On Friday 03 January 2020 10:35:14 Peter C. Wallace wrote: > On Fri, 3 Jan 2020, Gene Heskett wrote: > > Date: Fri, 3 Jan 2020 09:41:26 -0500 > > From: Gene Heskett <[email protected]> > > Reply-To: "Enhanced Machine Controller (EMC)" > > <[email protected]> > > To: [email protected] > > Subject: Re: [Emc-users] Somewhat of a puzzler with a 7i76D > > > > On Thursday 02 January 2020 15:59:09 Gene Heskett wrote: > > > > Reposted with some spelling corrections, clarifying language, and a > > thank you I forgot. > > > >> greetings & a happy new year to all; > >> > >> I finally figured > >> I felt good enough to go out to the garage and see if I could > >> resurect the 6040. When last putzing with it I had left it with > >> the circuit from machine motion to lights and coolant pump, and the > >> mist pump disconnected from the 7i76D field output 0 & 1 and was in > >> between sessions in the cath-lab fixing my ticker. > >> > >> One of these two was (I first thought) a dead short on the 12 volt > >> field supply, and when I traced the wire to the lights & motor > >> coolant relay, I found that I'd not installed a flywheel diode > >> across that coil because I was out of a suitable diode. So I > >> ordered a bag of 100 volt 4 amp diodes. > >> > >> While I'm keeping this chair warm, they came in, so I went out and > >> cut a 40" piece of light zip as that was more suitable wire, > >> crimping a diode into the hilitchi sleeves, pulled that relay loose > >> and connected it, throwing the other end of the wire toward the > >> interface box. Moved the ladder and hooked the other end up. > >> > >> Cycled it on and off from linuxcnc several times, worked perfectly. > >> Hooked up the mister motor power again, and it worked as designed > >> after a couple minutes to reprime the pump as it had a good month > >> to bleed back. > >> > >> By then my legs are telling me they are going to give me hell for > >> cramps in the night, I've been laying around too long. > >> But a tum calcium pill in each cheek headed that off. > >> > >> So thats the background. > >> > >> Question for Peter: > >> > >> Could the lack of that diode have triggered a substrate scr in the > >> 7i76D?, crowbaring the field power in the 7i76D, but which due to > >> the supply trying to cycle, never got clear to zero volts to reset > >> that invisible scr. > >> > >> Any other failure external to the 7i76D would not have been a > >> short. > >> > >> It could only be healed with a full powerdown, which is the only > >> theory I can come up with that makes sense. That same 7i76D output, > >> now with a flywheel diode seems to be running it flawlessly now. > >> The supply is rated at 12 volts and 2.1 amps so doesn't (probably) > >> have enough cajones to burn things up. All relays on, its a tad > >> under an amp total. > > > > I have seen this only 2 other times in my 70+ years of putting > > electrons to work. > > > > Thanks Peter. > > > > Cheers, Gene Heskett > > -- > > "There are four boxes to be used in defense of liberty: > > soap, ballot, jury, and ammo. Please use in that order." > > -Ed Howdershelt (Author) > > If we desire respect for the law, we must first make the law > > respectable. - Louis D. Brandeis > > Genes Web page <http://geneslinuxbox.net:6309/gene> > > I have not seen this before. usually if you switch an inductive load > with too much energy for the little MOSFETs in driver chip, they fail > in shorted mode > > The driver chips do have flyback protection but dissipate the energy > in the MOSFETs (if you have a flyback diode most of the energy is > dissipated in the coil) > I think I first saw the explanation in the mid 70's in one of Bob Pease's floobydust stories. Either in Electronics or Dr. Dobb's Journal. Unsure which at this late date since it's been a good 40+ years ago now.
So put this in your trivia file/box you use to impress the frogs with, I think I've found my 3rd instance. I had forgotten not putting in a flywheel diode, and it must have worked ok at least 50 times but was slowly damaging the scr-like layer in the substrate, similar to the slowly fading avalanche mode zener diodes which all zeners above 4.7 volts actually are. Now, with the flywheel diode (if its fast enough), even though the breakdown degradation that triggers the scr is still there in that output chip, the voltage at turnoff never gets to forceing the current level needed to trigger it again. Hence no additional damage will be done. I'd expect that same driver chip will now work normally until someplace in the far end of the bathtub curve, long after I've missed roll call. Moral of the story is of course that semi's can and WILL fail in the weirdest modes. Thanks Peter. Cheers, Gene Heskett -- "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) If we desire respect for the law, we must first make the law respectable. - Louis D. Brandeis Genes Web page <http://geneslinuxbox.net:6309/gene> _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
