Post by thread:[EE]: Automotive Circuit Protection

Hi Roman, Thanks greatly for the comprehensive overview of protection cktry you use on automotive devices. This is a great wealth of practical information, useful for design, reliability, and testing. It took me a while to digest it all. Anyone who has seen the underbelly of a transistor knows electronics design involves at least as much practical as theoretical. Good rules of thumb - simplicity, overkill, bipolar devices to buffer CMOS. I favor using the "smoking finger test" for determining whether a device is running too hot. If I can keep my little pinkie on it, then it is probably ok. Otherwise, I re-design for cooler operation. [sounds like you would favor using a baby's pinkie - figuratively speaking, of course]. I have one question. You mention using zeners in several places for spike suppression. Have you any experience using transzorb TVS devices [low-inductance, fast-breakdown diodes] to protect I/O lines? Should be superior to std zeners there. thanks greatly, - Dan Michaels Oricom Technologies http://www.users.uswest.net/~oricom =================================== ORIGINAL FOLLOWS (definitely worth re-reading): Roman Black wrote: {Quote hidden}>Dan Michaels wrote: >> >> Roman Black wrote: >> >> >Of course, if you wan't the *ultimate* in product reliability >> >you might want to use the extra safety parts. I do this in our >> >automotive applications. >> >> Hi Roman, >> >> Automotive environments are so incredibly noisy and spike prone. >> Could you give a brief rundown on the basic protection schemes >> used on the v.busses and I/O lines? >> >> best regards, >> - dan michaels > > >Hi Dan, now I'm not going to pretend to be some type of automotive >electronics expert, as that is not my main field. But, I will say we >have never had a product fail in service, and since many units we sell >are used in racing motorcycles which often have no battery(!) and >butchered charging systems I must be at least doing something right. > >I am a big believer in simplicity and overkill. Simplicity lets you >use a big resistor as opposed to a smaller more expensive semiconductor >or MOV spike suppressor etc. For the same parts cost you often get >greater durability. Overkill to excess, I will use a 2w resistor in >a circuit that dissipates 0.1w or even less. Resistors are cheap >compared to the cost to you of a product that fails. > >My theory behind this comes from 20+ years of servicing TVs, etc, >and it has been my experience that the most unreliable parts are >any semiconductors, any varistors, any components full of corrosive >(electro caps) and reliability is directly proportional to how cool >any device runs. With the new tvs, I can touch the heatsinks for the >the power semis and give you a pretty good estimate whether that semi >will fail in 1 month, 1 year, 3 years, or never. Any service guy who >fixes 50+ units a week will know what I am talking about. :o) > >For power filtering in auto applications I prefer simplicity of two >large series resistors, with a cap to ground at the point between >them. This is ideal for PIC apps which draw a small and regular >current. I only use metal film resistors of course. Carbon resistors >are a monstrosity! I use 630v polyester cap between the two series >resistors (to gnd), these caps have better reliability than just about >anything in that size and price range. With a 5v chip running from >12v auto, I drop about 4v across the resistors at normal running >current and the other couple of volts with a low voltage drop out >3 pin regulator. I also use a 5w 24v zener to absorb transients >that get through the two resistor ladder network. These big zeners >last forever when spike buffered by a couple hundred ohms, mainly >since it has zero temp as it doesn't conduct during normal >operation. Yes I could use a 1w zener, but the 5w has a much larger >silicon die and larger legs, and hence stronger mechanical coupling >during heat expansion etc etc. Always us a BIG part if you want it >to last forever, what the heck, costs 20c more per product! > >With a very low power app I would just use the two series resistors >to a 5.1v zener, with a 630v poly cap and 24v zener at the spot >between the two resistors, and obviously a large 22uf or more tantalum >(did I mention I hate electros?) and this should suffice. When I was >a kid I used to buy the ex-military and ex-commercial circuit boards >from the local scrap dealer, and strip the parts for my hobby use. >Surprise I found many of the electros were shot but NEVER found a >dead tantalum. Also noticed the military stuff always used tantalums. >How much price difference? How much do you want a little can of >corrosive sandwiched between your heasinks? Yes I will sometimes >use electros but only if no other choice, and I will use 105 degreeC >types which have better rubber plugs and different corrosive type. >Then I just prey that I mounted them far enough from any possible >heat source. :o) > >With input/output buffering, again use the largest ohms value >series resistor you can use, always use a buffer transistor, something >like a little BC337 (800mA 50v) is tiny and cheap, but has a HUGE >silicon die when compared to the in/out pins on a pic chip! If you >buffer this with a decent series resistor and cap, it will be pretty >bulletproof. If you need to run any decent output current from >the PIC use a transistor with about 10x the current/pwr needed, >again to use the maximum size silicon die and metal backing plate >(never use a transistor you haven't pulled apart) etc. I like >BD139/BD140, these have a huge backing plate and chunky die and >are dirt cheap if you can fit the physical size. :o) > >Remember that MOV varistors and the like sound nice, but many types >use an unstable metal "dust" compressed into a solid, and I have >replaced so many of these things over the last few years I reject >that as a solution. Nothing kills spikes as reliably as a RC network, >or a RZ network and with good choice of R and C (or Z) it will do >so forever. Many of the MOV type solutions are a trade off of "MOV >kills spikes, which at the same time slowly kill MOV" that suits >many TV/telecom manufacturers, but not suitable for a product I'm >putting my name on! > >I apologise to the many professional engineers here for my "bucket >science" solutions! I am not an engineer, I'm maybe a "re-engineer"?? >;o) >-Roman > -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! use spam_OUTlistservTakeThisOuTmitvma.mit.edu?body=SET%20PICList%20DIGEST

