Getting 5V3A DC power source from Colt Z27AG

I was trying to plug a Jetson Nano (later upgraded to Jetson Orin Nano) into my old, unintelligent Mitsubishi Colt Ralliart Version.R (What a stupidly long name), to allow for some GPU-powered AI fun.

What I wanted to do with the mini computer is not of much concern here, though. This entry is about how to get a 5V 3A DC power output from the car. To power the Jetson, obviously.

Power requirements

Both Jetson Nano and Jetson Orin Nano are capable of taking a barrel jack 5V DC power input. The older Jetson Nano can also be powered by a micro-USB socket which is also 5V DC. Less of a hassle for Orin Nano because it can only be powered by a barrel jack.

Either computer has a 'powerful' mode which consumes 15W. NVIDIA forum moderators or some actual employee decided to recommend a 5V4A power supply for that mode, having ~5W of redundancy in case of power peaks (or at least I guess so).

Also, it is very frustrating that NVIDIA for some bloody reason decided to change the dimensions of the barrel jack from Nano to Orin Nano as well. Jetson Nano uses a 5.5x2.1mm Center Positive barrel jack, and Jetson Orin Nano a 5.5x2.5mm Center Positive. It's absolutely not very difficult to find a converter but absolutely frustrating considering they are supposet to be of the same lineage.'

EDIT: I noticed that Orin Nano actually requires a 19V DC supply. No wonders they decided to use a different barrel jack.

Power from the car

Thoughts first

As many people have known, modern passenger cars usually run a 12V DC power for most, if not all, the electronics. However, at least for my Z27AG, it's 12V only when the car runs on battery. Once the engine is running the voltage rises to ~14.4V as the power is now supplied from the regulator and apparently it is rated at somewhere around 14V.

While I have faith in modern computers dealing with such tiny inconsistencies, the most important thing is still that the car is not directly having a 5V DC supply. So I'll have to use a buck converter or something similar to do a DC-DC step down. Hopefully, this converter also serves as a voltage stabiliser and keeps its output at 5V whether it is taking 12V or 14V.

Usually when people think about taking DC power from a car, they'll look at the cigarette lighter socket and plug something into it. I already have a Christmas tree in there and I don't want to burn that certain fuse by adding 15W load to it, so I decided to drain some power from another node in the fuse box.

After consulting the diagram shared by some kind people, I find that fuse #27 seems to be a good choice. It is supposed to be the fuse for the electric sunroof motors, so it's rated at 20A. What makes it good is I don't have an electric sunroof, so that fuse will by no means be loaded at all under normal circumstances.

Components

Aside from the wires and pins and jacks and connectors and all, there are 2 important things I will need:

• 12V-5V DC buck converter
• Fuse box power outlet

After some calculations and some Amazon browsing, I found a good DC converter, and it even comes with a decent casing. It is rated at 3A max which will give a somewhat mediocre 15W power, but if I run the Jetson at 7W this will have definitely no problem.

The fuse box power outlet is a rather easy purchase at any automotive store, or home centre with an automobile section (at least I think so). I bought mine at an Autobacs. As a 20A fuse, the outlet branch is fused with a 10A tube fuse, which is somewhat reasonable. Technically since the sunroof is nonexistent, I can even replace the tube fuse with a 20A one to allow more juice, but 12V10A is already a bulky 120W so I don't see the reason in doing it.