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On the topic of CIRCUIT damage prevention, please advise. - Page 2 — Parallax Forums

On the topic of CIRCUIT damage prevention, please advise.

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  • Peter JakackiPeter Jakacki Posts: 10,193
    edited 2020-09-01 15:49
    The HSP I used was the TBU-CA025-100 as part of the lightning circuitry on RS485. They cost around $1

    This is the 1A trip PPTC I used as well. Only about 6 cents each.
  • Clock LoopClock Loop Posts: 2,060
    edited 2020-09-01 15:52
    The HSP I used was the TBU-CA025-100 as part of the lightning circuitry on RS485. They cost around $1

    I try to avoid those DFN parts, not fun without an oven or a hot air gun.
    They are a lot cheaper tho.

    This is the 1A trip PPTC I used as well. Only about 6 cents each.

    A 1 amp pptc, if I ever repair that command station, that would be a good choice for the programming circuit. (perhaps even a 0.75 amp)
  • Peter JakackiPeter Jakacki Posts: 10,193
    edited 2020-09-01 15:55
    SMD parts are dead simple to solder. In fact they don't require any soldering skills at all. Just paste, place, and put it in a cheap toaster oven for four minutes. So easy. You'll never never know if you never never go and try it.
  • Clock Loop wrote: »
    I don't see the REV C schematic up yet, I figured it would be on the product page but it is not.
    REV A and REV B are here: https://www.parallax.com/downloads/propeller-2-es-eval-board-schematic
    I guess without the schematic, someone who owns the pcb will need to reverse it, to help you, sorry!

    That's OK. I need to learn about circuit protection. I have some high current projects I'd like to test. I could start by learning how to adjust current gradually. Two or three years ago I tried to design a spot welder. I charged a capacitor bank to maybe 9V though a 12V bulb and a diode. I bought a standard electrical switch from Home Depot to discharge the caps through three mosfets wired in parallel. First test I exploded the mosfets. I abandoned the project. Looking back I probably should have discharged the caps through a rheostat. I have other high current projects but I need to educate myself first.
  • Clock LoopClock Loop Posts: 2,060
    edited 2020-09-01 16:10
    SMD parts are dead simple to solder. In fact they don't require any soldering skills at all. Just paste, place, and put it in a cheap toaster oven for four minutes. So easy. You'll never never know if you never never go and try it.

    I said DFN, not smd.. (same diff?) I like most smt parts with exposed edge wing gull, or soic, or the like.
    I don't like to use the pcb as a sink to get the chip off the pcb.

    Production use, perhaps, but id still choose exposed lead package... Just personal preference, not lack of capability.
    I have a hot gun, I don't use it, just a precision temp solder station with fine tip gun, and some fine .38mm solder.

    I also don't like that you cannot inspect the solder bond very well.
  • DFN is SMD and works exactly the same way. It's like an oversized smd resistor with a center pad, that's all. Once you sit it on the paste then the heat reflows it and it is as simple as that. But I don't know how many times I to convince people that smd is dead dead easy the toaster oven way but they go "nah, it's too hard" without even trying.

    I also use a few layers of folded aluminum foil between the pcb and the tray though and that also makes it easy to pick up off the tray since it cools so quick too.

    I do use a gas iron as the precision hot air source to reflow parts or remove them. Hot air guns are too big and blowy.
  • kwinnkwinn Posts: 8,693
    I don't see any mention of using SCR's for circuit protection. They were particularly good for over voltage conditions, and would blow a fuse or open a circuit breaker fast enough to avoid damage to the rest of the circuitry in most cases.
  • DFN is SMD and works exactly the same way. It's like an oversized smd resistor with a center pad, that's all. Once you sit it on the paste then the heat reflows it and it is as simple as that. But I don't know how many times I to convince people that smd is dead dead easy the toaster oven way but they go "nah, it's too hard" without even trying.

    I also use a few layers of folded aluminum foil between the pcb and the tray though and that also makes it easy to pick up off the tray since it cools so quick too.

