Author Topic: EEVblog #1015 - Beware Evil (But Clever) DC Jacks  (Read 9406 times)

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Offline EEVblog

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EEVblog #1015 - Beware Evil (But Clever) DC Jacks
« on: August 14, 2017, 09:33:06 pm »
Trivia time. Dave explains one of the reasons why annoying centre negative DC power jacks exist.
 
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Offline RGB255_0_0

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Re: EEVblog #1015 - Beware Evil (But Clever) DC Jacks
« Reply #1 on: August 14, 2017, 09:42:10 pm »
Nice video. I always thought it was a Japanese thing.
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Offline Specmaster

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Re: EEVblog #1015 - Beware Evil (But Clever) DC Jacks
« Reply #2 on: August 14, 2017, 10:07:27 pm »
Nice video. I always thought it was a Japanese thing.
No, its used a lot on earlier radios like Hackers etc because of the internal batteries.
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Offline Cnoob

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Re: EEVblog #1015 - Beware Evil (But Clever) DC Jacks
« Reply #3 on: August 14, 2017, 10:33:23 pm »
I brought a brother P-touch H110 labeller, which I've had for 3 days now.
When I opened it on Saturday evening and looked at the power jack, I actually
thought "The evil bastards it's centre negative."
 

Offline Specmaster

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Re: EEVblog #1015 - Beware Evil (But Clever) DC Jacks
« Reply #4 on: August 14, 2017, 10:49:08 pm »
That must be a Brother thing then because I just checked on P Touch 60 and P Touch 300SP and yep, their both centre pin negative and 7V too.
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Offline jazz

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Re: EEVblog #1015 - Beware Evil (But Clever) DC Jacks
« Reply #5 on: August 14, 2017, 10:51:48 pm »
I think 9V center negative is actually the de facto standard for guitar effect pedals, and probably used in a lot of other musical gear as well.

I brought a brother P-touch H110 labeller, which I've had for 3 days now.
When I opened it on Saturday evening and looked at the power jack, I actually
thought "The evil bastards it's centre negative."
How convenient for printing a big warning label to stick on it :D
 
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Online sleemanj

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Re: EEVblog #1015 - Beware Evil (But Clever) DC Jacks
« Reply #6 on: August 14, 2017, 11:15:30 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.



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Offline Red Squirrel

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Re: EEVblog #1015 - Beware Evil (But Clever) DC Jacks
« Reply #7 on: August 15, 2017, 12:14:08 am »
I saw it on a hoverboard (the ones that catch fire, except this one was hopfully a newer model that does not  :-DD ) they used centre ground too.  I think they use the internal switch to make sure the device can't operate if it's plugged in. Like, so it won't take off on you while you're handling/charging it.  When I worked on it, I made sure to print a big label on it indicating the voltage and polarity.  If you leave the plug dangling somewhere so you can charge it, it would be easy for someone else to see that cord and plug something else in it.  Poof! 

One thing that has always annoyed me is that there is no voltage/polarity standard for barrel jacks.  You can plug as 12v adapter into a 5v device for example, because while there may be several sizes, they arn't standard to a specific voltage.  You'd think that there would be a standard, like either different sizes for different voltages, or perhaps even make it like a 5 pin design and the pin orientation differs based on voltage, or if it's AC or DC.
 

Offline boffin

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Re: EEVblog #1015 - Beware Evil (But Clever) DC Jacks
« Reply #8 on: August 15, 2017, 12:33:06 am »
Centre Neg is the standard on all music equipment (guitar pedals etc).

However, when I made pedals as a kid, they were always battery powered, and I switched the neg on/off by using a stereo jack for the input and put the battery neg on the ring connection and fed the circuit neg from the sleeve connection...
 
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Offline Specmaster

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Re: EEVblog #1015 - Beware Evil (But Clever) DC Jacks
« Reply #9 on: August 15, 2017, 12:58:57 am »
Maybe the reasoning behind the centre being negative goes back to the thought that its always best to switch the live or positive line and so that got carried through to the DC power jack as well?

