410757864530_dead_follicles [she/her]

  • 11 Posts
  • 44 Comments
Joined 7 个月前
Cake day: 2024年3月4日
  • 410757864530_dead_follicles [she/her]@hexbear.netOPtodiy@hexbear.net•Sphynx electrolysis development journal entry #5: the most annoying bug ever; also a websiteEnglish
    2·
    6 个月前

    Big update on killer bug squashing progress:

    I’ve narrowed down the source of the bug to the current source at the high side of the circuit. Many of the likely issues implicate the current sink at the low side of the circuit too. See entry #3 for schematics. I thought I could reproduce the behavior with the steps in the original post, but it turns out 1) it’s just flaky and it’s not as repeatable as working on first startup, and 2) it can be reproduced removing the entire digital timing board from the loop and tying the pulse line to ground, and even sourcing current from the output of the high-side current pump to ground instead of sinking it into the low side current sink. It’s not the instrumentation amp either, I built another mk2 without populating it and I’m having the same issues. I haven’t rigorously evaluated the mk1 version of the current source (see entry #1 for schematic) but I believe it behaves a lot better, and with the main two differences being the choice of opamp and the various resistor values, I think the latter is the culprit - I think there’s a lot of ways that using a very small R4/R2 ratio might cause instability, especially considering that tolerances could potentially have a larger than expected impact after all the algebra works out. That said, I do intend to try it both ways - both the new opamp on the mk2 board with the old resistor values, and the old opamp on the mk1 with the new resistor values. This will help me narrow down the source of the problem even further. I might also try to run some simulations with real opamp models instead of ideal ones, and model tolerances into the resistors. I still have confidence in this current source/sink topology so I’m not giving up yet. Of course my debugging options will be better once my scope arrives, but this is a thing I can do in the short term to avoid being stagnant.

    @macerated_baby_presidents@hexbear.net you always have really good EE advice - thoughts on the above? Any other potential failure causes you notice, any potential tests you’d like me to try, any other anything? Anyone else is welcome to comment too!

  • 410757864530_dead_follicles [she/her]@hexbear.netOPtodiy@hexbear.net•Sphynx electrolysis development journal entry #4: boards but IRL this time 🦾English
    4·
    6 个月前

    this is fantastic work, thank you so much! Since I have the F shank needles, I’m probably going to grab a couple of the Yasutomo’s and try to make myself a probe in the way you figured out - I’m currently holding on to the needle with an alligator clip which is completely a non-starter, they slip and fall out and just generally don’t work at all that way. It’s been fine for proof of concept but I’m excited to start testing something with a much nicer grip and a way to friction fit the needles in place!

    I’m not sure where I’m at with the 2.5mm jack, I’m starting to think I might want to just go with a loose wire and a screw terminal. It feels a little more hacky and it also makes it harder to move the device around, among some other issues, but it’s nice because it literally just takes wire - no other parts. Needing a screwdriver to install it is also a downside though. I’m still thinking on this one, 2.5mm jack and screw terminal are absolutely not the only two options. There has to be something available at hardware stores that’s a cheap and easy fix to this - maybe a spade terminal? Possibly some kind of spring-loaded wire terminal to avoid the need for a screwdriver? Things to think about, I’d definitely love to hear your thoughts too

  • 410757864530_dead_follicles [she/her]@hexbear.netOPtodiy@hexbear.net•Sphynx electrolysis development journal entry #3: More boards, more safety, and we're on sourcehut 🎉English
    1·
    6 个月前

    Hi thanks for engaging! trans-heart

    You honestly have a pretty good point about the board size, my original motivation was that ordering PCBs goes into price tiers - if I can keep it in a 100mm by 100mm square, it’ll be cheaper, but I think going up to 100mm by 200mm doesn’t add much more than a few bucks, so if it gets too cramped you’re right and I should definitely go up. Solderability is a real thing and you’ll hear some more about it when I make another post in a couple hours

    As for thermals, I think we’ll probably be okay in that regard, the whole board - LEDs, beeper, electrolysis front end, everything - should probably come in in the neighborhood of ~ 100 mW peak and ~20 mW average, so thermally it’s pretty chill and heat buildup won’t be too much of an issue - good looking out though!

  • 410757864530_dead_follicles [she/her]@hexbear.netOPtodiy@hexbear.net•Sphynx electrolysis development journal entry #3: More boards, more safety, and we're on sourcehut 🎉English
    7·
    7 个月前

    the parts count growing is a little bit of a bummer, but you’d be surprised at how cheaply I’ve been finding things, my 5V LDO is like $3, the LTC6993 is like $4.50, and then everything else combined is less than $10, so we’re not breaking the bank yet. Total BOM cost can probably stay under $40 🤞

    in order:

    • really good point on the transients, I think they should be pretty tame given that every device on the way up the chain has bypass caps, the voltage is all regulated by LDOs and there’s no inductors around, not to mention that a sufficiently brief transient doesn’t really pose a health risk (too low voltage to cause electrocution, too brief to cause over-electrolysis), but that all said there’s definitely no harm in adding some output capacitance with a bleed resistor!
    • test points kitty-cri-screm i’m used to poking legs of ICs with needle probes but I thought I was gonna be better but I forgot boohoo this is what I get for working while I’m sick kitty-cri
    • the end product will be one single PCB! I’m just debugging with separate ones so I can iterate on different parts of the topology without having to rely on an omni-board for testing. Once the three boards (battery/power, timing, current pump) are done, I’m just gonna merge all the designs and route it as one very dense PCB, so in total I’m thinking smaller than a 3x5 card!