Things derped engineers say.

[grumpydoc]

Quote from: ddavidebor on Today at 07:50:40 AM
In italy we have nutella!

That stuff is motherf-ing delicious too.

Oh <Deity>, yes! Especially with peanut butter. 

Just don't look at the ingredients list!!

[free_electron]

I have no arguments here but one, arent "current limiting" and "constant current" 2 different things ?

Not really. Both are in fact exactly the same thing.

Hold it ! .. rewind ....
they are NOT the same.
A curent limiting supply monitors the current it delivers. if you go over a preset limit the output turns off ( voltage goes to 0 , no more output) intil you reset the supply and turn the output back on. certian agilent( HP) supplies have such a mode. basically acts like a circuit breaker. go over the limit and we cut you off

constant current: if you hit the limit the supply modulates the output voltage so you don't go over that limit. if you hit the 2 ampere it will strt lowering the output voltage so you don't go over 2 ampere.

[free_electron]

Today my chemistry teacher completely  butchered electronics when teaching the electrochemistry unit.
In one 40 min period, he: neglected to mention ohms law, declared the mains to be dc, ridiculed electricians as being dumb asses when posting "danger high voltage" signs, and claimed that a car battery can kill.  A real nutjob...

laws.. who need's em. take a tunnel diode for example.. f*#$ ohm's law ...
for very short time intervals the mains can be considered DC yes ...
high voltage is not dangerous. jus't don't create a pathway to make the electrons flow...
a carr battery can kill. short a fully charged one with a big sturdy wrench and it may fling boiling sulphuric acied and lumps of lead your way potentially killing you. also if it were to be dropped of a 3 level building on your head it will kill you.

on the other hand. chemistry teachers knwo all about blowing themseves up , or otherwise poisoning themselves and their environment. maybe your teachers brain has been 'pickled' by snorting whatever vapors are produced in his lab...

[c4757p]

Just don't look at the ingredients list!!

I'm American - ingredients lists don't scare me.

[c4757p]

laws.. who need's em. take a tunnel diode for example.. f*#$ ohm's law ...

Agreed. Especially in electrochemistry I think it's more important to look at the real underlying physics, not just the simplified version. Ohm's law only applies to classical conductors. Part of me suspects that even copper probably could have extremely small semiconductor effects due to impurities. Chemistry is not the place to neglect the "what ifs".

That said, Ohm's law is nice for safety purposes, simplified to "increased voltage usually gives increased current".

for very short time intervals the mains can be considered DC yes ...

Sure. And for very small spatial sections I can be considered massless.

a carr battery can kill. short a fully charged one with a big sturdy wrench and it may fling boiling sulphuric acied and lumps of lead your way potentially killing you.

Car batteries scare the piss out of me. Working around that is like having a little bomb in front of me. I'll take HV any day, TYVM.

[SeanB]

Car batteries are not a problem, they do have internal resistance. Now take a wet cell NiCd cell, internal resistance tends to approach zero, for almost all currents and at almost any state of charge. Now add to this the demand of a jet starter motor/generator, which can both source and sink 8kA for around a minute without failure, and consider that the charge circuit is not temperature compensated, and the battery does have a negative temperature coefficient. That is why jet aircraft have a single very strict limitation of no more than 2 start attempts before removing battery and replacing, they do tend to go thermonuclear. Not good when you are sitting on top of it, and right next to it is a evaporating coil for the airconditioning, fed conveniently with LOX from the cryotank next to it, and used to give the pilot both oxygen and pressurise his g suit.

[c4757p]

Yeah, I'd say that's just a bit more dangerous than a car battery.

Still don't like the damn things, though...

[SeanB]

Yes, but the pilot got out of the plane ( with an armed ejection seat), opened the hatch and removed the battery and threw it away from the plane. Impressive when you consider the battery is held in the mounting with a M6 bolt, which is locked with a wire locking so it does not come undone. He had no tools other than Adrenaline. Needed a new frame and battery to fix that one, both were broken, one was very well cooked.

Only thing I did was jump start helicopters. Battery switch left on, so flat battery in the morning ( security wanted a radio to listen to FM, which aircraft AM radios can do to some degree) so next morning I brought up tthe DC plant and gave a quick 5 minute lesson to the pilots on the required procedure in case the battery got very unhappy, and had 3 big fire extinguishers nearby just in case. 20 minute later they left safely with a cool battery.

