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Author Topic: Basic Transistor Question?  (Read 3169 times)
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Well maybe you people who think "programming skills" + "arduino" = electronics should go and read some electronics theory.

What happens when the back emf kicks off and you have put the diode across the transistor instead ? go on think about it. What will happen is your saving the transistor and giving a negative spike a path to now run across the power rails of the whole circuit. This is part of good practice to put a diode on the supply, just in case someone did something stupid.

If you don't know electronics go read a book ! doing electronics wrong blows stuff up, unlike getting software wrong. If you like I can put up when i am home an oscilloscope screen shot that clearly shows the voltage on the load and positive and then when power is pulled shoot straight down from +24V to -400 and then slowly rise back to 0V, Or you can go get an education!
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Sparky,

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. smiley-wink

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.
« Last Edit: March 25, 2013, 04:38:37 pm by Retroplayer » Logged

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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

Or these flipping idiots:
http://pdf1.alldatasheet.com/datasheet-pdf/view/202145/TI/UN2003A.html

Stupid, manufacturers! Don't they know any better?!!!!!!!!

Must be a bunch of stupid programmers. lol
« Last Edit: March 25, 2013, 04:49:55 pm by Retroplayer » Logged

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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?
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I'm out of popcorn, damn.  smiley-wink

Lefty
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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

Or these flipping idiots:
http://pdf1.alldatasheet.com/datasheet-pdf/view/202145/TI/UN2003A.html

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.
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Northants - UK
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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.
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Sparky,

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. smiley-wink

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 !  smiley-mr-green

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
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I'm out of popcorn, damn.  smiley-wink

Lefty

Indeed !  smiley-razz
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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! smiley-wink

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.

Cheers!
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Oh and to answer your question: The cookie monster eats it. That's what happens to it.



Are you sure ? remember you can: 1) ask the audience, 2) phone a friend, 3) go for 50/50  smiley-mr-green smiley-mr-green smiley-mr-green smiley-mr-green
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Try this link http://www.csgnetwork.com/transistorcalc.html
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Are you sure ? remember you can: 1) ask the audience, 2) phone a friend, 3) go for 50/50  smiley-mr-green smiley-mr-green smiley-mr-green smiley-mr-green

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?

Please save us, Sparky!
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you confuse static, electricity and electromagnetic radiation. Get you tin hat on quick!  smiley-eek-blue
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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... smiley-wink

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...

{The Gremlin}
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