EEVblog #1015 - Beware Evil (But Clever) DC Jacks

[EEVblog]

Trivia time. Dave explains one of the reasons why annoying centre negative DC power jacks exist.

[RGB255_0_0]

Nice video. I always thought it was a Japanese thing.

[Specmaster]

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.

[Cnoob]

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."

[Specmaster]

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.

[jazz]

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

[sleemanj]

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.

[Red Squirrel]

I saw it on a hoverboard (the ones that catch fire, except this one was hopfully a newer model that does not  ) 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.

[boffin]

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...

[Specmaster]

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!     

[trophosphere]

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.

[Jay_Diddy_B]

Hi,

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

Regards,

Jay_Diddy_B

[Brumby]

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).

[nessatse]

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"   

[SL4P]

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!

[sleemanj]

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.

[bjcuizon]

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.

[Cnoob]

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.

[G7PSK]

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.

[tszaboo]

After charging a battery pack with an AC adapter (output AC 12V), I double and triple check the wallplugs.

[bjcuizon]

Haha, Just noticed the thumbnail for the video has the "evil inside" sticker.
[img width="320px" src="https://upload.wikimedia.org/wikipedia/commons/thumb/4/44/Intel_Inside_Logo.svg/2000px-Intel_Inside_Logo.svg.png"[/img]

[BBBbbb]

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.   

[Alex Nikitin]

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

Alex

[cengland0]

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.

[station240]

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.