Author Topic: why would anyone do it the other way around ?  (Read 7713 times)

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Offline siliconmix

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why would anyone do it the other way around ?
« on: February 03, 2012, 04:25:05 pm »
i think this should be made an industry standard . what you think ?
 

Offline deephaven

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Re: why would anyone do it the other way around ?
« Reply #1 on: February 03, 2012, 04:28:59 pm »
That is the usual way round. I have seen battery connections to broadcast TV cameras which have it the other way round. Don't ask me why!
 

Offline siliconmix

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Re: why would anyone do it the other way around ?
« Reply #2 on: February 03, 2012, 04:33:13 pm »
i have an old casio pocket t.v. thats minus in the centre ?
 

Offline siliconmix

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Re: why would anyone do it the other way around ?
« Reply #3 on: February 03, 2012, 04:38:38 pm »
set of "time" pc speakers the other way around.wierd i tell yah
 

Offline 8086

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Re: why would anyone do it the other way around ?
« Reply #4 on: February 03, 2012, 04:41:18 pm »
A lot of guitar equipment has negative centre, no idea why. It's almost got me a few times
 

Offline siliconmix

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Re: why would anyone do it the other way around ?
« Reply #5 on: February 03, 2012, 04:48:59 pm »
just looked at my zoom 50511 fx pedal never looked before as use batteries in it.it's weong as well
 

Offline Short Circuit

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Re: why would anyone do it the other way around ?
« Reply #6 on: February 03, 2012, 05:08:53 pm »
The socket usually have switching contact on the outter ring. So thinking straightforward and having a battery to disable with adapter plugged in, it makes sense to put positive on the outside and common negative on the center contact. Having a few dozens of different sizes around, I don't think standardizing this would be of much help...
 

Offline G7PSK

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Re: why would anyone do it the other way around ?
« Reply #7 on: February 03, 2012, 05:15:12 pm »
In a lot of older equipment the negative is center, I think that this was to do with cars being positive earth so equipment that could plug into a car had to be the same now its the other way round, so if you get a new positive outer plug its most like just so you have to use the adapter sold by the same equipment maker.
 

Offline siliconmix

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Re: why would anyone do it the other way around ?
« Reply #8 on: February 03, 2012, 05:23:42 pm »
The socket usually have switching contact on the outter ring. So thinking straightforward and having a battery to disable with adapter plugged in, it makes sense to put positive on the outside and common negative on the center contact. Having a few dozens of different sizes around, I don't think standardizing this would be of much help...
hmm maybe.but the connectors are power/ headphone/line in /line out.anyway.i wish someone would decide on a way and stick to it.we could have a vote.
 

Online Simon

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Re: why would anyone do it the other way around ?
« Reply #9 on: February 03, 2012, 05:29:03 pm »
about as anoying as the GB plugs or rather wallwarts with wires coming out the wrong side. I guess it is about making sure they sell you another adapter when it breaks or you loose it.
 

Online Zero999

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Re: why would anyone do it the other way around ?
« Reply #10 on: February 03, 2012, 06:31:40 pm »
The only reason I can think of for doing it the other way round would be in the case of a positive earth but most DC supplies are floating so it's irrelevant.

I think there should be a standard for DC connectors which should be keyed so it's impossible to get the voltage or polarity wrong and for AC non-polarised connectors could be used.
 

Online Simon

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Re: why would anyone do it the other way around ?
« Reply #11 on: February 03, 2012, 06:43:41 pm »
yes i agree, we have the mobile phone charger saga and then a parallel saga of all the other different types of power adapters, silly voltages and a myriad of connectors, not good in a "green" world where we could use standardized power adapters
 

Offline ivan747

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Re: why would anyone do it the other way around ?
« Reply #12 on: February 03, 2012, 06:56:40 pm »
A lot of guitar equipment has negative centre, no idea why. It's almost got me a few times

That's because the barrel jack connector has a normally closed switch (closed when nothing is plugged into the jack) on the outer contact. That can be used to switch to another power source when the power brick is disconnected. I suppose guitar equipment designers want to switch the positive rail over to a battery, rather than the negative but I don't know the reason for this.

Or maybe they it's just a convention used by guitar equipment designers. I hope they use diodes.
 

Online Simon

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Re: why would anyone do it the other way around ?
« Reply #13 on: February 03, 2012, 07:17:02 pm »
I hope they use diodes.

