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Author Topic: EEVblog #1015 - Beware Evil (But Clever) DC Jacks  (Read 3515 times)

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Offline station240

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Re: EEVblog #1015 - Beware Evil (But Clever) DC Jacks
« Reply #25 on: August 16, 2017, 12:46:42 AM »
I got given a uniden cordless phone 2000's era I think, which is center negative.
I accidentally used it on one of my projects, which luck would have it survived. The plug pack however didn't survive my next accident involving it, my hammer and the e-waste recycler.
 

Offline thm_w

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Re: EEVblog #1015 - Beware Evil (But Clever) DC Jacks
« Reply #26 on: August 16, 2017, 06:19:28 AM »
You are switching a battery cell (in Dave's example explanation), which is floating.  Once it's switched out (by the plug being plugged in), one end of the battery is connected to nothing, it doesn't matter what you plug in where, it's not connected.  It makes no difference if that thing that isn't connected is the positive terminal, or the negative terminal of said battery.

The only way it could was if you considered perhaps that you have the chassis at negative potential, and the battery cell is of a metal negative encased type, and the insulating covering over the cell is damaged or not present, and the cell is mounted in such a way that in some circumstance the now exposed metal case of the cell can come in contact with the chassis.... but if we go to that extreme the same can pretty much be said for a positive terminal coming in contact with the chassis.

yep. and battery sleeves may be negative or positive.

Quote
Dave should have named this video "what is the third terminal on DC jacks for", I think any link between center-negative and the pass-through connection of a DC barrel jack is very tenuous at best.

I think there is a connection, but its due to another convention of switching positive. Which does make sense, having it the other way is harder for most people to glance at the schematic and instantly know whats going on.
 

Offline nctnico

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Re: EEVblog #1015 - Beware Evil (But Clever) DC Jacks
« Reply #27 on: August 16, 2017, 07:09:06 AM »
I always but a reverse polarity protection in my circuits. In one of my recent designs I even went for a full bridge rectifier.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline NANDBlog

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Re: EEVblog #1015 - Beware Evil (But Clever) DC Jacks
« Reply #28 on: August 16, 2017, 07:57:32 AM »
This one had a reverse protection also. It is called a fuse. I dont have problem with crowbar, and similar circuits. I mean, you need to do something bad to trigger it. In fact, a little bit of punishment (blown fuse, not tons of smoke and fire) for the users is a good thing IMHO.
 

Offline mikerj

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Re: EEVblog #1015 - Beware Evil (But Clever) DC Jacks
« Reply #29 on: August 16, 2017, 08:36:25 AM »
I'm scratching my head at why the pass through being on the sleeve is a reason for making the sleeve positive.  It works just as well negative.  I do it all the time, battery negative on the pass through, disconnects battery when plug is inserted just the same. 

Either way one end of the battery is floating, so who cares where exactly it's floating.

I was also scratching my head during the video, so glad it wasn't just me.  I can't think of any reason why it would be problematic for the jack to isolate the negative side of a battery supply rather than the positive.
 

Offline Zerim

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Re: EEVblog #1015 - Beware Evil (But Clever) DC Jacks
« Reply #30 on: August 16, 2017, 05:13:44 PM »
I have been thinking it'd be nice to have a video on the differences of high side and low side switching, and what "ground" is in different contexts.

I know that road cars currently ground the negative to chassis and mostly switch the positive (except for e.g. coils), and I know that old cars switched the ground, ostensibly because they didn't want the electrons to leak out (another physics concept which tbh still slightly confuses me--feels like electrons should be interchangeable).
I'm working on a race car that switches "ground", and all the circuitry works just fine with rectifier negative and battery negative on one side of the primary switch, and the chassis ground and all other circuits on the other side. As a matter of fact I think the switched ground is a bit safer since accidentally shorting the main switch battery terminal to chassis will at worst turn the electronics on, vs maybe melting a wrench. So why did positive side switching cars replace the alternative?

Does there exist any black box combination of diodes, resistors, inductors, capacitors etc that behaves differently if the main DC power to the system is switched on the high side vs on the low side?

It's obvious that a mains-powered device needs to be high-side switched and have a permanently earthed chassis if it's made of metal, since the human body will short the chassis to earth if the device doesn't, but is it really necessary to connect DC "ground" with earth ground, other than maybe for EMI?
Plus, although I do know the difference between Earth ground, chassis ground, and common/negative/"ground" and their symbols, clearing up best practices for e.g. schematics symbols for everyone would be great since they all seem to get tossed around.

