Powering a Raspberry Pi with an ATX supply. Current limiting resistor values? - Page 1
Forum section: electronics / beginners

author: jayfehr
author_rank: Contributor
timestamp: February 05, 2015, 09:31:20 pm
content: Hello, I'm making a small desk sized arcade cabinet with a Raspberry Pi and a recycled 19" monitor. I have a bunch of old ATX power supplies kicking around and would like to repurpose one to run the entire unit. At this point I have yet to look to deep into the display, so I'm not sure what they max volt/amperage is (probably 12V, ? amps) so I'm not asking about that, yet. I would like to learn enough from the Pi to figure that out myself.I do know that the Pi is 5V, 2 amp max (the on board fuse is 2 amps) so I would like to add a current limiting resistor to prevent any current surges from frying the board (well blowing the surface mount fuse). I'm not sure how exactly to figure out the proper resistor value (I know ohms law applies but I'm not sure of any gotcha's to watch out for). I will also be adding an in-line 2 amp fuse just for added protection.If anybody could help me out it would be greatly appreciated.Thanks.

author: suicidaleggroll
author_rank: Super Contributor
timestamp: February 05, 2015, 09:44:46 pm
content: You can't use a resistor with a device that has a dynamic current requirement, the supplied voltage would bounce all over the place. I would lean towards just cleaning up the voltage before passing it on to the Pi; fuse, TVS diodes, etc.You could also try an integrated solution, such as an LTC4362.

author: LA7SJA
author_rank: Regular Contributor
timestamp: February 05, 2015, 09:51:21 pm
content: You should have everything you need from the ATX power. But use a500800-1000mA fuse one the +5V VCC line to protect the Pi.Check thislinkfor pinout and startup circuit.JFG

author: Zero999
author_rank: Super Contributor
timestamp: February 05, 2015, 09:56:07 pm
content: If the Pi is rated to 5V and the PSU outputs 5V then it won't be a problem, whatever the maximum current rating of the power supply is.2A is the maximum current the Pi will draw which is limited by the devices internal resistance.The only thing you need to protect against is short circuit which is what fuses and circuit breakers do. The fact that the ATX PSU can supply much more than 2A is not a problem when everything is working as it should but if a short circuit occurs, a large current will flow, causing bad things to happen and a fuse is needed to stop this.

author: jayfehr
author_rank: Contributor
timestamp: February 05, 2015, 09:58:20 pm
content: Thanks for the quick replies guys.@LA7SJA, I believe that the Pi requires 700mA to run, and according to Adafruit they are saying that you should run the Pi 2 (which I plan to get when they become more readily available) with at least a 1A power supply. With that in mind wouldn't a 500mA fuse blow regularly? I was assuming that the same value in-line as on the board would protect the board fuse from blowing. However, I'm far from an expert and that's why I'm here asking.Also thanks for the link. I've been using a few different pages for ATX info. I'm not sure why I didn't consider wikipedia earlier.

author: suicidaleggroll
author_rank: Super Contributor
timestamp: February 05, 2015, 10:02:36 pm
content: Quote from: Hero999 on February 05, 2015, 09:56:07 pmI was under the impression he wanted to protect the Pi from a malfunctioning PSU, which are known to occasionally send out high power surges when they die, destroying anything connected to them. I have pictures of a HDD that lit on fire after one such event...Quote from: jayfehrIf they're the same size fuse, then it's simply a race condition. One of them will blow, but you don't know which. The upstream fuse should be rated less than the downstream fuse if you want to guarantee the upstream fuse is the first to fail (it also needs to be the same or a faster type).

author: jayfehr
author_rank: Contributor
timestamp: February 05, 2015, 10:08:27 pm
content: Yes, I am a little concerned about about surges. However, the Pi is only $35. There is a complexity/cost tradeoff I'm considering as well. I will also make sure I get a fast fuse, probably 1.5A, but I'll take a look and see what's available.I believe that I will go with the just the fuse for now, and continue looking into your other suggestions when I go to power the monitor. Really, as long as I don't burn the house down I'll be happy. I don't have money much invested into this entire project.Once again, thanks for helping.

author: edavid
author_rank: Super Contributor
timestamp: February 05, 2015, 10:15:43 pm
content: Quote from: jayfehr on February 05, 2015, 09:31:20 pm"Current surge" doesn't really mean anything. What would make the current surge? The most plausible reason would be overvoltage, but a fuse isn't going to protect you from that. You would need a power zener or traditional "crowbar" overvoltage protector, but the ATX supply already has an overvoltage protector built in, so why duplicate it? Also, it's not cost effective to put a lot of effort into protecting a $35 Pi from an unlikely failure scenario.

