Post by thread:[PICLIST] H-Bridge Crowbar (was PIC reseting due

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 removed}

At 10:29 AM 8/29/01 -0500, you wrote: > >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. In essence you want to force one low when the opposite one goes high. Maybe you could use a transistor and base resistor. Without more information on your circuit (especially the motor voltage and current- I'd assume it is not the same as the PIC power supply), it's really hard to guess what you're up to. If you post your circuit, I'm sure we can make suggestions. Best regards, =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= Spehro Pefhany --"it's the network..." "The Journey is the reward" spam_OUTspeffTakeThisOuTinterlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com Contributions invited->The AVR-gcc FAQ is at: http://www.bluecollarlinux.com =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= -- http://www.piclist.com hint: To leave the PICList .....piclist-unsubscribe-requestKILLspam@spam@mitvma.mit.edu

This time with the right tag. ----- Original Message ----- From: "Lawrence Lile" <llileKILLspamTOASTMASTER.COM> To: <.....PICLISTKILLspam.....MITVMA.MIT.EDU> Sent: Wednesday, August 29, 2001 10:29 AM Subject: H-Bridge Crowbar (was PIC reseting due to relays) {Quote hidden}> 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 {Quote hidden}> I could not thinka one. > > -- Lawrence Lile > > > ----- Original Message ----- > From: "David VanHorn" <EraseMEdvanhornspam_OUTTakeThisOuTCEDAR.NET> > To: <PICLISTspam_OUTMITVMA.MIT.EDU> > 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 {Quote hidden}> > 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 {Quote hidden}> > 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 @spam@listservKILLspammitvma.mit.edu with SET PICList DIGEST in the body > > > > > > -- > http://www.piclist.com hint: To leave the PICList > KILLspampiclist-unsubscribe-requestKILLspammitvma.mit.edu > > -- http://www.piclist.com hint: To leave the PICList RemoveMEpiclist-unsubscribe-requestTakeThisOuTmitvma.mit.edu