start78
Posts: 43
Joined: Mon May 23, 2016 8:02 am
Location: Germany

powering question

Thu Oct 06, 2016 9:01 am

I got an old laptop display that i want to use for my RasPi by using an LCD Display Controller with HDMI out. The controller needs 12V/2A so i will have to use some AC adapter to power it.

Now i'm planning to build a wooden frame for it that can also hold my RasPi (Model 3). To save cables and AC adapters i'd like to use the 12V adapter for the RasPi as well. So i looked around and found this:
DC 7V-24V To DC 5V 3A USB Output Converter Step Down Module KIS3R33S

Now my question is: can i simply split up the cable from my AC adapter and connect it to the display controller and the step down converter? How much power will i need? Is 12V/4A enough? Could i even use 12V/3A?

The advantage of the above converter is that is gives 3A output (4A peak), so plenty enough to power a RasPi 3 and maybe even give supportive power to an external 2,5" hdd. All with a convenient USB plug. So my RasPi GPIO's would be free for any future projects. (And i'ts cheap.)

Another question: Is it OK to use a wooden housing or could the components get too hot (under normal circumstances). Maybe i could take some plastic for the base, build a wooden frame around it and take the display as a cover on top...

JimmyN
Posts: 1109
Joined: Wed Mar 18, 2015 7:05 pm
Location: Virginia, USA

Re: powering question

Thu Oct 06, 2016 11:20 am

Yes you can do that. I have a Pi3B with the RPF 7" touchscreen that's powered that way. If the Pi3 is under a heavy workload plus the additional ~450mA load for the display you need a real good cable and PS or you'll have low voltage issues. I decided the best way around that was to up the voltage on the supply and voltage drop wouldn't be a problem. So I dug around in my junk box of wall warts and come up with a 9VDC 4A unit. That powers a 5V 5A step down converter giving me a solid 5V at 5A available right at the Pi/Display and no more low voltage issues.

Your display will need 12V at 2A so that's 24 watts. The converter is 5V at 3A so that's 15W plus losses in the converter. That appears to be a switching type converter but will still have some losses so we'll use 15W x 1.2 = 18W. 18W for the converter + 24W for the display = 42 watts load. A 12V 3A supply is only 36 watts, but the 12V 4A is good for 48W which is enough and provides a little extra buffer.

start78
Posts: 43
Joined: Mon May 23, 2016 8:02 am
Location: Germany

Re: powering question

Thu Oct 06, 2016 11:52 am

Thanks for that info!

Does anyone know how hot that step down converter can get?

I guess the RasPi will be no problem in a wooden case and the display controller board also shouldn't get much warmer. But i didn't read anything about the working temperature of the step down converter.

Or does anyone know a better (cheap an easily available) way to get 5V USB out from a 12V power source?

tweak42
Posts: 536
Joined: Tue Mar 22, 2016 3:16 am
Location: Los Angeles, California

Re: powering question

Wed Oct 12, 2016 4:41 am

start78 wrote:Thanks for that info!

Does anyone know how hot that step down converter can get?

I guess the RasPi will be no problem in a wooden case and the display controller board also shouldn't get much warmer. But i didn't read anything about the working temperature of the step down converter.

Or does anyone know a better (cheap an easily available) way to get 5V USB out from a 12V power source?
Search around for people experiences with buck converters, and you might try searching/asking in the general forum.

ewaller
Posts: 32
Joined: Tue Oct 04, 2016 5:24 pm

Re: powering question

Wed Oct 12, 2016 2:53 pm

It is a switching regulator and they claim a (max) efficiency of 96%. I doubt you will just happen to hit that sweet spot in the operation curve, so let's assume the efficiency is 90%. At the max continuous output current (3A) the output power will be 15W. That means the module will be dissipating 1.5W. The converter itself looks like it is about 0.5 in^2, so you are looking at about 2.7W/(in^2). That might get a bit warm; my seat-of-the-pants-feeling would be a 30°C rise over room ambient. At that temperature, you should be able to but your finger on it indefinitely. If you were to get a 40°C rise, you would probably only be able to touch it for a few seconds -- in that case I would add a heat spreader to the little black converter module. Or, don't load it continuously at 3A :)

Edit: Note that the operating temperature range goes out to 85°C.

Edit: As to whether you can use a 3A supply at 12V. For the converter, we have established that the 5V output at 3A is 15W. With a 90% efficiency, that reflects to the input as: 15W/0.90 = 16.7W. From your proposed 12V, 3A supply, you need 2A for the display controller which leaves you 1A @12 for the converter. That is 12W. So, no, a 3A 12V supply won't quite cut it.

start78
Posts: 43
Joined: Mon May 23, 2016 8:02 am
Location: Germany

Re: powering question

Tue Mar 28, 2017 2:19 pm

It Works!

The display controller board has two different inputs: a round one, like on a laptop and one 4-pin. Power comes from a 12V 4A DC adapter and goes into the round power input. I use the 4-Pin connector as a power out (because both inputs are directly linked to each other on the pcb) to feed the step down converter with 12V, wich gives power to the RasPi. When i plug in my DC Adapter everything is powered at once and boots up just fine.


Additional Question:

Now I'd like to add a rechargeable battery system to the setup. It has to have 12v (for the display controller), so has anyone a practical solution or should i just hook up a bunch of standard AA NiMH-batteries until i get 12V?

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