I agree, it would make so much more sense to come up with a standard configuration for the polarity and also the voltages, after all there enough sizes available to allow that. It would stop manufacturers having their own variations as well, like the HP laptops etc are different again to anyone elses!     
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Offline trophosphere

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Re: EEVblog #1015 - Beware Evil (But Clever) DC Jacks
« Reply #10 on: August 15, 2017, 02:24:04 am »
I accidentally fried one of my projects in the past when I connected an adapter that had a center negative pin. From then on I placed a full-wave bridge rectifier into every project that had a DC-jack input. Granted I had to contend with a 2x diode voltage drop but most of the projects used a lower voltage for their power rails so it was not a big deal. Now they have chips like the LT4320 which makes implementing the MOSFET variant much easier.
 

Offline Jay_Diddy_B

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Re: EEVblog #1015 - Beware Evil (But Clever) DC Jacks
« Reply #11 on: August 15, 2017, 02:37:10 am »
Hi,

Having the center (centre) negative would make sense in the early days when PNP transistors were popular.

Regards,

Jay_Diddy_B
 
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Offline Brumby

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Re: EEVblog #1015 - Beware Evil (But Clever) DC Jacks
« Reply #12 on: August 15, 2017, 04:49:08 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.

The negative is quite often a common.  Switching a common can be defeated by plugging things together that provide an alternate path.  This can't happen with switched positive (unless a very unusual topology).
 

Offline nessatse

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Re: EEVblog #1015 - Beware Evil (But Clever) DC Jacks
« Reply #13 on: August 15, 2017, 06:05:43 am »
Just to add another layer of evil, there is also 2.1mm vs 2.5mm centre pin DC jacks.  "Doesn't fit" vs "sometimes works"   >:(
 
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Offline SL4P

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Re: EEVblog #1015 - Beware Evil (But Clever) DC Jacks
« Reply #14 on: August 15, 2017, 06:23:00 am »
My sentiment on the 'standardisation' of centre-positive comes from the inevitable supply lead flopping around when the device is open...
The pin is 'almost' isolated from accidental contact - while the outer sheath is exposed.
When you drop that loose lead onto an active circuit - you're dropping 0V in - rather than an unexpected raw +V source.
Neither is likely to be a 'positive' experience - but I'd rather hit 80% of the possible accidental drop points with 0V than a hot +lead.
The remaining 20% of potential drop target candidates - well... that's the risk!
« Last Edit: August 15, 2017, 06:28:28 am by SL4P »
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Online sleemanj

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Re: EEVblog #1015 - Beware Evil (But Clever) DC Jacks
« Reply #15 on: August 15, 2017, 06:27:12 am »
The negative is quite often a common.  Switching a common can be defeated by plugging things together that provide an alternate path.  This can't happen with switched positive (unless a very unusual topology).

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.

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.
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Offline bjcuizon

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Re: EEVblog #1015 - Beware Evil (But Clever) DC Jacks
« Reply #16 on: August 15, 2017, 07:09:57 am »
Another type of DC power connector I encountered was the 3.5mm connector. Yes! its the one like the mono headphone jack. I saw this on an old transistorized radio and also its plug pack or "wall-warts"(for the yanks) used these "headphone connectors".
Any music device with a aux 3.5mm input can be damaged by applying power to its inputs via the plug pack.
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Offline Cnoob

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Re: EEVblog #1015 - Beware Evil (But Clever) DC Jacks
« Reply #17 on: August 15, 2017, 07:23:29 am »
I would totally agree about the 2.1mm v 2.5mm jack, the  BBB springs to mind I had power up issues with that SBC.

I brought my brother H110 off Amazon.    I read the reviews and none of those evil bastards mentioned it was centre negative.
I have now rectified that now.
 

Offline G7PSK

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Re: EEVblog #1015 - Beware Evil (But Clever) DC Jacks
« Reply #18 on: August 15, 2017, 07:49:52 am »
The center negative goes back to things like PNP transistors and positive earths in cars. The most likely reason for the switch to center positive is due to laptops where the negative is earthed referenced on the power brick.
 

Offline NANDBlog

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Re: EEVblog #1015 - Beware Evil (But Clever) DC Jacks
« Reply #19 on: August 15, 2017, 08:08:36 am »
After charging a battery pack with an AC adapter (output AC 12V), I double and triple check the wallplugs.
 

