Don't be like me - stupid, impatient, dumb, irresponsible, etc. 💀 - Page 1
Forum section: electronics / beginners

author: mindcrime
author_rank: Unknown
timestamp: July 13, 2022, 02:11:02 am
content: Alternate title: "Adventures in Fried Raspberry Pi"Alternate, alternate title: "Use your programmable load, dumbass!"...I've been working on this project. It involves powering a Raspberry Pi using some circuitry I cooked up to allow hot-swapping power sources. This has been going on for a while, and I've had a few different iterations of this working to varying degrees. I finally got something usable a few days ago, so life is good. But (there's always a "but")... it was based on a relay, and relays suck for various reasons, not the least of which is the physical space they take up. And in this case, having that big ole clunky relay hanging around bit me, as I broke one of the leads on the relay due to physical stress when assembling the overall system.No worries, just de-solder it and put it back, right? Yep. Did that, but changed the mounting orientation a bit to avoid the stress that broke the first one. And I don't know if it's because I did change the orientation of the relay, or if it's down to manufacturing variances, but the new board now causes the Pi to reboot when going off of the AC charger and back to the battery.Soooo... I figured I'd finally learned about enough to go back to attempting to do this the semiconductor way. So I baked up a new board, and started testing. Now one of the things about all of this all along, is that I've been using a boost converter as the final stage of the power setup to compensate for the voltage drop in my board (mostly due to diodes, but there's a "but" to that as well). Anyway, I have this boost converter wired up, and I set up my new board, connect the Pi and... <crickets>. Huh? Here's where I make my fatal mistake, and learn a valuable lesson. I throw a multimeter on the output of the boost converter and it's way lower than expected. So I grab a potentiometer trimming tool and start messing with it --- with the Pi still connected!You can probably guess how this story ends. I tweaked the dial a little too hard, or something glitched, or whatever and the output voltage briefly spiked to more than 7 volts. Not a lot in absolute terms, but guess what? The Pi has a Power Management IC (MXL7704)  that has an absolute maximum input voltage of 6V. Yikes.One might think "Great Phil, this is where you learn that you should order a programmable load to use for testing power supply related circuitry before testing with a real device - especially something that's becoming somewhat scarce and slightly expensive like a Raspberry Pi 4". Uuuhhh. Yeah, no. I already *had* a programmable load - aRigol DL3021A. I was just being stupid and didn't use it to validate all of this stuff before connecting my precious Pi.So there you go. Lesson learned. And now I have the opportunity to break out my SMD rework station and try my hand at Raspberry Pi repair (that is assuming the problem really is the MXL7704).Thankfully I bought like 4 Raspberry Pi 4's last year before the big shortage started, and I managed to secure 1 more earlier this year. So even if I can't repair mine, I have backups. But I sure want to be careful not to repeat this experience. Can't be blowing through those things.: or at least it used to. Searching for a possible replacement part, I see that these chips are apparently no longer manufactured. So presumably the newer revs of the Pi 4 have moved on to a newer PMIC? Will have to break out the magnifying glass and check out my fried one and see if I can identify what's on it. But in either case, I expect whatever it uses has similar characteristics. And *something* on the Pi is definitely borked one way or another.EDIT: looks like the manufacturer replaced a couple of parts in the MXL7704 series with a newer part, which is still an MXL7704, just with different suffixes tacked on. I guess theMXL7704-BQB-Ris the new one. Not 100% sure that's what the newer Pi boards use, but it seems like a good guess. Anyway, just a slightly interesting sidebar to this whole topic.

author: jwet
author_rank: Frequent Contributor
timestamp: July 13, 2022, 03:15:49 am
content: We all do bone headed things in this hobby. I can't count how many times I've decided to "fix" something that was slightly broken and made it completely broken. Luckily most of the raw bits and pieces that we work with are relatively cheap. Recently, I went to clean the touch screen LCD on my Whizoo Toaster Over Reflow oven. It had been sitting around and while and had a couple of sneezes on it. I took the plastic bezel off so I could get to the edges and not just push all the shmoo into the corners and spritzed it lightly with windex. The display is held with thin double sided tape and connects with a wide fine pitch kapton connection that solders down. My vigorous cleaning dislodged the display and broke some of the solder connection. A little hot air, some fine soldering and nothing but more disconnections and now a small tear in the kapton. Of course, its 2 am and I'm trying to reflow some boards urgently. Luckily, I emailed Whizoo and they had a board for me in a couple of days- $70 lesson for the evening. It just happens. Take a little time away or take a run.Regards- I feel your pain.

author: mindcrime
author_rank: Unknown
timestamp: July 13, 2022, 03:20:59 am
content: Quote from: jwet on July 13, 2022, 03:15:49 amHah! I knowexactlywhat you mean!