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Forum Index : Microcontroller and PC projects : Expensive Boo Boo..... (MM2)
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GoodToGo! Senior Member  Joined: 23/04/2017 Location: AustraliaPosts: 188 |
Howdy all, ***Warning! Lengthy rubbish following!*** A while ago I was in the process of designing a Hybrid relay circuit to be controlled by a Micromite. The purpose of it is/was to remotely control a pool pump via a HC12 bolted to the Micromite. Grogster’s Wireless Node 1A looked promising for the brains of the unit, coupled to a Mosfet via optocoupler and a 12v relay via a transistor. On paper it looked magic.  The code would energise the Mosfet, then the relay, then de-energise the Mosfet. The Mosfet would absorb the current surge from the pump on start-up, then the relay would take over the running current. This prevents the relay contacts from burning out. Did I mention that it looked magic on paper? I carried out lots of research into Static relays, hybrid relays, Mosfet circuits, blah blah blah. It all looked so good on paper. The power side of things was taken care of by a linear power supply consisting of your usual transformer, Bridge rectifier, caps, voltage regulator. etc. All this dropped the incoming 230VAC to 12VDC. Man, it looked so good on paper! Almost Magic. I hocked up a schematic in Tinycad with all suitable values and double checked my calculations. I printed it out and took it out to the garage. It looked Magic! I cobbled all the components together onto a piece of Veroboard, being careful to ensure proper isolation of the bitey-bits from the not-so-bitey-bits. Much continuity and insulation checking ensued. I’ve been belted by too many ergotrons over the years and have developed a healthy respect for it. The Wireless Node 1A was connected to the Veroboard for power and 2 switching wires for the opto and transistor. In turn the Wireless Node 1A was connected to my garage Mini-pc via a standard USB cable. It even looked magic in the flesh! I confidently applied power. The Wireless Node 1A lit up and talked to the Console. Voltage checks proved that everything was right in the world. (except some sort of pandemic thingy) So, I set the appropriate pins to digital out and switched them on. Nothing.  Confused, I consulted the Magic Paper.  All looked as it should, no nasties were evident, just the optocoupler and transistor weren’t operating like I expected. I guess the Mosfet may not have switched without a load, but surely the relay should of? More voltage checks, nothing obvious. The Magic Paper was losing its shine….  So, comfortable with the level of isolation I had incorporated on the Veroboard, I did what probably no one would do……. I shorted the Collector and Emitter of the Transistor together.  Well, lucky I had the brown pants on. I was convinced the Police Riot Squad had broken into my garage and thrown a Flash-Bang or similar onto my desk, under my face. Many bright sparks, much loud noise, and much molten copper from the Veroboard spread forth over my bench before the garage circuit breaker called time out. Many “Golly Gosh Darn It’s” issued from my verbal hole while my vision and hearing returned to normal.  I unplugged it all, reset the circuit breaker and walked away for a few days. Eventually I returned to it and surveyed the damage. The Magic Paper had 3 big pretty burn holes in it and the Veroboard was missing a fair swath of its copper. Tracing the arcs on the board, I still couldn’t make out the mode of failure. It was obviously a short but from what to where I dunno.  Anyway, The Mosfet, bridge rectifier, optocoupler were all trash. In fact, I salvaged only the relay and transformer and turfed the rest into the bin. Concerningly, the Wireless Node 1A didn’t fair well either. Both PIC chips somehow copped it and were now short circuited. They followed the Veroboard into the bin too. However, the one thing that really upset me, and I still am not sure how it did it, was the garage Mini PC was now trash. The power brick for it had blown up, and after repairing it, I found one of the main PC motherboard 5v rails was also short-circuited. I’ve traced it to a faulty voltage regulator IC, and I’m hopeful that replacing it will fix it, but I’m not holding my breath…… It’s on the slow boat from China and should be here hopefully before Christmas…. 2021. The not-so-Magic paper has been archived as a reminder to me on how things can go real bad real quick.  I must have had some luck at least on my side because sitting on my desk, less than 6 inches away from where Chernobyl 2.0 let loose is a plastic container full of isopropyl alcohol. Now if that had of gone up, well best not to think about it. Stay safe all! Cheers! GTG!  ...... Don't worry mate, it'll be GoodToGo! |
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CaptainBoing Guru  Joined: 07/09/2016 Location: United KingdomPosts: 2170 |
vero board... I blame vero board. Sorry to hear you are having troubles, it's never nice when a lovingly designed and built project goes up in flames. So many times I have been on a hunt for cuts I should have made etc. That's why I went to bare perfboard (copper dots) and back-wiring (which I find quite therapeutic) with a netlist. Nowadays, I have got etching a pcb down to a fine art in under an hour or so. I do a PCB even for the simplest of things, guaranteed glitch free - the only wiring errors must be in the schematic. Did two small DC-DC converters yesterday with a laser and then after cleaning, etched them with ferric (rubbing with a sponge) in 68secs! From PNG to board in about 25 minutes. Bigger boards take longer natch. That said, I am curious to see the outcome when you get time to analyze things. Keep us informed! |
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| robert.rozee Guru Joined: 31/12/2012 Location: New ZealandPosts: 2442 |
SSRs and plugpacks are your friend! that way you can keep tinkering with the mains to a minimum  https://www.ebay.com/itm/371707212542 cheers, rob :-) |
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| Shadamus Newbie  Joined: 26/07/2020 Location: United StatesPosts: 17 |
Nicely written GoodToGo! Thanks for sharing a potentially embarrassing story. It may serve as a warning to those playing along at home, but if not, at least it was an entertaining read. Condolences on your lost gear/components, but congrats on still having a shed/house! |
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| Volhout Guru Joined: 05/03/2018 Location: NetherlandsPosts: 5089 |