I also just recently found out the functional difference between PNP and NPN transistors and I've been making Arduino circuits and watching EEVblog videos for months. I figured everything was just high-side switched, though I still don't know if/why NPN's are "better". I've been missing some pretty critical information in hindsight.
« Last Edit: August 16, 2017, 05:29:48 PM by Zerim »
 

Offline nctnico

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Re: EEVblog #1015 - Beware Evil (But Clever) DC Jacks
« Reply #31 on: August 16, 2017, 10:41:39 PM »
This one had a reverse protection also. It is called a fuse. I dont have problem with crowbar, and similar circuits. I mean, you need to do something bad to trigger it. In fact, a little bit of punishment (blown fuse, not tons of smoke and fire) for the users is a good thing IMHO.
I disagree. A device shouldn't break if the user does something wrong. It will reflect bad on the manufacturer (how many people know about electrical polarity these days?) and it takes time to deal with returns and confused customers. The device I mentioned earlier may be wired by the end user so I put in a rectifier in to make it work even if they get the polarity wrong.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline cloudscapes

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Re: EEVblog #1015 - Beware Evil (But Clever) DC Jacks
« Reply #32 on: August 17, 2017, 12:15:20 AM »
I build guitar pedals as a hobby. The defacto standard by a very large margin in guitar pedals is center negative. Even new designs. If we changed that standard, it would be chaos, since people tend to like using power supplies they already have on new pedals.

 

Offline John Coloccia

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Re: EEVblog #1015 - Beware Evil (But Clever) DC Jacks
« Reply #33 on: August 17, 2017, 12:02:30 PM »
Before going back to a normal engineering job, I was building guitar pedals for a while.

If you don't plug a battery in to the 9V snap, it can short against the metal case (which is nearly always tied to whatever you're considering ground), and that would of course short the power supply if you didn't switch the positive side of the battery snap.
 
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Offline amyk

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Re: EEVblog #1015 - Beware Evil (But Clever) DC Jacks
« Reply #34 on: August 17, 2017, 12:49:22 PM »
I remember reading that corrosion was one of the main reasons to make the chassis the more positive potential. Also, this.
 

Offline vk6zgo

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Re: EEVblog #1015 - Beware Evil (But Clever) DC Jacks
« Reply #35 on: August 17, 2017, 08:12:19 PM »
Quote from: Zerim link=topic=93528.msg1281718#msg1281718 date=1502867624ese
I have been thinking it'd be nice to have a video on the differences of high side and low side switching, and what "ground" is in different contexts.

I know that road cars currently ground the negative to chassis and mostly switch the positive (except forties e.g. coils), and I know that old cars switched the ground, ostensibly because they didn't want therthe electrons  to leak out (another physics concept which tbh still slightly confuses me--feels like electrons should be interchangeable).
When telegraph systems were first set up, they operated with "earth return" circuits, so one side of the battery supplying the system was returned to (real Earth ---the dirt).
It was found that if the negative side was earthed, the wire connecting to the "earth stake" would corrode badly.
With the opposite connection, this corrosion did not happen, so "positive earth" became the standard in that service.

When cars were first made, some manufacturers , mainly in the UK,followed this standard, & others did not.
Over time, it was found that this corrosion was not a problem with cars, &
eventually, after many years, the industry standardised on the " negative earth" connection.

Switching on the ground side of devices in cars was done for cost reasons.
It's cheaper for most stuff which is close to the battery, to just direct connect one side of the device, & run a wire to the switch on the dashboard.
OK, you will say, how about the accessories & other stuff that comes off the ignition switch?
It's still cheaper for most as you only need half as many wires in the form going to them.
From memory, lights were normally "supply side"switched.
Quote

I'm working on a race car that switches "groundsheet", and all the circuitry works just fine with rectifier negative and battery negative on one side of the primary switch, and the chassis ground and all other circuits on the other side. As a matter of fact I think the switched ground is a bit safer since accidentally shorting the main switch battery terminal to chassis will at worst turn the electronics on, vs maybe melting a wrench. So why did positive side switching cars replace the alternative?

It is probably because a lot of devices in modern cars are not just simple electrical or electromechanical things.
Switching the supply  side is standard with the neg ground circuitry used with
Electronics, so maybe that just seemed to be the way to do it.
Quote

Does there exist any black box combination of diodes, resistors, inductors, capacitors etc that behaves differently if the main DC power to the system is switched on the high side vs on the low side?


Passive devices like that,  no,  but there may be some active circuitry where it makes a difference (can't really think of any offhand,though).
Quote

It's obvious that a mains-powered device needs to be high-side switched and have a permanently earthed chassis if it's made of metal, since the human body will short the chassis to earth if the device doesn't, but is it really necessary to connect DC "ground" with earth ground, other than maybe for EMI?
It isn't actually necessary, nor always used,  but is  useful, & has become a standard over many years.
If all the devices you are using have the same DC ground,  you may avoid  some interference & noise problems.
Of course, cars don't have any real connection to the real  Earth.( no connection to the real dirt).
Quote

Plus, although I do know the difference between Earth ground, chassis ground, and common/negative/"ground" and their symbols, clearing up best practices for e.g. schematics symbols for everyone would be great since they all seem to get tossed around.