    I do use a gas iron as the precision hot air source to reflow parts or remove them. Hot air guns are too big and blowy.

    I hope I'm reading that correctly. Can a QFN IC can be soldered with a toaster oven? There is an IC I want that is only available in a qfn package. I have also heard that SMD capacitors must be heated evenly which rules out irons.
  • Cluso99Cluso99 Posts: 17,970
    Never had a problem hand soldering smt parts, caps included. Tiny tipped temp controlled iron, fine solder, and use a flux pen. And have solder braid handy. I’m not including QFN parts here. No problems with 0603 parts or the quad 0603 resistor packs, 0.5mm pitch ics (extend the pads out 0.5mm when designing the pcb). 0403 are much harder.
    Start off with larger 1206 and graduate down as you learn. Practice makes perfect.

    A stencil to place solder paste helps enormously. Keep you paste in a sealed ziplock bag in the fridge. My paste is still fine many years later. I bought a jar maybe 8 years ago. I decant a small amount into a miniature jam jar (seals nicely) and use that for a couple of years before replacing with fresh paste from the main jar.

    However, I do have a $5K Torch reflow oven that works wonders. I setup the temp profile once. No need to change the profile as it just works.
  • Cluso99 wrote: »
    Never had a problem hand soldering smt parts, caps included. Tiny tipped temp controlled iron, fine solder, and use a flux pen. And have solder braid handy. I’m not including QFN parts here.
    If 1206 caps can be hand soldered then I'll include them on my next pcb.
  • lardom wrote: »
    Cluso99 wrote: »
    Never had a problem hand soldering smt parts, caps included. Tiny tipped temp controlled iron, fine solder, and use a flux pen. And have solder braid handy. I’m not including QFN parts here.
    If 1206 caps can be hand soldered then I'll include them on my next pcb.

    Very doable.
  • Peter JakackiPeter Jakacki Posts: 10,193
    edited 2020-09-01 23:27
    @lardom - if you want to use QFN then I'd advise extending the recommended pad out enough that you can view and access the solder joint, and still be able to use an iron to touch it up if necessary. While you could hand solder these parts with the right pad, they are better soldered with paste and reflowed, either in the dedicated toaster oven (don't ever use it for food!) or a hot air pencil. I have a gas iron I use without the tip simply for hot air reflow when I need to touch-up or replace parts.

    It may help to smear a thin layer of flux on the bottom of the QFN to help it reflow cleanly too. This aids the soldering process and also helps to prevent any bridging.

    Because I build my own prototypes including a small number of preproduction types, I have had decades of experience with hand soldering smd but when it comes to irons I use a larger chisel type tip rather than a fine tip which btw, I never use. The reason is simple, the fine tip has a much smaller contact area and cannot transfer heat as well as a larger tip nor can it regulate the temperature as well. The chisel tip may contact more than one pin but that's not a problem. I use a syringe of flux gel I apply to areas that need touching up or may even have a short, and just putting the hot chisel tip on it reflows everything cleanly. I can even run the tip along the pins and watch them reflow. After testing you can clean up with IPA which I pump spray on and use an old toothbrush, then rinse off at an angle with more IPA.

    btw, you can solder caps and other parts with an iron and solder but the difficult part is keeping it aligned and flush to both pads. It helps to wet one pad beforehand, place and hold the part with tweezers while briefly heating that wetted pad will tack it in place. Solder the other pad and come back and touch up the first pad with an extra bit of solder, but not too much.

    I find 0603 easy to handle these days and 0805 I avoid simply because I want to be able to squeeze more, in less space! Use resistor arrays where you can since you can squeeze 2 or 4 much smaller resistors in an easy to handle package. Beware the metric vs imperial sizes! For 4 resistor arrays I use 1206 (3216 metric = 3.2mm x 1.6mm) but I see no reason why I can't use much smaller packages either. Resnets are cheap and you can put them in series or parallel to get different values too. For instance I use 2x100k in series plus another 100k as the voltage divider for my switching regulator and the other as the enable. Beats placing four resistors that take up much more room.