Dan Michaels wrote: {Quote hidden}> > Hi Roman, > > Thanks greatly for the comprehensive overview of protection > cktry you use on automotive devices. This is a great wealth of > practical information, useful for design, reliability, and > testing. It took me a while to digest it all. Anyone who has > seen the underbelly of a transistor knows electronics design > involves at least as much practical as theoretical. > > Good rules of thumb - simplicity, overkill, bipolar devices > to buffer CMOS. I favor using the "smoking finger test" for > determining whether a device is running too hot. If I can keep > my little pinkie on it, then it is probably ok. Otherwise, I > re-design for cooler operation. [sounds like you would favor > using a baby's pinkie - figuratively speaking, of course]. > > I have one question. You mention using zeners in several places > for spike suppression. Have you any experience using transzorb > TVS devices [low-inductance, fast-breakdown diodes] to protect > I/O lines? Should be superior to std zeners there. > > thanks greatly, > - Dan Michaels > Oricom Technologies > http://www.users.uswest.net/~oricom > =================================== Thanks Dan, them's nice words! And I have got 8 years of "theoretical" too hee hee! Re the transzorbs, no I don't think I have ever used them. I would guess I've replaced some of the dead ones in TVs with big zeners though! ;o) Seriously, we currently buy the big 1w/5w zeners in bulk, and I have used them for enough years to be real confident with what they will take and what they won't. I know roughly what mass of metal is inside them and what to expect. I feel a bit unsure of new technology stuff, often in my work I find that what should be a higher performance part due to new operating principles is often "exploited" in a manufacturing attempt to save silicon, make it smaller etc, that the benefits get a bit foggy. ie, "yes it's a whiz-bang new design that will safely conduct 10 times the current, so we will use 1/20th the size of silicon inside it". Mosfets especially fall prey to this kind of thinking. Even more so when they put huge current/power specs in a tiny package. Laws of physics just don't work like that... Now I don't know if this relates to the transzorbs, but to me they are still an unknown... If I had been using them (or working with them in other peoples designs) for 10 years I would know pretty much what they take and what kills them. Now the resistor, zener, cap solution I know well and it works. We use beautiful high quality metal film 2w resistors, good caps and big zeners of a known brand. At bulk quantities we get these for a few cents and THEY WORK. Really well. Not only do they protect well, but there is no reason to think they will ever fail. Now with the transzorbs, do you use a large series resistor? With the R/Z/C system it may not clamp it as tightly, but the BULK of the power of any spike is dissipated by the resistor, which as a piece of ceramic with metal film wrap, is probably my part of choice if something has to take the strain and dissipate the power, especially intermittant power with the associated heat/cool expand/contract that semis really don't like. Ideally if you can run a part at 10% of its ratings (I,V,and P) and also allow in P for the part to run cold, it will last forever. I have stuff that was built by me 20 years back that has never had a part failure, save for replacing some electros as the rubber caps perish after 5 to 10 years. Dissipating heat itself is not bad, I often make linears that give off 100W+, the trick is using enough mass so that temperature rise is minimal. You can have four big transistors giving off 100w at a temp rise of 3 degrees celcius, and they will last forever as they run cool. However a small switching chip that gives off only 5w and runs at 10 degree celcius rise will fail in a year or two, as it is running hotter AND heating/cooling thousands of times every second. Then you get an engineer who says "great, this chip is rated at 2A, and I only need 1.2A" I work all day fixing 2 year old TVs with multiple blown semis, then I sometimes get a 15 year old TV with dry joints. Why has it failed with dry joints? Because in 15 years it has NEVER had a component failure. The weakest link was the actual metal properties of the solder itself. There's an engineer that did his job! :o) -Roman -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! use .....listservKILLspam@spam@mitvma.mit.edu?body=SET%20PICList%20DIGEST

> > I have one question. You mention using zeners in several places > > for spike suppression. Have you any experience using transzorb > > TVS devices [low-inductance, fast-breakdown diodes] to protect > > I/O lines? Should be superior to std zeners there. > Re the transzorbs, no I don't think I have ever used them. > I would guess I've replaced some of the dead ones in TVs with > big zeners though! ;o) That probably isn't such a hot idea. Although a tranzorb/transil has very zener-like characteristics, their application is quite different. The zener-resistor-cap is ideal for I/O and signal lines because normally there isn't much current flowing and the influence of the resistor is negligible. When something like a load dump comes along, that form of protection allows the (outside) signal line to go as high as it wants with the inside being protected by the clamp. The power is disipated by the resistor but it is limited by the value of the resistor which can be set fairly high. However, if the same circuit is applied to the power supply input (or a relatively high current load switch) the drop across the resistor in normal operation costs too much in voltage overhead so it can't be used. That means that when the surge comes along, there is nothing to limit the clamping current so much more will flow. The transil can cope with these types of currents while a zener will be vaporized. Obviously a zener is cheaper and generally smaller and you are getting a bit of input filtering at the same time. If you can't afford the impedance then a tranzorb is required. One thing that is particularly impressive about the zener protection is that it covers the spectrum from low speed, high energy disturbances through to high speed ESD pulses. Steve. ====================================================== Steve Baldwin Electronic Product Design TLA Microsystems Ltd Microcontroller Specialists PO Box 15-680, New Lynn http://www.tla.co.nz Auckland, New Zealand ph +64 9 820-2221 email: .....stevebKILLspam.....tla.co.nz fax +64 9 820-1929 ====================================================== -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! use EraseMElistservspam_OUTTakeThisOuTmitvma.mit.edu?body=SET%20PICList%20DIGEST