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Almost bulletproof
salbayeng   6/19/2014 2:59:13 AM
I once had a client frantically ring me because they had reversed the battery on a motor controller, wanting to know if they had killed it and what to do next. Simple: plug it in the right way. The controller included a large PTC on the incoming battery line and a unidirectional transorb. (The unidirectional transorb is just a zener, so you get a free reverse conducting diode) . I knew they would eventually plug one in backwards!

n.b. If you ensure the PTC and TVS are thermally coupled, then a sustained overvoltage will heat up the TVS, which heats the PTC and it goes high resistance.

On a related solar powered buck converter design, the collector of the switching transistor connects to one leg of the coil (and a freewheel diode). As I wasn't 100% sure of what might happen in all voltage/current/ambient combinations, I added a SMD PTC between, and thermally connected to both the coil and the transistor. This way if the transistor or coil (or diode) overheated for whatever reason they would heat up the PTC and minimal current would flow. 


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Re: Ratings of IC's and caps
salbayeng   6/19/2014 2:32:06 AM
Hi Jim, 

While you are probably correct regarding the origin of the caps, there is a lesser known feature with tantalums related to surge current. This can cause their destruction when hard switched to a low impedance source, consider an ESR of 10mΩ, when switched across a 12v lead acid battery, I=V/R = 1200Amps , instantaneous power = 1440000W.

What the wise old mentor would tell you is to only use a tant that is gauranteed for surge operation   (these cost more, so be wary of someone in purchasing doing  a downgrade). The other thing the sage would do is to add a little "seasoning" to the circuit, in this case add a PTC,(and a ferrite bead), this not only protects from shortcircuits but minimises the surge current too. 

Also be wary of shortcircuiting a tant with screwdriver or similar, as it can cause the same damage. 


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Ratings of IC's and caps
jimfordbroadcom   6/18/2014 6:54:09 PM
Back in the old days, there was an IC company called Comlinear that put Max and Min values in its datasheets for every parameter.  Since sold to National Semconductor which in turn was sold to Texas Instruments, of course.  Were Comlinear's parts better than the other guys'?  Not always, but I liked to use them because they at least had the guts to stand behind the specs.

And somebody mentioned capacitors.  Used to be you would derate the working voltage by a factor of 2.  Derating of tantalum caps was particularly critical, and the factor of 2 stopped working.  About 10 years ago, I had a 20 WVDC (working voltage DC) cap with 12 V on it blow up in my face (no, the polarity was not reversed).  It seemed that the derating factor was going up to 3 or 5 or more, and at that point I was wondering if the WVDC was worth anything at all!  My suspicion is that the newer caps were coming from China, known for low prices by not for quality control.

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Reliability - most all the information is out there...
MWagner_MA   6/16/2014 7:29:25 AM
Jack, music to my ears.  In the same time or less, I can select parts that will last longer than going through the "Trial and error method.".  From a personal stand point, by spending just a little more on "the good stuff", everything works in my house, and I don't spend every weekend fixing stuff.  For those willing to read and learn the information about how to select components for high reliability is out there for free - you just need to want to do it right the first time.

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