Hi, this is one of the most annoying product quirks in all of electronics. The Center Negative DC Jack 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 Comments and links down below to the Eevblog forum: YouTube Comments: All that sort of jazz. If you liked the video, please give it a big thumbs up cuz you know that helps and you got to engage in videos.

and You know so leaving comments is the best way to do that. Thanks Catch you next time.