author: ovnr
author_rank: Frequent Contributor
timestamp: February 06, 2015, 02:14:50 am
content: Don't bother.A) A fair number of Pis are run from an USB port (yada yada, don't do it, whatever). They're thus directly connected to the PC's +5V rail thru a simple polyfuse.B) Remember that fuses are not instant, magical devices. If you're trying to protect the Pi, a fuse alone will do squat, as it's too slow to respond. You will need a voltage clamp after the fuse.C) The Pi is, as you said, $35.Lastly, consider the fact that if you have no other loads for the PSU, it will likely - if it's as crusty and old as I imagine it is - eat 10-20W by itself; the quiescent power loss is so high you could run several Pis on it. Just get a cellphone charger rated for a couple amps (... but not a shitty deathtrap from china).

author: darkness
author_rank: Newbie
timestamp: February 06, 2015, 10:55:31 am
content: I'd just connect it to the +5VSB rail. It can deliver at least 2.5A while the powersuppy is "off", and will be current limited.See page 27 ofhttp://www.formfactors.org/developer/specs/atx12v_psdg_2_2_public_br2.pdf

author: jayfehr
author_rank: Contributor
timestamp: February 06, 2015, 03:28:21 pm
content: @ovnr, I do plan to run the display off the same power supply. I'll be disconnecting the mains power and wiring the 12v (most likely, we'll see once the plastic is removed) directly to the dc in on the display.Obviously, I need to probe the display before I connect anything to it. I'll probably be back here with some more questions once I get to that point.

author: ivan747
author_rank: Super Contributor
timestamp: February 06, 2015, 05:15:12 pm
content: The raspberry pi has it's own fuse (a PTC fuse) built in, just so you know.Extra protection would be your choice.

author: ovnr
author_rank: Frequent Contributor
timestamp: February 07, 2015, 11:18:04 am
content: Quote from: ivan747 on February 06, 2015, 05:15:12 pmEver looked at the spec sheet for one? It's quite illuminating.They're not very good fuses.

author: Stonent
author_rank: Super Contributor
timestamp: February 07, 2015, 09:24:00 pm
content: Unless you needed the 12V from the power supply as well, you could just use a cell phone charger and save on the power.If you're worried about voltage spikes, you could feed from the 12V line and use a separate 5V regulator.

author: lapm
author_rank: Frequent Contributor
timestamp: February 07, 2015, 09:49:20 pm
content: I run my Raspberry Pi from mobile phone charger. 1,5 A max current capacity... No problems.. Heck i can even run it from one of those mobile powerbanks for few hours if i absolutely need to...

author: LukeW
author_rank: Frequent Contributor
timestamp: February 08, 2015, 04:19:03 pm
content: If you're prepared to run hundreds of dollars worth of a PC's hardware (and potentially an enormous value worth of data) from a standard ATX power supply, and you accept that the probability of a power supply failure that somehow destroys the rest of the system is sufficiently small for an ordinary consumer-grade ATX standard PSU, then obviously you should be fine to take that risk with a $40 raspberry pi.Assuming that the monitor runs off 12V, you should be able to just run the monitor off the 12V rail and the RP off the 5V rail.Extra overcurrent protection devices are probably optional, if you want to add them.Even a cheap ATX PSU will have ample power capacity to run a raspberry pi and a monitor.

author: Kjelt
author_rank: Super Contributor
timestamp: February 08, 2015, 04:59:06 pm
content: Older atx power supplies needed a minimum load to operate properly, i am not sure if your 3,5W will bevsufficient. Why not buy a decent industrial PS like a Puls or TDK-Lambda they are not that expensive.

author: jayfehr
author_rank: Contributor
timestamp: February 08, 2015, 05:01:35 pm
content: Thank you everybody for the responses. But I do want to clarify one point. I will be using a single power supply to power the entire unit. The Pi is where I am starting. However the display and fluorescent light will also be wired directly to the PC power supply. This means one switch will power the entire unit.The other option is a power strip and an extension cord. Although a much simpler solution it's not as much fun, nor do I learn anything. I also don't think it'll look as clean in the end.

author: jayfehr
author_rank: Contributor
timestamp: February 08, 2015, 05:04:55 pm
content: @Kjelt, other than the Pi and buttons everything I am using is being recycled. It's a fun way to get rid of some of the spare parts I've had kicking around.