short circuit protection diode

doggiedoc

Hey all -

I may be about to express my greenhorn gene, but here goes.

I've been worried that I might accidentally reverse the connection on my PEK when connecting the battery. The datasheet on the LM2940 lists reverse "battery protection" as a feature so does that mean adding a diode in series with the battery is redundant?

Thanks in advance,
Doc

Mike Green

Yes. The LM2940 is designed to protect its load from a reverse connected power source. It's a low dropout regulator intended to provide regulated power close to the supply voltage (about 0.5V drop). Using a protective diode would at least double the voltage drop (to 1.1V or more) which is partly why built-in protection is so useful. Note that an electrolytic capacitor may be used as an input filter capacitor and would not be reverse connection protected. You would need to use a ceramic or mylar capacitor.

doggiedoc

Thanks Mike.

I understand about the added voltage drop but you lost me on the input filter capacitor. Are you saying a filter cap would help somehow or that if I was using a filter cap that I should consider a diode to protect that capacitor against reverse battery connections?

Mike Green

What I'm saying is that electrolytic capacitors have a polarity and, if you connect them backwards, they may go POP! and all the "magic smoke" will come out and they won't work anymore. If you're trying to allow for accidental reverse connection, you should use a ceramic or mylar or other non-polar capacitor for the input filter capacitor. That's usually not a problem because this sort of regulator doesn't need much of an input capacitor. The manufacturer's datasheet recommends a 0.47uF input capacitor. That's readily available in ceramic.

doggiedoc

Thank you for clarifying! As for the "magic smoke" - I've never been able to figure out how to put it back in after I let it out.

Mike Green

You need a reverse temporal transmogrifier spell. That's the magic part.

doggiedoc

Awe. I was afraid of that. Unfortunately I have not accumulated as many "Magic Points" as you have and therefore can not purchase that spell.

Beau Schwabe

"You need a reverse temporal transmogrifier spell. That's the magic part." - I wondered what that ringing in my ears was.... Thanks Mike!

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

Alan Bradford

I thought when electrolytic caps popped thet were transformed instantly into 'Confetti Generators'
Maybe that is part of the transmogrifier effect.
Is Morgrifier still alive, or was he(she) one of the good old crowd with Hertz and Tesla?

Alan Bradford
Plasma Technologies

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Alan Bradford

Plasma Technologies
Canaan NH 03741
www.plasmatechnologies.com

Beau Schwabe

Alan,

"I thought when electrolytic caps popped they were transformed instantly into 'Confetti Generators'..." - They do, and sometimes let out white smoke in the process. I had a couple about as big around as my pinky finger 'pop' in a rapid fire avalanche failure mode once, and I thought that I had been shot. They sounded like a gunshot and, being totally unexpected, scared the you know what out of me.

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

Mike Green

trans·mog·ri·fy
Pronunciation: \tran(t)s-ˈmä-grə-ˌfī, tranz-\
Function: verb
Inflected Form(s): trans·mog·ri·fied; trans·mog·ri·fy·ing
Etymology: origin unknown
Date: 1656
transitive verb
: to change or alter greatly and often with grotesque or humorous effect
intransitive verb

Also the name of a compiler-compiler: www.geocities.com/ResearchTriangle/2363/tmg011.html

CounterRotatingProps

Beau Schwabe (Parallax) said...
Alan,

"I thought when electrolytic caps popped they were transformed instantly into 'Confetti Generators'..." - They do, and sometimes let out white smoke in the process. I had a couple about as big around as my pinky finger 'pop' in a rapid fire avalanche failure mode once, and I thought that I had been shot. They sounded like a gunshot and, being totally unexpected, scared the you know what out of me.

Beau,

years ago, I was taking one of the original "online exams" at school.·(Online in the sense that it was an old TVI-100 terminal, I got used, hooked up to my 1200 baud telephone modem connected to the schools Unix machine. Yes, this is c.a. 1980.)

I was under tremendous stress to finish the exam on time - on literally the LAST question, I heard this tremendous bang, and the top of the terminal erupted a·cloud of *magic* smoke with a stream of some black/brown stuff inside it shooting upward (the genie in the bottle?).·· The terminal - again literally - went "fizzzzuuut!" and the screen went from nice ASCII text to the infamous green dot in the middle. (The precursor of the Blue Screen of Death.)

A stunned second later, I looked up on the ceiling directly over the monitor -- a black blob of goo was there --- Yes, it was Tantalium Goblin Blood.

A day or so later, when I told the Prof. that I couldn't complete the exam because "My TVI Terminal blew up!" he laughed and said, "that's better than the old 'the dog ate my homework'!"

The next office day he had, I drug in the terminal, and pulled off the loosened case to show him the offending Evil Capacitor --- it was a biggy --- the top had blown off, and it was completely empty.

I got to retake the exam.

And I really did not blow any magic smoke on·the Professor!

cheers
- Howard


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No matter how much you push the envelope, it'll still be stationery.

Philldapill

Wow. I've seen a few OT posts, but this one is added to my top ten. I love it!

A pleasure to read. LOL