Retroplayer:
BTW, you may want to start writing some nasty letters to IC manufacturers, because they have been putting clamping diodes across the transistors for decades. Like these guys, for example: http://www.mitsubishielectric-mesh.com/products/pdf/m54530p_e.pdf
Stupid, manufacturers! Don't they know any better?!!!!!!!!
Must be a bunch of stupid programmers. lol
Diodes are often put on inputs and outputs for protection, but they are to protect the transistors or IC's they are on. it is your responsibility to look after the rest of the circuit. Why not get a detailed diagram of one of the mcu's used on your beloved arduino - oh what a surprise, they have TWO diodes on all inputs, that is to help protect the device (and in theory unnecessary if everyone does their job right). and you might be really surpruised to find that most other mcu designers do it.
Next you will be showing me the built in diode across mosfets ? before you do I'll forewarn you, those are part of the manufacturing process, cannot be helped and can be a nuisance in some situations but you can't change the physics that govern the manufacturing process.
sparkylabs:
You have just contradicted yourself. No matter what the load any inductive load will produce a voltage spike with reverse polarity when power is removed. be it a simple power on, power off, or the off periods of a PWM drive. The diode provides a shortcircuit path for the destructive voltage.
I have run test for my work place and shocked them with osciloscope graphs of -400v spikes running around the system because of lack of diodes
Who contradicted whom?
go re-read it, you said the same thing twice but drawing different conclusions for both.
I happen to be an electrical engineer for a major aerospace company. So I guess if you lack confidence in my education, perhaps you should avoid flying.
Having a bad day, are we?
Honestly... it doesn't make a flipping difference, really. Back EMF gets shunted to ground with the diode. It follows the path of least resistance and the diode provides that. That super scary 400 VOLTS!!!!!! you are so eager to show us has practically no current capacity to it at all to it. It is just absorped by ground within femtoseconds. I can one up you and show you 1000V!!!! by walking across a carpet!!! But very doubtful I will blow a BJT by touching it. Why? Because it has practically NO current to it at all, not even close to enough to breakdown the junction in a BJT. FETs on the other hand... and even that is actually more theory and suspicion than even actually proven.
The danger is all in the voltage itself and the way semiconductors are formed. A high enough potential across the collector to emitter MAY be enough to breakdown the junctions and the transistor self-destructs. MUCH more likely with a FET than a BJT. In fact, do you have any idea how many thousands of everyday products that you use that do NOT even have a diode in the circuit? Yet they keep on clammering away, happily. Imagine that!
It CAN damage a transistor, and it is good practice to have a diode whichever way you choose to install it. But it is not even close to being as dramatic as you are making it. If one were to run a 12V motor (especially just a fan) and not even put a diode in there, I doubt they would ever even have trouble.
You go read a book, Sparky.
Yes I am concerned about flying now !
You are now "thrashing around" into other arguments an situations. How much current does it take to destroy cmos circuits ? yes you are right, there is not a lot of current in those spikes but more than enough to damage control circuitry. Why and how are you dumping the spike to ground ? what do you think ground is ? A sponge ? yes static (your 1000V on a carpet - how about the 20'000V from combing your hair) can be easily "absorbed" as the energy is low but back emf spike are a different thing. Again I ask, tell me where that spike is going and what is eating it up when you have a diode only across the BJT, it hits the power rails, then what ? The testing I refered to where i got -400V was to work out why thermostats in the system where being broken when a certain relay turned off a certain fan (24V 15A), so much for little energy and that was not even using cmos stuff AND the spike was arcing across relay contacts before getting the the cheap automotive thermostat
sparkylabs:
The testing I refered to where i got -400V was to work out why thermostats in the system where being broken when a certain relay turned off a certain fan (24V 15A), so much for little energy and that was not even using cmos stuff AND the spike was arcing across relay contacts before getting the the cheap automotive thermostat
Sounds like I should be more concerned with driving!
Have a good day, Sparky. I hope you don't end up with writer's cramp with all those angry letters you are going to need to write.
P.S. And yes, high voltage (even with nano-amps of current -- negative OR positive) can spark across AIR, such as in a relay. I hope you don't believe that you are the first to have discovered that, because I believe ben Franklin and Leyden have you beat there (I learned that in a book!) I have no idea how that compares to what we are talking about here.
Oh and to answer your question: The cookie monster eats it. That's what happens to it.
sparkylabs:
Are you sure ? remember you can: 1) ask the audience, 2) phone a friend, 3) go for 50/50
Can I just roll over and ask your mother? lol. Ok, ok... maybe that was too far (but funny, yes?) lol
Hey Sparky, I was just playing with my oscilloscope and noticed that there is 120V at 60hz just floating around me. I am afraid I am going to get electrocuted. Where should I put the diode to protect myself? Wait, apparently I didn't learn... Why be so selfish.... where do I put the diode to protect the world from these stupid utility companies? I'm beginning to see stupid programmers all around me! Would a tin-foil hat protect me? Perhaps copper?
Silly me... and here I thought voltage was just the difference in potential between two points. I had no idea there were different 'flavors' of voltage. Learn something new every day!
Seriously though, people make entire careers out of claiming that the sky is falling and there will always be someone that comes along and believes it. We wouldn't have politicians if not.
BTW, you may want to check the board layout for your thermostatic fan/relay thingy-majig...
Thanks for the laughs, Sparky.
With all sincerity, have a wonderful day (afternoon there isn't it?) No actual harm was intended.
I've gotta go design faulty electronics for airplanes now...
1N4007 or similar. if you are doing PWM above 500Hz I'd suggest a schotky that can handle a few hundred mA, where you put it ? where ever you like just make sure it's in back to front or in one case the fan will never turn off and in the other you will short your transistor out.
Look at it this way. The coil of the fan can be likened to the spark coil of any modern internal combustion engine. Anyone care to grab a spark plug wire with the motor running? I didn't think so.
Static electricity will blow a hole in the glass insulation inside a CMOS device. Unbelievable? Try going out to the desert with lowest humidity, wearing your nylon running clothes, and run for 100yds. Now pick up a known good CMOS device by the leads. After you feel the shock (maybe) put the device in the circuit and see if it still works. Or... Just skoot your feet across a carpet, wearing your fuzzy kitty slippers. Same thing.
The old 3 finger rule still applies. (MFC = motion, flux direction, current direction)
The inductive kick is in reverse polarity to the current that expanded the field in the first place.
That's because the motion direction changed by 180 degrees.
The coil starts looking like a source. (e.g. high voltage battery)
Well, that's my 2 cents worth. Now back to the kiddie playground.
The point of the snubber diode is to contain that back EMF to a closed loop containing the coil and diode. It's not shunting anything to ground. The field collapses and generates a high negative spike, dependent on field strength and the speed it collapses. The diode contains this to that mini circuit and the energy is dissipated as the circulating current and respective magnetic field decays. The rest of the circuit is thus protected from that spike.
tack:
The point of the snubber diode is to contain that back EMF to a closed loop containing the coil and diode. It's not shunting anything to ground. The field collapses and generates a high negative spike, dependent on field strength and the speed it collapses. The diode contains this to that mini circuit and the energy is dissipated as the circulating current and respective magnetic field decays. The rest of the circuit is thus protected from that spike.
Thank you, now go ask the "aeroplane designer" which planes he meddled with and make sure you don't fly on them