    I have a $29 toaster oven with the timer jammed on, an extra tray on the bottom, and use folded aluminium/aluminum foil as the carrier for the pcbs to sit on the middle tray. I leave it on max with an oven thermometer sitting inside and put the pcbs in, hit the kitchen timer preset to 4 mins and when that's up I remove the tray gently and place it on a rack to cool. Now that the pcbs are safe, close the door on the oven and normally I leave it on until I'm sure I don't need it.


  • @Peter Jakacki, thank you.
  • P.S. While I might use optos or solid-state isolaters, most of this is avoidable if you have proper routing for common grounds, keeping the heavy currents away from the signal currents, and thereby also the "spikes".

    I 'just' saw this. That's good to know. I struggled with electrical noise which is why I chose to use opto's.
  • evanhevanh Posts: 11,844
    Perhaps he is after some input on actual "circuit protection" rather than the semantics and terminology of it.
    It's not semantics to say that the fuse is there for safety. Passing that off as unimportant is why someone might use a piece of copper wire in its place.

  • Clock LoopClock Loop Posts: 2,060
    edited 2020-09-03 23:02
    evanh wrote: »
    Perhaps he is after some input on actual "circuit protection" rather than the semantics and terminology of it.
    It's not semantics to say that the fuse is there for safety. Passing that off as unimportant is why someone might use a piece of copper wire in its place.

    I didn't pass it off as unimportant, clearly the image with a BULLET as a fuse was an OBVIOUS joke.
    And my step dad who wrapped a fuse in tinfoil for Christmas lights was a MORON, and he even said "never do this".

    I have changed the thread title.

    "On the topic of CIRCUIT damage prevention, please advise."
  • evanhevanh Posts: 11,844
    Clock Loop wrote: »
    I didn't pass it off as unimportant, clearly the image with a BULLET as a fuse was an OBVIOUS joke.
    And my step dad who wrapped a fuse in tinfoil for Christmas lights was a MORON, and he even said "never do this".
    Huh, I don't see any such image anywhere.
    I have changed the thread title.
    Thanks.

  • evanhevanh Posts: 11,844
    edited 2020-09-04 04:16
    Oh, yeah, that picture would've made all the difference to me saying anything at all. :)

    PS: Because of all the tracking scripts these days, I have third-party domains blocked by default. I probably miss seeing many posted pictures on the forums.
  • Not to hijack the thread, but I want to underscore Peter’s assertion of the utility of a toaster oven. They work really well once you get the hang of it.

    I bought a $279 “professional reflow oven” on Amazon. Try as I might, it either left burn marks on the boards or left components in the middle unsoldered. Eventually I got tired of cussing it and spent $60 on a Walmart toaster oven, wired it “hard-on”, and within 4 attempts had some good quality boards. I sold the “professional reflow oven” for half of what I paid for it. Lesson learned!

    This effort also led to me learning KICAD. Rather than pay $7/ea for test boards, I rolled my own and made many “free” mistakes (the oven doesnt care if you misplace a trace, goof a hole diameter, etc. Lol).

    So yeah. What Peter said.
  • @JRoark, I'm glad you posted. Now I know not to try to reflow a $3 QFN chip without prior testing first. Did you have less than 100% success even using solder paste? Should the oven be preheated? Is there an optimal temperature, time?

    @"Clock Loop", I promise not to hijack your thread!
  • Clock LoopClock Loop Posts: 2,060
    edited 2020-09-04 17:31
    lardom wrote: »
    @"Clock Loop", I promise not to hijack your thread!


    I doughnut care.

    mmmmm-donuts.jpg

  • I can't speak to fuses/protection in someones project, but as @"Peter Jakacki" stated above, you will always need the fuse you don't have. I will say that as a rule I try to use very few fuses when troubleshooting a problem. Because of "Peter's Law". I may eat one, possibly two if I guessed wrong. But generally, I have a set of breakers from 0.5A up to 30A with alligator clips on them. That way if I am having to retry a few times, I just reset it. This was a trick from my TV repair classes at Voc Tech in a galaxy far, far away and long ago. Of course with this fuse substitution kit, you could easily put the 30A in place and you will likely find the defective circuit fairly quickly. Or at least "a" defective circuit.
  • lardom wrote: »
    @JRoark, I'm glad you posted. Now I know not to try to reflow a $3 QFN chip without prior testing first. Did you have less than 100% success even using solder paste? Should the oven be preheated? Is there an optimal temperature, time?