Good luck with that, although there are standard symbols, but they are not always used.


Quote
I also just recently found out the functional difference between PNP and NPN transistors and I've been making Arduino circuits and watching EEVblog videos for months. I figured everything was just high-side switched, though I still don't know if/why NPN's are "better". I've been missing some pretty critical information in hindsight.

When transistors were first made, they appeared in both  NPN & PNP types.
These first ones were made using Germanium, & it was found easier to achieve consistent good performance with PNP devices.

When the early Silicon transistors were made, it was found that it he opposite applied, with NPN being the easiest to achieve  consistent results.
Silicon transistors are superior to Germanium in all but a few special applications, so they took over in most jobs.

The difficulties with good performance with PNP Silicon devices was overcome, but NPN had already become the default type of device for most circuits.
« Last Edit: August 17, 2017, 09:15:10 PM by vk6zgo »
 
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Offline Red Squirrel

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Re: EEVblog #1015 - Beware Evil (But Clever) DC Jacks
« Reply #36 on: August 18, 2017, 04:33:18 PM »
I'm scratching my head at why the pass through being on the sleeve is a reason for making the sleeve positive.  It works just as well negative.  I do it all the time, battery negative on the pass through, disconnects battery when plug is inserted just the same. 

Either way one end of the battery is floating, so who cares where exactly it's floating.

I was also scratching my head during the video, so glad it wasn't just me.  I can't think of any reason why it would be problematic for the jack to isolate the negative side of a battery supply rather than the positive.

Or why does the location of the positive on the jack even matter, the switch would just need to be set up to work differently?  So you could have it so the positive is centre but the switch still switches the positive.  Am I missing something?
 

Offline Brumby

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Re: EEVblog #1015 - Beware Evil (But Clever) DC Jacks
« Reply #37 on: August 18, 2017, 04:53:09 PM »
While there are versions available that can do what you suggest, you should look more closely at the common switching mechanism.  The operating lever is also an electrical connection.

Here is a 1/4" jack (SPDT version), but the same approach is used in barrel jacks...
« Last Edit: August 18, 2017, 04:54:42 PM by Brumby »
 

Offline jonovid

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Re: EEVblog #1015 - Beware Evil (But Clever) DC Jacks
« Reply #38 on: August 18, 2017, 08:15:18 PM »
so thats Why they did it,  I lost a brand new flatbed scanner from this type of DC power jack  >:D
as the same 1990s PC used the opposite DC power jack on its powered speakers at the time.  :palm:
Hobby of evil genius      basic knowledge of electronics
 

Offline Red Squirrel

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Re: EEVblog #1015 - Beware Evil (But Clever) DC Jacks
« Reply #39 on: August 20, 2017, 08:19:17 PM »
While there are versions available that can do what you suggest, you should look more closely at the common switching mechanism.  The operating lever is also an electrical connection.

Here is a 1/4" jack (SPDT version), but the same approach is used in barrel jacks...


Oh I see, so really it's just a physical thing as to how the connector is designed. Makes sense if they are using the switch as a connection too.  I guess it simplifies the design and reduces chance of failure. They could have some kind of extra plastic lever etc that is independent but it makes it more complicated I guess.
 

Offline Brumby

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Re: EEVblog #1015 - Beware Evil (But Clever) DC Jacks
« Reply #40 on: August 20, 2017, 09:53:00 PM »
That's it.

There are ones with isolated switching - but you would have to look a little harder to find them.
 

Offline John Coloccia

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Re: EEVblog #1015 - Beware Evil (But Clever) DC Jacks
« Reply #41 on: August 20, 2017, 11:41:05 PM »
While there are versions available that can do what you suggest, you should look more closely at the common switching mechanism.  The operating lever is also an electrical connection.

Here is a 1/4" jack (SPDT version), but the same approach is used in barrel jacks...


Oh I see, so really it's just a physical thing as to how the connector is designed. Makes sense if they are using the switch as a connection too.  I guess it simplifies the design and reduces chance of failure. They could have some kind of extra plastic lever etc that is independent but it makes it more complicated I guess.

Getting 1/4" jacks that switch a different circuit is not so difficult though it's less common and they generally come in the enclosed style. Incidentally, I tend to stay away from the enclosed ones. Switchcraft makes nice ones, but the smaller form factor forces them to use much smaller/weaker contacts. The open frame ones grip the jack with authority!

Finding the little power plugs with a switched pin is a lot more difficult. In fact, I don't really know of any off the top of my head, or at least none in the similar form factor. As I mentioned earlier, I suspect the driver in 9V battery applications is you really don't want to let the battery + short against the case, so you really want to switch the + side...and the availability of suitable power jacks drove the decision from there.
 


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