I bet they don't, having the user at fault for blowing up their equipment makes more sales. After all not using the original adapter invalidates warranty bla bla
 

Offline ciccio

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Re: why would anyone do it the other way around ?
« Reply #14 on: February 03, 2012, 08:25:06 pm »
This type of plug is manufactured in many sizes (Maplin stores about 15 types: http://www.maplin.co.uk/dc-power-plugs-43084).
I'm not sure if there is a standard, but most of the times I see the larger one (2.5 mm pin diameter) with negative outer shell and the mid-size one (2.1 mm pin diameter) with positive outer shell.
I don't remember how is for the smaller ones (i see them only occasionally).
Sometimes they are used for ac power, such as in many modems and routers. and so they will work with  ac or with both dc polarity.... no more problems!
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Offline PStevenson

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Re: why would anyone do it the other way around ?
« Reply #15 on: February 03, 2012, 11:04:19 pm »
to be honest whenever I design anything, I put a bridge rectifier on the DC input so it automatically corrects the polarity at the cost of a volt, cause I'm sick of getting people emailing me to say "I blew it up with my powerpack" so yeah I would love a standard
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Offline MarkS

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Re: why would anyone do it the other way around ?
« Reply #16 on: February 03, 2012, 11:08:46 pm »
to be honest whenever I design anything, I put a bridge rectifier on the DC input so it automatically corrects the polarity at the cost of a volt, cause I'm sick of getting people emailing me to say "I blew it up with my powerpack" so yeah I would love a standard

That's not a bad idea! 8)
 

alm

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Re: why would anyone do it the other way around ?
« Reply #17 on: February 03, 2012, 11:54:35 pm »
Diodes drop voltage, which is not always an option. Of course they also add another $0.001 or so to the BOM ;). A fuse plus crowbar diode is another option, and I believe Bob Pease also published a circuit with two MOSFETs for reverse polarity protection without voltage drop.
 

Offline vk6zgo

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Re: why would anyone do it the other way around ?
« Reply #18 on: February 04, 2012, 12:09:16 am »
That's what all your outlandishly expensive DMMs are for----check first,plug in later!! :)
But seriously,you've got a history of manufacturers going their own way for around 40 years with these connectors,I don't think you'll get them to change their ways now!

By the way,have you notice that some barrel connectors have thinner centre pins than others?
They sort of fit,then at the worst possible moment lose contact!

VK6ZGO
 

Offline gregariz

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Re: why would anyone do it the other way around ?
« Reply #19 on: February 04, 2012, 01:13:10 am »
I've got an old wavetek signal generator with a PH-163 connector. With those connectors, some manufactures wired live and neutral one way and others did it the opposite.

http://richardhess.com/notes/2006/03/15/round-pin-power-cords-for-older-equipment-using-ph-163-connector/
 

Offline Jimmy

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Re: why would anyone do it the other way around ?
« Reply #20 on: February 04, 2012, 02:43:47 am »
The thing with standards is there is so many to choose from
 

Offline ivan747

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Re: why would anyone do it the other way around ?
« Reply #21 on: February 04, 2012, 02:46:46 am »
This is the reality of standards:

 

Offline electrode

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Re: why would anyone do it the other way around ?
« Reply #22 on: February 04, 2012, 05:06:37 am »
It's easy to use a P-channel MOSFET for reverse polarity protection - much better than a diode too, as forward voltage drop is maybe 0.1 V instead of 0.7 V (eg. Fairchild FQP17P06).

Connect as follows:
+ve of power supply to MOSFET drain
GND to gate
source of MOSFET becomes the +ve supply for circuit

If you connect it backwards, it simply doesn't turn on. No messing with fuses and no wasting voltage with diodes.
 

Offline PStevenson

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Re: why would anyone do it the other way around ?
« Reply #23 on: February 04, 2012, 10:24:37 am »

the reason I chose the bridge just to correct polarity rather than a method that only works when you plug the right thing in is because
I don't design things which are voltage critical in that way, I'd rather have the thing work with whatever you plug into it rather than
having to piss about looking for the right plug pack of which I'm sure we all have bloody draws full of, I know I do

my time loss is more critical than a volt loss :)
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Offline rr100

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Re: why would anyone do it the other way around ?
« Reply #24 on: February 05, 2012, 08:40:07 am »
Funny thing I have a special "kit" with 16 of those small plugs (apart from ones salvaged from dead wall-warts, universal wall-warts, etc). They've been invaluable when one or other router, external hdd or something else dies (as they usually do - I tend to keep a lot of equipment around even from 1990's).
HOWEVER just after I bought the kit I instantly found that at least two sizes were missing!!! One was the "large" Nokia type! What the heck, how many different mechanically incompatible hollow cylinders can you design in half a centimeter or so of space!
 


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