    All of my “recipes” start with a cold oven and cold board. It generally it takes 4-5 minutes of wide open throttle heat to get the whole board reflowed. I have a lamp dimmer inline with the supply but I basically only use it as a switch anymore. The little fan in the oven makes for an even heat. Once everything has flowed, power off and open the door all the way and you’re good in another 3-4 minutes. You can do QFN’s this way. In truth, I have never had a (new) component that I couldnt reflow successfully. Almost all of my “bad” boards are due to paste problems (too much paste usually). Cleanup is usually pretty easy with some GootWick and an iron. Then it gets a bath in IPA followed by a dunk in the ultrasonic cleaner. Makes some beautiful boards!
  • sam_sam_samsam_sam_sam Posts: 2,263
    edited 2021-02-22 14:31

    I hope that this question is not off topic

    What is the correct way to size a tv diode for a given voltage that you want to protect
    What is the correct way to size a reset able fuse for a given current that you want to protect

    For me rest able fuse has been trail an error approach but is there an easier way to figure this out
    I have tried to use tv diodes before but I do not understand how to correctly use them

    Could someone please give an example of the correct way to use both of them

    Thanks

    One method that I use when troubleshooting a switching power supply that has issues is to use a incandescent light bulb instead of a fuse for this purpose

    Or anything else that is blowing fuses

  • evanhevanh Posts: 11,844
    edited 2021-02-23 04:14

    Transient-voltage-suppression (TVS) diodes (aka: Transils and Tranzorbs) are high pulse current rated zenor diodes. If you don't need/desire high frequency on the signal path then the fatter it is the better. For high frequency operation, there is ultra-rapid parts available just like with regular diodes.

    For true spike suppression, there is potentially short high-current spikes. The current flows through the TVS and through whatever is on the other leg, usually the ground plane. If the grounding is not well planned, ie: just a thin track, then that'll rise plenty during the spike and therefore a significant extra voltage can exist on the signal track also.

    In tougher environments you'll want to add inline high voltage resistors in front of each TVS.

    As for choice of rated voltage, there is AC rating and DC rating. Bipolar parts (pair of back-to-back zenors) are usually AC rated while unipolar parts (single zenor) are usually DC rated. The main thing to work out is what is the maximum possible signal voltage then choose a rating a little higher than that. The more undefined the expectations are the more leeway you'll have to provide. For example, with mains power the expectations are so wild that no one uses TVSs for suppression. There they use things like gas-arrestors and self-healing capacitors.

  • evanhevanh Posts: 11,844

    @sam_sam_sam said:
    One method that I use when troubleshooting a switching power supply that has issues is to use a incandescent light bulb instead of a fuse for this purpose

    Yep, very handy to have current limiting abilities on the bench. A variac can stand in there too.

  • evanhevanh Posts: 11,844

    Fuses and circuit breakers is a wide ranging topic. There's upstream protection, downstream protection, electrocution factors and designed load limiting that all play into the choices of type and capacity. It's not something I'm up to speed on.

  • sam_sam_samsam_sam_sam Posts: 2,263
    edited 2021-02-23 16:08

    @evanh said:
    Fuses and circuit breakers is a wide ranging topic. There's upstream protection, downstream protection, electrocution factors and designed load limiting that all play into the choices of type and capacity. It's not something I'm up to speed on.

    Thanks for your reply’s this helps a lot

    How is the best way to use this “gas-arrestors” I have seen these before but I do understand how you would use them correctly

  • evanhevanh Posts: 11,844

    Sorry, not used those either. They're for heavy duty use, particularly in lightning prone situations.

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