I would like to use a buck converter between a 12V DC power supply and my RPi4, and found this car power supply to be appealing. It claims to step down from 12V to 5V 3A, which is almost the same as the recommended power supply for RPi 4 (5.1V 3A).
Would the 0.1V difference matter? The RPi4 I have is loaded with sensors and the like and I am worried if the 5V 3A would be "underpowering" the Pi.
Thanks!!
3 Amperes would almost certainly be more than sufficient power for your RPi. As @Milliways has pointed out in his answer however, an issue that you should be more concerned with is the performance of the power supply: How will it perform in my project, and has it been designed and produced properly? This is the point of my answer:
It is not clear to me why anyone deals with the junk traders that infest Amazon, eBay and the other well-known online retail outlets. The power supply you have found is being sold by complete morons, and possibly designed and built by more of the same! Yes - harsh language, but consider the facts... the facts are available in the car power supply link provided in the question. Consider this:
this item has no specifications for line or load regulation - what happens to the output voltage if the input voltage changes, or if the load changes?
what are its thermal characteristics? What are the operating temperature limits? Will it need a heat sink to keep it in a safe operating area? How does the output voltage vary over a range of temperature?
what happens if the output is shorted - does it have any protection built in?
I call this item "junk" because virtually no specifications were provided. Further, the "sellers" seem to be clueless; e.g. the Input Voltage spec is given as:
DC 6.3V-22V (12V 9V)
What does that even mean?
Fortunately, there are alternatives; you don't have to buy from ignorant fools in Internet retail cesspools. There are numerous outlets that sell electronic components online, at reasonable prices, designed and built by competent organizations, with specifications you can rely upon. Here are two examples, but there are many more available for the cost of a search.
If you want something small (~11mm2 footprint; could be integrated into a case for an RPi), inexpensive ($6.99 for QTY 1), with full specifications, an I2C interface (RPi-to-power supply comm & control?), and more. The datasheet also provides some application notes & other information to support integration. Here's one place to buy this part.
If you want more modular, less DIY this unit has a footprint of approximately 2" x 3", and full specifications. It's a bit more expensive - $14.95 from this source.
These are only two examples - there are thousands more. You can easily find reputable vendors selling quality products - all it takes is a free search. You are of course free to trade with anyone you wish, but a smart buyer will shop around. Learn to recognize quality.
The absolute maximum rating of the Pi power converter chip is 5.45V. This must include any transient voltage peaks, noise, measurement errors, safety margin etc. Going over this limit will not necessarily destroy the Pi, but it will be a gamble.
E.g. if you are able to measure the voltage with 0.05V precision and your DC-DC converter regulation precision is 0.2V, you can safely set the output voltage to 5.2V maximum. Setting it to 5V provides a 0.2V safety margin, which is good to have, but keep in mind that the lowest possible voltage will then be 5-0.25 = 4.75V, which is just 0.1V above the Pi's undervoltage limit (4.65V).
If you can measure the voltage drop in the wires when the Pi is idle (lowest current consumption), you can compensate for it by increasing the output voltage by the same amount. That's typically around 0.2V with adequate wires. In my example above that would mean the target voltage range on the power supply side should be set to 5.1-5.4V, depending on whether you prefer a safety margin for overvoltage or undervoltage.
The official power supplies (which have attached cables) are set above 5V to allow for the inevitable voltage drop in the cable, even though these have decent wiring.
Use of most USB cables is more problematic, as they use inadequate wire. They work for their intended purpose (which is generally to charge batteries) and the voltage drop is not a major issue.
Power sources SHOULD provide 5±0.25V but many of these buck converters have poor regulation. You would need to confirm that the units you are proposing are adequate - the "documentation" is limited.
The best thing you can do is to place the converter as close as possible to the Pi and use heavy gauge wiring, as short as possible.