Offline bjcuizon

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Re: EEVblog #1015 - Beware Evil (But Clever) DC Jacks
« Reply #20 on: August 15, 2017, 08:50:42 am »
Haha, Just noticed the thumbnail for the video has the "evil inside" sticker. :clap: :-+
[img width="320px" src="https://upload.wikimedia.org/wikipedia/commons/thumb/4/44/Intel_Inside_Logo.svg/2000px-Intel_Inside_Logo.svg.png"[/img]
« Last Edit: August 15, 2017, 08:53:32 am by bjcuizon »
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Offline BBBbbb

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Re: EEVblog #1015 - Beware Evil (But Clever) DC Jacks
« Reply #21 on: August 15, 2017, 09:14:42 am »
The evil "N type" connector, I've learned it almost the hard way about half a year ago.

I'm just thankful Ophir did an input protection on their Nova meters. I've lost its charger and just connected it to my PSU to charge the crappy batteries they put inside, and was scratching my head for about 15-20min until realizing reversed polarity. I was convinced it was a standard to have the negative on the outside.   
 

Offline Alex Nikitin

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Re: EEVblog #1015 - Beware Evil (But Clever) DC Jacks
« Reply #22 on: August 15, 2017, 11:04:49 am »
I've repaired many Sony WM-D6C Walkman Pro cassette recorders because of the central negative DC connector. No reverse polarity protection, so when a central positive supply is connected the motor servo chip dies - resulting in the motor running at a very high speed. No other damage usually and as a part of that repair I always install a protection diode  ;) .

Cheers

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Offline cengland0

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Re: EEVblog #1015 - Beware Evil (But Clever) DC Jacks
« Reply #23 on: August 15, 2017, 11:41:48 am »
This brings back memories of when I worked for a well-known electronics store and sold universal AC adapters.  They came with many different diameter tips and you had to insert the plug into the end of the wire carefully to insure you have the polarity setup correctly.  The adapter had a switch to control the output from 3V, 4.5V, 6V, 9V, or 12V. Problem was that many of the products of that era did not disclose the voltage or polarity on the actual product.  If you still had the original adapter, it usually did tell you the voltage and mA rating but usually not the polarity.  The reason the customer was buying a replacement was due to a defective OEM or it was lost so you couldn't test the original AC adapter to figure it out.

I could sometimes help the customer by using a multimeter to check for continuity with the battery compartment and the jack but that was a dangerous thing to do.  If I was wrong and it destroyed the customers device, they could hold me accountable.  I would have to explain it to them and they would usually accept my technique as an educated guess and promise to not sue me if it damages something.

I'm old enough to remember when AC adapters didn't use coaxial plugs but used 1/8" phone jacks instead.  Those got damaged all the time because the two connectors get shorted out for a split second when you're plugging it in (can even see sparks sometimes) and people would plug it into the wall before plugging it into the device.
 

Offline station240

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Re: EEVblog #1015 - Beware Evil (But Clever) DC Jacks
« Reply #24 on: August 15, 2017, 02:46:42 pm »
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 #25 on: August 15, 2017, 08:19:28 pm »
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 #26 on: August 15, 2017, 09:09:06 pm »
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 #27 on: August 15, 2017, 09:57:32 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.
 

Offline mikerj

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Re: EEVblog #1015 - Beware Evil (But Clever) DC Jacks
« Reply #28 on: August 15, 2017, 10:36:25 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.
 

Offline Zerim

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Re: EEVblog #1015 - Beware Evil (But Clever) DC Jacks
« Reply #29 on: August 16, 2017, 07:13:44 am »
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, 07:29:48 am by Zerim »
 

Offline nctnico

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Re: EEVblog #1015 - Beware Evil (But Clever) DC Jacks
« Reply #30 on: August 16, 2017, 12:41:39 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 #31 on: August 16, 2017, 02:15:20 pm »
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 #32 on: August 17, 2017, 02:02:30 am »
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 #33 on: August 17, 2017, 02:49:22 am »
I remember reading that corrosion was one of the main reasons to make the chassis the more positive potential. Also, this.
 

Online vk6zgo

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Re: EEVblog #1015 - Beware Evil (But Clever) DC Jacks
« Reply #34 on: August 17, 2017, 10:12:19 am »
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, 11:15:10 am by vk6zgo »
 
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Offline Red Squirrel

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Re: EEVblog #1015 - Beware Evil (But Clever) DC Jacks
« Reply #35 on: August 18, 2017, 06:33:18 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.

