DC barrel jack/connecto with swithcable center pin

[stenzer]

Hi,

out of curiousity I'm wondering if there is a DC barrel jack available with a "switchable" center pin, similar to that one in the image below [1], which has a switchable outer connection. I wasn't able to find any. The reason I'm asking is that IMHO using the outer pin as a PLUS pole is a bad practice.

I want to install a DC barrel jack to my lebelwriter to power it from a mains adaptor. Preferable I would switch over the plus pole between the battery and the DC jack. Without any mechanical switching functionality, I could simply use two diodes as shown in the picture above. I will probably use an external 5 V or 6 V power supply, and the labelwriter houses four AA batteries, which results in a voltage of ~4.8 V (NiMH) up to 6 V. So to ensure the power is delivered by the mains adaptor the battery has to be disconnected. The picture above shows a practicable realization which ensures there is no unintended current flowing towards the battery when plugging in the mains power adaptor.

I'm aware of the relative large current consumption during printing a lable, thus I will probably using two high current schottky diodes [2] to keep the voltage drop low (if there is sufficient place, I have some of those at home).

Is there a more elegant/simpler way to switch over between a battery and an external supply while using the center pin of a DC barrel jack as plus pole? I think for this kind of application the use of a power multiplexer is not needed (at least for me).

[1] https://electronics.stackexchange.com/questions/193847/dc-barrel-battery-schematic
[2] https://www.vishay.com/docs/96457/vs-40cpq050-n3.pdf

BR

 

[betwixt]

The outside diameter of the plug pushes against the switch and operates it so I don't think you will find one that operates from the fixed inner pin.
If the polarity worries you, why not simply wire it with the polarity reversed.
Why do you think a positive outer pin is bad practice?

Brian.

 

[stenzer]

The outside diameter of the plug pushes against the switch and operates it so I don't think you will find one that operates from the fixed inner pin.

I think so as well.

If the polarity worries you, why not simply wire it with the polarity reversed.

I do not get that.

Why do you think a positive outer pin is bad practice?

Every mains supply I have in my lab (and those are dozens) use the inner pin as plus pole (expect those where the connector itself can be changed). So I might excidentially use a wrong one, if I would now use the outer pole as PLUS pole. But of course this is my opinion, maybe somone else would say using the outer pin as plus pole is more convinient.

 

[BradtheRad]

When you ask for increased versatility, you must put forth greater effort in design and construction. It's unavoidable. Then to install safeguards is further effort.

houses four AA batteries, which results in a voltage of ~4.8 V (NiMH) up to 6 V

You're inserting more components in the current path. Suppose the battery voltage sags at moments when the printer operates? What does the 7805 regulator put out in such conditions? Does the printer work on 4.8V (that's four times 1.2v per cell)?

You may find it necessary to add a fifth cell to the pack.

Then, do you wish to make it more versatile by charging the batteries in the unit? Of course it will require further effort in design and construction.

 

[stenzer]

When you ask for increased versatility, you must put forth greater effort in design and construction. It's unavoidable. Then to install safeguards is further effort.

Yes of course, but this is a simple modification most likely only used by me. A diode bridge would be a solution to use both possibile polarity combinations.

You're inserting more components in the current path. Suppose the battery voltage sags at moments when the printer operates? What does the 7805 regulator put out in such conditions? Does the printer work on 4.8V (that's four times 1.2v per cell)?

Sorry, the included image and especially the 7805 regulator is some kind of missleading. I was only interested in the implementation of the barrel jack. The printer itself works fine with NiMH batteries. I have not disassembled the printer until now, so I do not know which kind of regulator is used.

Then, do you wish to make it more versatile by charging the batteries in the unit? Of course it will require further effort in design and construction.

Yes especially with respect of using NiMH as well as ordinary batteries. I will open the printer and will have a look how much space is left. Do you have a "small" circuitry in mind which can be used to charge 4 NiMH batteries. I know there are several DIY projects based on a LM317. By having a quick look at e.g. Digikey there are quite a lot of dedicated ICs available as well.

BR

 

[BradtheRad]

Do you have a "small" circuitry in mind which can be used to charge 4 NiMH batteries.

Just by installing a voltage regulator set to deliver 5.6V (based on 1.4 v per cell). That way the charge rate automatically tapers to zero as the battery pack rises to full voltage.

Make fine adjustments as needed so the printer gets adequate voltage.

And if you find that a depleted pack receives too much current, install a safety resistor.