Powering a PI (and accessories) from a 12v 15A rail.

Hi Joshua,

My first thought is that you should continue to use the feed wire for the radio. You can change the fuse to a higher value of perhaps 3 Amps. If you use switching converters like the ones you mention this should be plenty of power. 3 Amps at 12 Volts is 36 Watts. Another good reason for using the Radio Power feed is that it may have some noise decoupling built in. This could provide a cleaner power source for the converters. Otherwise the power distribution that you mention should work for your application. Get good quality converters and you can add a couple capacitors, (1) 1000 uF at 16 V and (1) 100 nF across the outputs of the converters. This will protect the Pi from spikes and fluctuations.

John

Hi Joshua,

The Raspi 3 has a maximum current demand of 2.5A. The USB hub is included in the figure as are all the devices powered from it.

If you don't need wifi and BlueTooth then use a Raspi 2, instead (maximum draw of 1.5 Amps).

Try your desired system with a 1A mains PSU and a quarter metre USB lead and see if it works. If you can manage it, use a full powered PSU and measure the maximum current draw with a multimeter.

You might well find that you can get away with a standard 12v lighter socket USB power supply. Use a very short USB cable.

There are some adaptors that have dual 2.1A or a 2.1 and 1A output.

Most portable USB drives come with a double USB lead for powering the drive from two USB outlets.

This could be an option.

The alternative is to find a proper 12v to 5v step down regulator.

Personally I would use the cigarette outlet and change the fuse to something like 5A.

It's probably time to rip apart the 2.5 A supply to see if it's a switch mode version.

If the design is what I'm thinking then it could run with 12v input.

Mark

Hi Mark,

Are there reasons other than personal preferences for your choice of the cigarette outlet?

John

John, I think that's because he thinks that the 1A fuse in the radio power lead will limit him.

Joshua, all power-requirements are usually "maximum required". It should be safe to calculate with those. But on the other hand, it pays to quickly look at what you are REALLY going to use. So for example the pi3 power requirements include drawing 500mA on each of the four USB ports. Similarly your USB hub specifies a similar powerbudget that includes lots of maximally loaded ports. My guess is that if you look a bit more carefully at what you are going to use, then you can quickly decimate your power requirements. First you're not going to use ALL of the USB ports. And secondly you're not going to use them all at 500mA.

Officially USB devices should report their power requirements in 2mA increments. That USB camera can be examined easily. Just connect it and use lsusb -v to list its requirements.

Cheap LM2596 based DCDC converters are fine to power a pi or a hub. If you really want to go for the "full 5A" you can split the work between two of them. One for the pi, one for the hub. And, as has been said: just put in a 2A or 3A fuse, that should be plenty.

Several reasons ...

The wiring size is usually less.

It may also be that the connection further back shares a connection that feeds something else, so while you can draw more, the connection point at the fusebox may become overloaded.

Generally radio outlets are fed from the accessories, so everytime you go to start the vehicle, it will drop power while it is cranking.

Mark

In theory, the size of the wires dictates the fuse or the other way around. So if you see a 1A fuse, they could've used a wire that is so thin that it would carry ONLY "well over 1A" when shorted at the end. In practise it is much cheaper to just use the same wire as everywhere else.
Also, in general it is not common to have a cascade of fuses and then say a 15A fuse at the start and then subdivided into exactly 15 smaller groups of 1A. Even if that were the case you can safely get away with upgrading one to 2A or 3A.

But.... That third argument is quite valid. Just for that one, I'd say: Yes, do that!.

(On the other hand, if it is on the accessory line, you can have the pi up and running in the acessory position of the key. That would allow you to listen to the radio while waiting somewhere without the motor running.  Does the cigarette lighter work in that position? When off? I've seen both. A cigarette lighter that would go off with the car and one that would stay on even when the car was off..... ).

Does the cigarette lighter work in that position? When off? I've seen both

Without more info it is a guess what version the OP has.

The more modern cars switch it off with the key to stop children setting fire to cars by playing while left 'unattended'.

Fusing and the currents are an interesting science.

Domestic House wiring fuses/breakers are there to protect the wires, not the load.

in this case the fuse/breaker will run at it's rated current for 4 hrs before rupturing, but the curve for above that varies between manufacturers.

My advise to halkun is load up his Pi and then measure the current.

This will give him some ideas about his options.

Mark

When you say "decimate your power requirements" do you mean?

verb (used with object)

, decimated,

decimating.      

I assume you are not out to destroy people even every tenth one (#1 & #2), so do you want 1/10 of the power requirements as in #3?

Clem

Wow. I didn't think I would get so many thoughtful and useful replies! Here I just wandered in because of Ben Hack and this community is great.

Turns out I jumped the gun and have been spending my time on the Maker/Pi side and didn't do much research in the Vehicle side. Turns out my problem has been already solved by many manufacturers. First I apologize, and I read my wiring diagram wrong it's 12v 15A going to the radio. (The 1A line went to my A/C control deck, under it.. Helps if I follow the lines right ) My model also has a sense line that stays high after I turn off my car and will drop to low after I open my door or after 10min has passed. (So the radio stays on after the car is turned off to you can listen to the end of you favorite song)

However, there are aftermarket modules for cars that take the 12v line and steps it down two 5v 3A USB ports. It also has a considerable filter on it to clean up of voltage. It appears the 12v DC coming off the alternator is *super* dirty and really all over the place.

The Bluetooth is needed so I can link into my car's CAN network so I can pull ECU and telemetry data from my vehicle

Wifi is needed as I'll be running a VNC client so that I can redirect my cell phone's display to my car's screen for GPS and access to my apps...maybe Still researching that one.

The only other issue I have is a safe startup/shutdown watchdog system... I'll Look into that later too.

Anyways thanks for the input.. I'll probably be doing a youtube series on this.