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Topic: diode in usb cable (Read 9752 times) previous topic - next topic

kevinkroon

Hi there,

I've recently started using arduino, and I've been having fun with it. I have just purchased a new macbook pro, and seeing a classmates macbook air + arduino literally dying after plugging in the arduino (baffled everyone incl. the professor in the room), I went on an internet search to protect my new computer from any harm. I found this topic here: http://arduino.cc/forum/index.php/topic,122300 and the second poster states this:


I took apart the usb cable or rather cut into the wire placed a diode in series with the positive or negative wire to allow current to flow in to arduino not out...


As no one seems to disagree with this post, I assume this works. However, my knowledge about electronics is close to nothing. I know the basics about diodes, and about series, but that's all. Could anyone explain to me the quoted sollution above?

thanks in advance!

Grumpy_Mike

I would say it wasa bad idea because you get a voltage drop across the diode and so you only supply the USB device with 4.3V or so.

Back feeding the power to the Mac should not happen, was the arduino being powered? Was the Mac per mentally damaged or was it alright after a restart?

kevinkroon

the 4.3V.. I was afraid that would happen. Would an externally powered usb hub be a safe option? The other option, an usb isolator, seems to cost more than an arduino itself.

On the case of my classmate, both the macbook and the arduino are damaged, the arduino isn't found in any com ports anymore and the macbook harddisk is permanently damaged/only fixable with a replacement.

Grumpy_Mike

So what was the setup that caused the damage, what was powering what?

CrossRoads

Use a schottky diode, voltage loss reduced to ~0.3V range. Only get 500mA from the port, pick a diode accordingly.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.


Far-seeker


Would an externally powered usb hub be a safe option?

Since we don't know exactly what cause the problem, I wouldn't say for sure.  If the hub is truly powered from a separate source then that would it would protect the computer from the Arduino somehow forcing a higher voltage on either the power and ground pins for the USB connector.  

However, that wouldn't stop a short from a higher than 5 VDC VIN loop or input pin to either of the USB's data lines from reaching the computer.  Although this last situation is possible, it seems unlikely given the layout of the Uno's circuitry.  If the soldermask was still intact over the traces there just aren't that many places for such a short to occur.

kevinkroon


Use a schottky diode, voltage loss reduced to ~0.3V range. Only get 500mA from the port, pick a diode accordingly.


I'm sorry to be asking so many questions, but when I search for a schottky diode I get over 300 different items. I figured I needed to search for a 500mA but then I get names with 50v or 30v in them. Also, will the voltage loss interfere with my projects?

As for the macbook, it short circuited or something the moment it was plugged in, it was the first class in arduino

dhenry

Quote
it short circuited or something the moment it was plugged in, it was the first class in arduino


If Apple follows the usb standard, there must be a 500ma fuse in there that even if the arduino is shorted, it should  not have burned the usb port; Even if the usb port is burned, the computer should still function.

Far-seeker


I'm sorry to be asking so many questions, but when I search for a schottky diode I get over 300 different items. I figured I needed to search for a 500mA but then I get names with 50v or 30v in them. Also, will the voltage loss interfere with my projects?

Those values are probably the reverse breakdown voltage, or in other words how much voltage in the wrong direction the diode can block before it fails.


As for the macbook, it short circuited or something the moment it was plugged in, it was the first class in arduino

Yes, but was anything plugged into the Arduino's barrel jack?


If Apple follows the usb standard, there must be a 500ma fuse in there that even if the arduino is shorted, it should  not have burned the usb port; Even if the usb port is burned, the computer should still function.

They might not, at least in their smaller Macbooks like the Air, space must be at a premium there. 

<sarcasm>Besides if you are a good Mac user and only use your Mac with Apple or other pre-approved products, why would you ever need such protection in the first place?</sarcasm>

majenko

<sarcasm>Besides if you are a good Mac user and only use your Mac with Apple or other pre-approved products, why would you ever need such protection in the first place?</sarcasm>


Have they made the iDuino yet?

Grumpy_Mike

#11
Sep 27, 2012, 03:33 pm Last Edit: Sep 27, 2012, 03:36 pm by Grumpy_Mike Reason: 1
The casual inaccurate statement like:-
Quote
it short circuited or something the moment it was plugged in,

does not cut it in electronics land. You are implying that a perfectly good Arduino was connected to a perfectly good Mac and the pair were damaged. That simply does not happen, period.

There must be something wrong with either piece or something wrong about what was connected to, or powering, the arduino.

Unfortunately you don't seem to be able to give any more information so I think this will remain a mystery as to who screwed up and how. But I will tell you that it wasn't the Arduino crew nor the Apple crew.

You said:-
Quote
and the macbook harddisk is permanently damaged

and before you said :-
Quote
and seeing a classmates macbook air + arduino literally dying after plugging in


The whole point about a Macbook Air is that it does not have a hard drive in it!!!!


Far-seeker


The whole point about a Macbook Air is that it does not have a hard drive in it!!!!


I share your annoyance at the vague description of the events and the technology involved, but since this was in a post criticizing casual inaccuracy... ;)  Up until 2010 many Macbook Airs had magnetic hard disk drives.  So it's not unlikely that college student could be using a Macbook Air from a few years ago with an actual HDD. 

Also, while it is very inaccurate to call a solid-state drive a "hard drive", it's an understandable confusion for non-technical people (just like calling an entire desktop or tower case full of specific computer component's a "CPU").  It's worthwhile to correct this inaccuracy of course, but maybe not quite so much vehemency. :)  If kevinkroon was making this mistake, having an SSD might make catastrophic failure more likely because SSDs are constructed using either flash memory or DRAM ICs.

kevinkroon

#13
Sep 27, 2012, 05:58 pm Last Edit: Sep 27, 2012, 06:10 pm by kevinkroon Reason: 1
I can only give a vague description as it wasn't my macbook but someone in my class. Also, yes, I am one of those non-technical people who call an ssd a harddrive.  :*

But I feel this topic is derailing. I only referred to the macbook air situation as I didn't want it to happen to my new macbook pro. I too feel it's highly unlikely for this to have anything to do with the arduino, maybe it was just a really bad coincidence.

Anyways, my question would something like this:
http://www.ebay.nl/itm/Lot-5-1PS59SB20-SCHOTTKY-Diode-40V-500mA-SC-59-Philips-B3174-/250919466173?pt=LH_DefaultDomain_0&hash=item3a6bf734bd
be sufficient and will the voltage loss interfere (much) with my projects?

Also I was thinking would it be possible to have something like a seperate breadboard or something with a switch on it, turning the arduino on or off, but still alowing the user to upload code to it while it's "off". In that case I would be able to unplug it, hook it up to a battery and turn it on, hence removing any possible harm to the computer.

Grumpy_Mike

Wow that is an expensive diode. You don't need a fast switching one.
I would say yes a diode drop in the USB line is going to give you trouble some where down the line.

The point is that you are trying to protect your Mac from something you don't know, and it looks like we will never find out. Therefore it is impossible to give any measures that will protect it.

I have used a MacBook pro for over 5 years on many different makes of arduino and have not encountered any problem. And I think that goes for every other member on this forum. So I would advise against doing this modification to your cable.

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