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 #36 on: August 18, 2017, 06:53:09 am »
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, 06:54:42 am by Brumby »
 

Offline jonovid

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Re: EEVblog #1015 - Beware Evil (But Clever) DC Jacks
« Reply #37 on: August 18, 2017, 10:15:18 am »
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 #38 on: August 20, 2017, 10:19:17 am »
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 #39 on: August 20, 2017, 11:53:00 am »
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 #40 on: August 20, 2017, 01:41:05 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.
 

Offline graphtronics

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Re: EEVblog #1015 - Beware Evil (But Clever) DC Jacks
« Reply #41 on: October 30, 2017, 12:44:31 am »
A&D a Japanese company started with analog to digital converters, hence the name. They usually build high quality balances and scales. The FX-400 a 400 g x .001 g is a precision scale. It may not be a balance, but an scale. Precision refers to 1 mg readability and Analytical requires 0.1 mg readability. I was fortunate to locate an data sheet for this long discontinued item. Dave if you need the user's and or service manual I might be able to find them.
 

Offline Richard Crowley

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Re: EEVblog #1015 - Beware Evil (But Clever) DC Jacks
« Reply #42 on: October 30, 2017, 01:17:34 am »
This brings back memories of when I worked for a well-known electronics store and sold universal AC adapters.  They came with many different diameter tips and you had to insert the plug into the end of the wire carefully to insure you have the polarity setup correctly. ....
That "well-known electronics store" still sells those power connector adapters and a variety of wall-wart power supplies (both AC and DC).  Of course, since their storefronts are all gone, they are now sold on their website.  Alas, the one irreplaceable feature of those shops was the big bundle of "tester plugs"  where you could try the various plugs in your gadget to discover which one fit best.  Ain't no way of doing that over the interweb.

If I were in the plastics business, I would create a product with a bunch of extruded plastic tube pieces. With the I.D. (inside  diameter) and O.D. (outside diameter) of the various "DC barrel jacks".  You could sell the bundle for a couple of bucks and it would serve the same function as that big bundle of tester plugs.
 

Offline IanMacdonald

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Re: EEVblog #1015 - Beware Evil (But Clever) DC Jacks
« Reply #43 on: November 06, 2017, 06:25:18 pm »
Another evil feature is that there are 2.1mm and 2.5mm center pins. The difference is often too small to see easily, and you can't very well get a caliper  into the socket to measure it. Use the wrong one and it may work but cause a bad connection, which can eventually overheat the socket and damage the case around it.

BTW, the jack shorting as it's put in or pulled out also applies to 1/4" speaker jacks on guitar amps. A bad idea to use these here, especially on solid state amps. Valves will tolerate a brief short.

It never ceases to amaze me that so many low voltage connectors have been devised, but none are entirely satisfactory. In cars we end up using cigarette lighters, which is just plain daft. Those aren't very reliable either, a slight tug on the lead and you have a bad connection.
 

Offline John Coloccia

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Re: EEVblog #1015 - Beware Evil (But Clever) DC Jacks
« Reply #44 on: November 07, 2017, 10:53:46 am »
Another evil feature is that there are 2.1mm and 2.5mm center pins. The difference is often too small to see easily, and you can't very well get a caliper  into the socket to measure it. Use the wrong one and it may work but cause a bad connection, which can eventually overheat the socket and damage the case around it.

BTW, the jack shorting as it's put in or pulled out also applies to 1/4" speaker jacks on guitar amps. A bad idea to use these here, especially on solid state amps. Valves will tolerate a brief short.

It never ceases to amaze me that so many low voltage connectors have been devised, but none are entirely satisfactory. In cars we end up using cigarette lighters, which is just plain daft. Those aren't very reliable either, a slight tug on the lead and you have a bad connection.

The general rule is to let solid states go open, but short tube amp outputs. Tubes kind of self limit themselves on the output and usually won't do anything too terrible when the output is shorted except raise your electric bill, but if you disconnect the speaker while it's on you could get all sorts of high voltage spikes due in part to the output transformer. I'm not sure how effective shorting actually is, and there are probably some better strategies for dealing with it, but that's the conventional wisdom anyway.
 


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