means the required input power required is:

48.5 Watts (out) / 0.80 ≈ 61 Watts (in)​
So long as the laptop can supply in excess of 61 watts you have a chance to make this work.

At this point, what you want to do is look for appropriate DC-DC converters that you can purchase. I would not recommend that you try to design and build your own DC-DC converters at this point. When you find some candidates check back to see if you have understood what the vendor is telling you.
 

Thread Starter

Thomas Robinson 1

Joined Jun 7, 2018
6
Thanks for the information... I was hoping it was "theoretically" do-able... I figured a laptop has a screen, hard drive, motherboard and fans to power... I'm replacing the motherboard/fans/harddrive with the RaspPi which is basically an even smaller motherboard so, the PSU from the laptop should be enough.

The idea is to put the screen and RaspPi into a thin wide wedge shaped box, that can sit on a table or on your lap, joystick on the left, buttons on the right, screen in the middle. Basically, a 2 foot wide handheld game console. That's why weight is a concern as is the number of cables out the back.

So, something like a pair of these? Then split the input 19v using something like a terminal block.... run a pair of wires to one board set at 12v and another set to 5v?

https://www.ebay.ca/itm/223524573272?hash=item340b1a5058:g:zX8AAOSwcD5c4ynR
 

Thread Starter

Thomas Robinson 1

Joined Jun 7, 2018
6
Says 19.5v 4.6A on the AC adapter in front of me... I have a few here, but this is the one from the laptop I'm now dismantling.

So.... in theory so far....

- AC adapter (19V / 4.6A)
- Terminal block for splitting
- Two DC-DC step-down boards (set to 12V and 5.1V)
- RaspPi v3 B (Plugged into USB joystick controller and small USB fan)
- LCD controller board (Plugged into LCD screen)

LCD controller and RaspPi both have 3.5mm audio jacks for speakers. Don't want to put too much a load on the RaspPi else get that lightning bolt error.

But... with that list, some wire and solder... doable?
 

Thread Starter

Thomas Robinson 1

Joined Jun 7, 2018
6
Just took the laptop apart... and yes! I can get an adapter....

https://www.ebay.ca/itm/353424067240?hash=item5249b74aa8:g:qOgAAOSwKThgUxlI

LCD Controller Board:

HDMI DVI VGA AUDIO LCD Board Work for LVDS Interface LCD Screen
Input Power Adapter: 12Vdc More than 2A.
Come In Standby Model When Not Input Signal
Standby Model <1W
Video Input: HDMI DVI VGA
Audio Input: HDMI
Audio Output: Speaker Connector
OSD: Brightness Contrast Auto Language And So On
Language: English French German Spanish And So On
Board Size: 125mm x 58mm x 17mm
 
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