Hi, this is one of the most annoying
product quirks in all of electronics the Center negative DC jack
(annoyed grunt)
Now you'll find this on a lot of old gear which we are looking at here
So what they do is your typical DC barrel Connector like this
Whatever you want to call it, the outer one is actually the positive, hence
why the actual like the diagram is very descriptive there and the inner central pin is negative and this is
opposite to
Pretty much the De-Facto standard these days which is the center positive pin.
And if you go buy a plug pack on the market almost every modern product is centre-positive
So why are some of them centre-negative?
Well let's take a quick look at it. Now the product I'm actually looking at here is an A.N.D. or "And"
Electronic balance or analytical balance it's one of these laboratory ones 400 grams maximum -
fantastic, they make really good
Balances and this one is centre-negative. How did I find out?
Well, some idiot who shall remain nameless um
went and plugged in a 12 volt plug pack and just got the plug back out of the
big bin of plug packs that you have and
plugged it in, centre-positive
wah-wah-wah-wah blew the arse out of the fuse. doh.
So now depending on who you ask. This, ah centre-negative thing
Which is very quite uncommon these days in Mainstream products is that it this actually used to be
Maybe even the de Facto standard way way back, and somewhere along the line it's flipped
And it's become centre-positive
Why is it so? Now there might be several reasons for this, but there is one particular reason
Which is actually a very convenient design decision
to actually do this, to make it center negative. So what we'll do is take this apart and have a look inside.
There's a quick teardown inside for those who want to see it. Now this one actually has an optional battery pack
And that's the key here
And thankfully we can just get access to the nice little board down here
which has our DC barrel Jack and the fuse which somebody blew the arse out of
And a little jumper connector over here now the first thing you'll notice about these DC Barrel Connectors
Is that they're usually three pins like this
And if you have a look at the uh schematic for this you can see that
It's fairly descriptive of how it operates there's three separate contacts in there and one of them is
effectively a switch.
So when you insert the jack it can make or break that internal switch.
Pin three is actually the switch
contact you can just ignore that and not use it or you can short out pins two and three if you're just
not using the switch
It doesn't matter you can just use pins 1 and 2 or pins one and two and three connected
but if you want to use the internal switch in this thing then pins two and three with no
DC plug plugged into it
It's normally closed, but then if you plug in the DC plug
Then it will actually break pins two and three and this is important. So I'll demonstrate that here
I've got my probes on pins two and three and you can see that's normally closed, but if I plug in my
Power Connector Bingo it breaks
Hmm. Let's go to Dave CAD because this is useful.
All right so let's have a look at what's going on in this little board in here. Now, here's our
DC jack here with the three pins one two and three like that and
This goes into a four-way jumper connector on here, and I'll explain this in a second.
But basically we're utilizing these three different contacts. Now, we've got ground which is connected to the center pin
so it's that Evil centre negative
that we're talking about. We'll see why it's useful now
We have an optional internal battery this one doesn't have it
But if it did then the positive of the battery would be connected over to here and hence
Pin three that switch contact on the DC Jack and the ground of the battery is just thing
It's just connected directly to the regular common
Which is the center pin, so let's have a look what happens if we don't plug in the jack
So nothing's plugged in here. Well pins two and three are normally closed due to the internal Jack in there and
Bingo you can see that the positive of the battery
Gets shorted and routed through to this pin here now
there's a jumper inside the product which is just connected between
Here and here, and you can see that jump a link down in there
It's the center two pins
But now let's plug our DC plug into this thing and the
Schematic is quite descriptive because you can see when you push it in it literally pushes this lever arm
Down like that when you actually push it in so it's quite descriptive
And it actually breaks the contact between pins 2 & 3 there
So I just love that symbol
Whoever came up with that, genius, great, anyway you plug that in and two and three are now disconnected
So the positive of the battery is completely
Disconnected from anything and the positive outer
positive here
then goes through the Pin 2
just like that so it's effectively a double pole single throw switch that selects between either the battery or
the external
DC Jack
Brilliant and that's one of the reasons
Why one of the Advantages of why having a center negative
actually is useful especially back in the old days you could just use the contacts on the switch button to switch between
battery and external power use with no
additional
Circuitry required there's no diodes. No nothing. No active circuitry, nothing fancy pantsy just a simple switch
Built in to the DC Jack. It's fantastic, and that's one of the reasons there might be other reasons out there
I don't know if you know of them if you have ever designed products back in the day
And you know why they you centre-negative
Maybe because like everyone else did that had battery products and then it just like rolled over to other products
and if you know exactly when the tide turned and
We went away from the center negative
And it basically centre positive dominates practically every product on the market
But I believe there's still some products out there like in coms gear people have told me on Twitter and other
Musical I think Yamaha gear or something like that, somebody was saying
Umm, yeah, if you know of gear that still uses the center negative
But try and find it on a modern consumer product you won't - it's always
Center positive because they probably have active stuff built in for the battery these days well
You know with Lithium-ion batteries lithium polymer
It's all active charging and everything's you know like it costs cents to do that nowadays, but back in the day
You know it was a big deal
Don't waste money on a diode or something like that. So you use or a couple of diodes you use
the Switch built in the Jack -
Fantastic. So there you go. I hope you found that interesting. You know one of the reasons. Why they might have used sent a
negative back in the day
anyway
If you want to discuss it as always
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