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Author Topic: 24V DC back fed my USB port  (Read 958 times)
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South East USA
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My motor voltage (24 volt) got back fed through my Arduino UNO to my USB cable to my old MacBook. It let the magic smoke out of the tiny 8 pin soic chip (3.3 v regulator?) on the UNO. But mainly it (fried?) my MacBook!  It doesn't turn on, it won't do anything...  Tried removing the battery awhile... Nothing.
Anyone have any knowledge of what exactly would go wrong from back feeding 24 volts into the USB port?  Would it just be a power supply board fried, the motherboard, would my hard drive still be salvageable?
Thanks.
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Manchester (England England)
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Quote
would my hard drive still be salvageable?
Probably it is hard to say.
Putting such a large voltage into a system is likely to get onto the power rails and blow most things.
While your hard drive logic could be gone it is likely that the actual disk itself can be salvaged.

Do you know why this happened? It is not the sort of thing that just happens without the design of your add on circuitry being wrong.
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South East USA
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After a little more reading, maybe I didn't get  24 volt on it...
I'm working with an electric wheelchair.
When things smoked, I just assumed I had the BK & the BK/WH wires mixed up (one being ground the other being 24 volt). But I'm not positive.-- what I thought I had was the five volt wire from the wheelchair controller tied to the 5 volt pin on my uno.  Would this cause problems? 
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Manchester (England England)
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Yes it might but not as dramatic as that. It is never a good idea to connect both power and ground of two supplies together because two voltages are never the same and current will flow between them. This can lead to high current especially with low impedance power sources. However if you mistook your 24V wire for a 5V one and connected 24V into the +5V of the USB cable then that indeed might result in magic smoke release.
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South East USA
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Ok. Well here are facts:
The UNO smoked. 8-pin chip just below the crystal.
MacBook quit working. Completely dead, tried battery removal, holding power button...

So, it is possible that connecting the 5 volt from the chair to the 5 volt pin on the UNO do this?
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Manchester (England England)
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Well pin 8 is the ground pin, so it looks like what ever happened put a lot of current into the ground, therefore I think, it is unlikely to be caused by simply connecting the two 5V together.
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No,no, I mean the "8-pin SOIC chip" is what smoked...
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http://twitter.com/SouthernInHeart/status/251326558957096961/photo/1

Would 5 volt backfed do this?
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That is the 3V3 regulator, it is meant to have 5V fed to it, so no feeding 5V o that would not cause that to happen. However, feeding 24V to it would, as it can only stand 16V on the input.
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Ok, so just trying to troubleshoot myself--is it pretty safe to say that having the 5 volt from the chair connected to the 5 volt pin on the Arduino did NOT fry my MacBook then?



If so, then the problem WAS a crossed wire.
« Last Edit: September 27, 2012, 10:09:50 am by SouthernAtHeart » Logged

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Its a good precaution to put 4k7 or similar in series with any logic signals from micro controller to a higher voltage controller circuit. But you must always check before connectly high voltage power, one silly mistake can, as here, be expensive.
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Also, make sure to hook up ALL the ground wires to a common ground before connecting anything to a power source or voltage rail. If you don't have a common ground, ANY voltage can destroy other components, as they may not be using the same reference. This can cause really nasty amounts of current to flow through electronic components in places that were not desinged to handle any current.

As far as the macbook is concerned: most likely you blew the rectifiers/regulators on the mainboard. That's not an easy fix. Most stuff that is not directly connected to the mainboard power should be okay though.

regards,

Pieter
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