I have a Rasp pi 4 that will be insatalled on a boat with a 12V system.
I need a power supply to step from 12 to 5v @ 3A through a USB-C to the Pi
Second, I need a second power supply for the screen, converting 12V to 14V@ 2A.
Can this be done with one transformer, splitting the voltages or do I need 2 transformers?
What are the options?
no need for transformers but you will need to make an usb c power cable…
not hard to do…
pls read below …all can be powered from your boats supply…
however it would require an main power switch for your rpi-4 and the screen for times when you are not using your boat…so it will not discharge your battery.
1: step down regulator…
or
2: step up still in question…
is your screen 12-14 volt or 19 …could you please check its original power supply adapter…
if you cannot interpret your supply packt then could post some pics so we can be of better assistance.
Hi Colin,
How interesting, sounds like you need a couple DC to DC converters as Brian said, should be pretty quick and easy to set up.
Another option (depending on the max current capabilities of your boat) would be to pick up an inverter and setup a 240VAC port, you’ll want to use pure-sine wave if possible if you want to minimize the noise, but most modified wave should do the trick too. Unfortunately, due to their high-powered nature, we don’t supply them ourselves, but you should be able to grab one from your local Jaycar (you can also throw a powerboard on these as long as you’re not going above the max power, although being on a boat it would be a good idea to make sure that there’s a protection circuit and no way that any of this can get wet).
Then once you’ve setup a few ports you can use the recommended power supplies for each and don’t have to worry at all about setting up the DC-DC converters manually. If there’s anything else that you need please let us know.
You’ll also want a 14V PS with the appropriate output, I’m not sure exactly what kind of jack you’ll need to I haven’t added a link here. 
All the best with your project!
check the plug on your screen power adapter is it a male or female plug…and often they are custom build by the manufacturer
It’s sounds that I will need 2 separate supplies then.
12v to 5v and 12v to 14v
Inverters are a poor option, very inefficient to convert dc to avoid and back. Plus the cooling fan in the inverter
Screen is definitely 14v. 1.78a I
Transformers and Pi will redide behind a panel. Boat is powered off at main isolators when no one aboard and will power up system
When isolators switch on.
https://www.jaycar.com.au/dc-to-dc-step-up-voltage-converter-module/p/AA0237
i found this at jaycar
all or any of the devices here and in above posts are most likely not suited to marine usage,as they are not water resistant devices,
i would be looking at a marine supplier /engineer …for a much better guidance
I agree completely, @Colin153015 before ordering any parts it would likely be a good idea to check in at your local boat store to confirm that it’s safe to hook this up in the cabin. It’s all quite low voltage, but with conductive saltwater about, it could create an electric shock or fire risk if not properly waterproofed.
You’re right, and the noise would be a bit of a pain, thought I’d make the suggestion in case there’s anything else that you wanted to run from it at the same time 
Brian is spot on in suggesting some buck and boost converters depending on the voltage required. Yes, I’d take a look at the Pololu range on our site depending on your expected current draw. You’ll want two converters running off the 12V supply as it will be much more efficient than trying to step up to 14V then back to 5V. Best of luck with the project!
This not assisting in resolving my questions, and eroding my confidence with the forum as being a source of expertise.
It’s deflecting away into other unrelated areas.
From phone
Hi Colin,
I apologize for the tangent, the forums are usually used for discussion as well as support in order to help customers set up their projects which is why I’d suggested the inverter set up initially and let you know about the risks of setting up an exposed quite temperamental system like this in a boat. For some part recommendations to get this solved I’d check out these DC-DC converters from our stock. Should provide enough current for each at the appropriate voltage regulated from your existing 12V boat supply.
This buck converter should step down to voltage efficiently and silently to act as a suitable power supply for your Raspberry Pi, you can hook this up to the appropriate Vcc and Gnd pins on 2 and 6.
As for the display, you should be able to adjust this boost converter to a 14V output, should have more than enough current to run the display, and some peripherals if needed.
I don’t want to put a downer on any of this but it seems the Amps are adding up.
If the 12V source is the starting battery make sure there is some battery low voltage cut out somewhere or it is possible that you could be fresh out of starting power when out in the ocean or other quiet (no passers by) place. If all this is only running when the boat is running the charging system should have enough headroom to cope.
Cheers Bob
Thanks Bob. No concern there. 500Ah in house bank. Separate to starter. Plus solar panels.
Thanks Bryce.
I’ll read into those a little more.
Screen is 1.8A at 14v, so that step up unit at 150w would be huge (unstressed).
Cheers
I agree,
You’re not going to be generating much heat from that only creating a 2V differential, you’ll need to adjust the potentiometer onboard until you’ve got the appropriate output from your 12V-in before hooking it up, you could go smaller than that converter if you need to, but personally, I’d say that’s the easiest to use in that application, and if you ever take it apart or need to replace the display, etc, you should be able to easily change the output voltage.
Hi Colin.
You seem to have it well and truly covered. Better too much than too little when it comes to boats.
I brought this up as quite a number of people with small boats run everything off the same battery and I didn’t know the size or use of your boat. Commercial boats go to a bit more trouble to do things right possible because they have to to pass strict survey conditions.
Don’t know about now but it used to be illegal anyway (using starting battery for everything). Starting battery for starting, separately charged usually from main engine. House battery (separately charged) for everything else. In the past people have been in some trouble with a flat battery creeping up on them.
Cheers Bob
A marine environment can be very harsh on electronic equipment.
If it was my boat I would be looking at something like the link below.
Wide range input, 5V 3A output, waterproof, dust proof.
Might be worth a visit to a marine shop. Pretty sure they would have something to suit.
https://www.amazon.com.au/Converter-Voltage-Waterproof-Regulator-Step-Down/dp/B07Y2V1F8V
Assume the screen 14V 2A is the output of the power supply that came with the screen. eg 19" Phillips monitor I have needs 20V DC input. 14V DC is not a normal standard voltage and you would need something like those listed above; where you can adjust the voltage to get what you want. Weather proofing could be a hematically sealed box. Similar to:
https://www.jaycar.com.au/ip65-sealed-polycarbonate-enclosure-with-mounting-flange-115-w-x-90-d-x-55-h-mm/p/HB6251
Again, a marine shop should be able to help.
Regards
Jim
Thanks James, the manufacturers label on the screen specifies 14V, 1.78A. I’ll take that rather than second guess.
Yes the marine environment is harsh. But this is a 13m sail vessel so the exposure in the salon inside the instrument panel is nothing like what would be expected on a tinnie.
I have now purchased a step down and a step up. And boxes to mount them in. These will be sealed once ready for install.
Cheers
colin
pls do keep us up with to date with your project with some pics if you can
were always interested in our op`s ingenuity…
Will do. 
From phone
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