[smashedProton]

I wasnt clear enough.  He said that a 12 volt battery would push 600 amps through your heart.  He never mentioned the chemical danger

[c4757p]

I figured that, we're just having some fun derailing enhancing the thread.

(How did he get his job, exactly?)

[smashedProton]

I'm afraid to ask.

Like always, google saves the day:
http://data.kitsapsun.com/wa-school-staff/324166H#axzz2Se38JkqC

[ivan747]

So, he earns 80 grand a year... What should I do with my life now 

[smashedProton]

Its common for teachers to get a pay raise in the order of 5 grand by hosting a club.  He hosts science club.  That, combined with the head of the science department, tenure, and a safe track record raise his salary.  It doesnt seem honest.  That's the same as my dad.  He is a senior engineer...

[c4757p]

Does he whine about his salary like every high school teacher I've ever had? (Mine made about that much too, and I live in an area where the cost of living is incredibly low...)

They like to whine about time spent grading, too. Dude, that was my job last year. I did more papers than them in half the time and didn't bitch once, and that was diff eq...

[lemmegraphdat]

Does he whine about his salary like every high school teacher I've ever had? (Mine made about that much too, and I live in an area where the cost of living is incredibly low...)

They like to whine about time spent grading, too. Dude, that was my job last year. I did more papers than them in half the time and didn't bitch once, and that was diff eq...

These folks knew what the pay was gonna be. They took the job. What they are is a bunch of trouble makers.

[prazzb]

When i was in college, a gal came in late to join academics. So the professor made me explain all the previous practicals.

Colleges start from basics, so I was explaining a non-inverting op-amp circuit. After my complete explanation,

Q: Do all these grounds need to be connected together on the breadboard ?
Me: 

[free_electron]

Q: Do all these grounds need to be connected together on the breadboard ?
Me: 

my answer. no , they could also be connected together elsewhere ....

[tom66]

Isn't it more fun to have grounds at multiple potentials?

[Flávio V]

Isn't it more fun to have grounds at multiple potentials?

It is even more fun if the Gnd is at a higher potential than the input voltage(Vcc)...

[tom66]

Or if it's at 50 or 60Hz!

[smashedProton]

I once drew one amp from a smps between the 5 and 12 amp rails to get 7 volts 70 watts.  I won't do thaf again..

I leatned that my smps had its protection on the ground path

[c4757p]

And it's not designed to sink current on the 5 volt rail.

[ConnorGames]

I learned what happens when the output ground from a power adaptor for a RepRap becomes disconnected while the center positive pin remains connected ... The ground current (~3A) all flows into the USB port of the connected PC. This results in a drop nearly equal to the Vgs(th) of the external mosfet it is controlling(properly grounded), causing it to operate in linear mode. The mosfet also starts to have a mind of its own because the stepper motors turn on and off, changing the voltage drop across USB ground. Wont do that again. Amazingly, nothing blew in the ~30 seconds it took me to realize I had a problem, but the unheatsinked D2PACK FET, which was supposed to be switching 24v at ~4 amps did get QUITE hot!

Grounding problems are just SO FUN! Don't you agree!

[c4757p]

I learned what happens when the output ground from a power adaptor for a RepRap becomes disconnected while the center positive pin remains connected ...

A power adapter like that should not be earth ground-referenced. This is why.

[ConnorGames]

I learned what happens when the output ground from a power adaptor for a RepRap becomes disconnected while the center positive pin remains connected ...

A power adapter like that should not be earth ground-referenced. This is why.

Originally, it wasn't earth grounded, but I had made a separate connection to the output ground to get it at the same potential as the external PSU for HBP, an ATX supply with a boost converter attached. Without that the MOSFET tended to be... Unhappy. And I was not about to spend the extra money for an isolated 24v supply that can put out 4 amps!

Maybe a resistor between the two grounds would be a good idea. (The only current that SHOULD be flowing between them is the MOSFET gate drive)

Never mind. I connected the ground wire "properly" now, to the main pcb instead of the outside of the barrel jack connecter. This horrible clip wire connection was what allowed the grounding issue, it would need a picture to explain.