On the H-Bridge Crowbar effect: I've got an H-bridge made up out of discretes driving an itty-bitty DC motor. I used discretes because of low cost and low power requirements. I have one side of the H-Bridge driven from one port pin, the other side of the H Bridge driven from the other port pin. Valid inputs to the H-Bridge are 1,0 (forward); 0,1 (Reverse) or 0,0 (Do nothing). If I input 1,1, the H bridge crowbars the power supply. This could be a handy feature if it was one of those self-destructing messages they give to James Bond, but it's not. I really hate having invalid states that are not locked out, either in software or hardware. I always like to check to see if variables are in range, setpoints are not some crazy number, EEPROM contents are sane when recalled, etc. Now I'm careful to make my software never get even close to driving both inputs to 1, with a nice delay of a few cycles, and a mirror byte so I can't get hosed by the read-write-modify bug. So far so good. But what about when a little glitch comes along and this thing begins to crowbar? Maybe I do want to destroy the device at that point... but let's assume I don't. It would be simple to add a little logic circuit to prevent both inputs from going high at the same time. Digikey sells a 2-input Xor gate for US$0.35, two of them would do the trick. $0.70 might sound cheap to you rocket scientists, but I build whole PC boards that cost less than that. Can anybody think of a simple, cheap way to implement this, say using diode logic or a To-92 transistor? Kind of a design challenge. What I had originally wanted was an H-Bridge that would go forward for a 1 input, reverse for a 0 input, and do nothing for a high impedance input, but I could not thinka one. -- Lawrence Lile ----- Original Message ----- From: "David VanHorn" To: Sent: Tuesday, August 28, 2001 8:54 PM Subject: Re: [PIC]: PIC reseting due to relays > > > >Setup: pic chip controlling a motor in a typical h bridge. PIC just > >happened to be an EPROM variety compleat with window. > > > >System was on and running, customer said "Looks cool, gotta get a photo for > >the folks back home". He grabbed his camera, click, flash, bang, smoke jets > >out, motor stops, shows over. > > > >Seems the flash latched up the photo sensitive pic and caused just the > >right outputs to go on to fire top and bottom of the h bridge simultaneously. > > > ROTFLMAO!!! They DO recommend that you cover those windoes! > > I've never had THAT experience, but I did have an ericsson 3717 crater, > while I was sitting talking to the rep about excessive cratering. It was > just sitting idle, power on. Turns out there is a max switching frequency > on the choppers, which is poorly documented, and apparently none of their > tech guys were aware of, that we were exceeding. So, in some parts, one > part of the bridge was not quite off when the other part was turning on. > > Good timing though, we had been talking for maybe 15 minutes about this > very problem, and suddenly smoke is pouring out of the product, little > flame jets inside, but the case masked that from view. > > -- > Dave's Engineering Page: http://www.dvanhorn.org > > I would have a link to http://www.findu.com/cgi-bin/find.cgi?KC6ETE-9 here > in my signature line, but due to the inability of sysadmins at TELOCITY to > differentiate a signature line from the text of an email, I am forbidden to > have it. > > -- > http://www.piclist.com#nomail Going offline? Don't AutoReply us! > email listserv@mitvma.mit.edu with SET PICList DIGEST in the body > > -- http://www.piclist.com hint: To leave the PICList mailto:piclist-unsubscribe-request@mitvma.mit.edu