I am not sure where the magic happened, but I have an idea what could have gone wrong. When I read your description, I think you need 3 power domains (isolated power circuits). 1/ The pool pump (230Vac) 2a/ The USB power from the Garage PC and the Micromite. 2b/ A 12V (connected to 2/) to power the mechanical relay 3/ An isolated voltage (i.e. 12V) that allows the optocoupler to drive the gate of the MOSFET. Regardless if you use a bridge rectifier, or 2 back-to-back MOSFET's this voltage needs to be isolated. I have the feeling (the text speaks of a bridge rectifer, not 2 bridge rectifiers)you could have combined 2b/ and 3/. Depending on the way your power plug was inserted in the wall outlet, potentially nothing would happen, until you start the pump. Hybrid relays.... danger... Volhout P.S. sometimes a simple relay (slightly more expensive) is sufficient. P.P.S. When the relay is switched ON, there may be no need to switch the MOSFET off. It is bypassed anyway. Edited 2020-08-08 00:40 by Volhout PicomiteVGA PETSCII ROBOTS |
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| GoodToGo! Senior Member Joined: 23/04/2017 Location: AustraliaPosts: 188 |
Same here..... and stupidity. Re-reading my post, I realised I stuffed one important thing up. The Mosfet wasn't a Mosfet, it was a TRIAC. (Well, it used to be) Other things of note, the Optocoupler was a zero crossing type. (Well, it used to be) There were 3 distinct current surges when the unit was in it's death-throes, this rapid surge on the mains is, I believe, what caused the mini-pc powerbrick to go bang, and thus causing the mini-pc itself to go bang. The PIC's dying must also have been due to a surge on their voltage lines, but the two voltage regulators on the Wireless Node 1A survived?? I dunno.... Cheers! GTG! ...... Don't worry mate, it'll be GoodToGo! |
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mikeb Senior Member Joined: 10/04/2016 Location: AustraliaPosts: 174 |
You're still alive with your hearing, and more importantly your sight, still intact. I once had a Wima electrolytic capacitor expel its insides, like a party streamer, in a vertical direction once. Glanced off my temple. That was as close as I ever want to come to losing an eye. The circuit seemed innocent enough while I was standing looking over it. I use safety glasses now when energising unproven circuits. Especially around switched mode power supplies (proven or not). All electronic components have the ability to launch shrapnel at you when things go pear shaped. Eyesight can get taken away in an instant. PPE is as useful at home as it is as work. Glad to hear you're still with us. And you learnt what ?..................................................  There are 10 kinds of people in the world. Those that understand binary and those that don't. |
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Poppy Guru Joined: 25/07/2019 Location: GermanyPosts: 486 |
Even "simple" soldering or cutting off anything can harm our eyes catching some splatters ... safety glasses are worth being recommended for nearly any practice!   Andre ... such a GURU? | ||||
Grogster Admin Group  Joined: 31/12/2012 Location: New ZealandPosts: 9610 |
Electro caps in reverse polarity make pretty good detonators! Last one that I blew up by accident was a 470uF from memory, and when it blew it's top, the cap left a dent in the ceiling, so yeah - they go off with some force, and not a good place to put your eyes if/when that happens. That is also followed by a horrendous smelling smoke. Very yucky smell. On the plus side, you generally only do that once in your electronic career, cos you remember the first one so vividly, that you never want to do that again!  If you need a couple of programmed 1455 chips for further experimentation, I can send to you. That would allow you to program a new 170 chip and get the Wireless Node unit going again. Can you program the 1455's yourself? (you need a PicKit3 and the Microchip IPE software + the HEX for the chip) Shorting a transistor C-to-E is acceptable, so long as you have suitable collector-side resistance in your circuit(for an NPN), and I do that very thing in one of my designs. If you don't have suitable collector-side resistance...... Good story, thanks for sharing. Smoke makes things work. When the smoke gets out, it stops! |
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| CaptainBoing Guru Joined: 07/09/2016 Location: United KingdomPosts: 2170 |
good advice and some I really ought to follow more closely. |
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| CaptainBoing Guru Joined: 07/09/2016 Location: United KingdomPosts: 2170 |
+1 got a really nice pair of polycarb ones.. on the bleedin' shelf. I really ought to push this more. |
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vegipete Guru Joined: 29/01/2013 Location: CanadaPosts: 1132 |
Many years ago, a fellow university engineering student demonstrated the fun that could be had with an electrolytic capacitor soldered to the end of a power (mains) cable. Some modicum of safety was followed in that the capacitor end of the cable was passed under a closed door into an empty adjacent lab room before the cable was plugged into the wall. The resulting bang and cloud of smoke was very appealing to us trouble-makers. ================ Some years ago reading a trade mag, I discovered the ideal eye protection. Safety glasses are a must but as eyes age, reading glasses are also required. I was very pleased to discover a combination of the two:  Dentec Bifocal Safety Glasses, example retailer At the time, a box of 12 glasses was 60-odd (Canadian) dollars, including shipping. Available in diopters of +1.5, +2.0, +2.5. Clarity with a new set is often so good that I occasionally have to try poke myself in the eye to confirm I actually have them on. If you can find them in your local market, I highly recommend them. I pretty much wear a set of +1.5 all the time as general purpose glasses. I'll even put 2 pairs on if I need more magnification. The silicon nose pads tend to vanish but that doesn't bother my nasal anatomy. Perhaps my only complaint has to do bifocals in general. There is no magnification when you are looking up which can be awkward sometimes. Putting the glasses on upside down is less than ideal. The other thing I did was put dabs of paint on the arms so I could identify the diopter of a given set by colour since the imprinted text on the bridge is too small to read without glasses. Visit Vegipete's *Mite Library for cool programs. |
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