Raspberry Pi Power Consumption and PTC resettable fuses - Page 1
Forum section: electronics / general

author: GromBeestje
author_rank: Frequent Contributor
timestamp: July 01, 2020, 02:57:55 pm
content: When I look at my extension cord, I see a row of power supplies, all 5 volt.Now, my idea is to take a big 5 volt power supply to replace them.Among things I like to power are raspberry pi's.Hooking one up at a big power supply, say 10 amps, if things go wrong, they go very wrong,so, to prevent problems, I would like to incorporate a PTC resettable fuse in the circuit.The thing I haven't figured out yet, is what value to take.When I look athttps://www.raspberrypi.org/documentation/hardware/raspberrypi/power/README.mdit gives a recommended power supply.I could take the values in there as my fuse value. The thing is, the trip current is usually the double of the hold current.If I would, for a Raspberry Pi 4, take a 3 A fuse, the trip current is 6 A. I'm wondering whether that wouldn't be too high.Furthermore, when I look athttps://www.raspberrypi.org/documentation/faqs/it mentions a power consumption under stress for a Raspberry Pi 4 of only 1.25 A. If that is the case, the mentioned fuse with a 3 A hold current would be way too high, and a 3 A trip current would make more sense.I am planning to hook up a Respberry Pi 1, 3 and 4. Looking at these values, what value of fuses should I use?

author: wraper
author_rank: Unknown
timestamp: July 01, 2020, 03:51:37 pm
content: Quote from: blueskull on July 01, 2020, 03:38:42 pmIt does not have any input overcurrent protection. Previous models had PTC and OCP in series, Pi 4 has TVS right across USB-C connector and no OCPhttps://www.raspberrypi.org/documentation/hardware/raspberrypi/schematics/rpi_SCH_4b_4p0_reduced.pdf. And I've seen plenty of raspberry Pi having shored TVS and other parts connected to 5V.

author: wraper
author_rank: Unknown
timestamp: July 01, 2020, 04:02:43 pm
content: Quote from: blueskull on July 01, 2020, 03:54:41 pmDunno why it shows so and I have zero reasons to look into it but you could simply look into official circuit diagram I provided a link to. And there is no protection on the input if you simply look onto PCB.EDIT: even 5V on GPIO pin header is directly connected to VBUS on USB-C.

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 01, 2020, 10:45:11 pm
content: Since an overcurrent condition most likely means a catastrophic fault, save cost and voltage drop by using regular fuses. Also set the 5V supply to something like 5.25V to add margin for voltage drops.Back in the Perk mining days, it was common for large clusters of cheap smartphones to be run from ATX PSUs that can easily supply 30A. I did not see any mention of fuses to protect against overcurrent faults since they say the PSUs have built in overload proection, but they do often use 1N400x diodes to drop the voltage closer to the 4.2V of the original batteries. Prior to the ATX PSU trick being common knowledge, there were many reports of expanding batteries (which were very close to some processors being run at high load 24/7) and one instance of a counterfeit replacement exploding, which is what prompted the community to look into running the phones without the batteries. I guess eliminating the batteries is a big improvement in safety...

author: Siwastaja
author_rank: Super Contributor
timestamp: July 02, 2020, 10:18:52 am
content: PTC resettable fuses are pain-in-the-ass because every trip event increases their resistance and lowers the trip level on further trips, to the point they are constantly tripping for no reason.Hence, you really need to oversize that trip rating. I would definitely use at least a rated 5A PTC for Raspi4. Actual tripping with a virgin PTC then might be at 10A, but you can't do anything about it.Note that traditional fuses are a bit, but not much more accurate. They don't have the problem with repeated trips, however, so you can likely use a 4A fuse, which will actually blow at 6A given enough time (minutes), or at 8A a bit more quickly (seconds).If you need more accuracy, the only option is to design an active e-fuse circuitry.The huge plus with traditional fuse is that it forces you to find the cause for the catastrophic failure.In case of Raspi, there are approximately two possibilities for overcurrent (that can be handled with fuses):1) Somebody drops a spanner directly at the voltage lines2) Raspberry Pi itself converts into a spanner due to catastrophic failure.In case 1, Raspberry Pi might be saved.In both cases, the fuse protects things from setting in fire if the PSU does not have hickup or latch mode overcurrent protection, or if the PSU is grossly oversized.Such events are rare enough to warrant a self-resetting fuse, IMHO. Just change the fuse if it blows.

author: GromBeestje
author_rank: Frequent Contributor
timestamp: July 05, 2020, 04:59:42 pm
content: Quote from: blueskull on July 01, 2020, 03:38:42 pmThose look interesting, overcurrent AND overvoltage protection. I'll put them on my list to investigate.At this point, I haven't done anything about my Raspberry Pi 4 setup yet.For my Raspberry Pi 3 setup, I have now a 1A Hold current fuse. I was planning to put it on a 2A one, but they seem not to trip as expected. That's one thing, a resettable fuse can be verified to work, while a regular fuse can't. Furthermore there is a DVD drive and Amplifiers on there.Quote from: Siwastaja on July 02, 2020, 10:18:52 amThat's the scenario I'm looking at, an oversized PSU, as I am replacing an extension cord of PSUs with a single PSU. Replacing a couple of 5V 2A with a single 5V 10A, ( or even a 20A one, as I have one 10A and one 20A).

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 05, 2020, 05:54:36 pm
content: Look at the power supply to see if there's a voltage adjustment trimmer. If there is, you can boost it to about 5.25V to compensate for voltage drop.

author: james_s
author_rank: Super Contributor
timestamp: July 05, 2020, 06:04:53 pm
content: Quote from: Siwastaja on July 02, 2020, 10:18:52 amThere is a third that may or may not be important. Somebody connects several USB devices such as hard drives that collectively draw more current than your supply is fused for.

author: GromBeestje
author_rank: Frequent Contributor
timestamp: July 06, 2020, 08:55:46 pm
content: Quote from: NiHaoMike on July 05, 2020, 05:54:36 pmYep there is one.Quote from: james_s on July 05, 2020, 06:04:53 pmI doubt that will happen. It's connected to the TV, and it's supposed to gets its content from the network. (or the connected DVD drive, to a powered hub)