Brute force beats the barrel power-connector barrier

Article By : Bill Schweber

This engineer modified a label maker PCB so he could connect any suitable power adapter.

Like most of you, I have accumulated a random collection of AC/DC and AC/AC power-adapter units – often referred to as “wall warts” – over the years. After all, what engineer is foolish enough to throw one away if it still works, even if the unit which it powered is gone?

Further, even if the adapter itself is defunct, there’s still that barrel (coaxial) connector that might come in handy in the future. At one time, I even dreamed of taking the time to number each of my adapters, then set up a database calling out each one with output type (AC or DC), voltage, and current, plus connector type, polarity, and dimensions. But I decided that doing so would be a sign of an unhealthy compulsion.

I can understand the need for adapters with different voltage and current ratings, but the need for so many different barrel connectors like those shown in Table 1, puzzles me. (The Wikipedia reference lists even more of them). I’ve done some research online and found two explanations.

table comparing barrel connector dimensions Table 1 This is just a subset of the many defined barrel (coaxial) connector inner and outer dimension pairings in use. Note the slight but still consequential differences. Source: CUI Devices

The charitable rationale is that vendors were concerned that users might plug in an adapter with too high a voltage and damage their product, or plug in one with insufficient current output and then assume the product was defective when it was not. In other words, having the many distinctive barrel connectors was a crude way of “keying” the adapter to the product for what is now referred to as “user experience.”

Of course, there’s also a more cynical explanation. The use of different connector sizes was a way to tie users to a specific pairing of power unit and connector, so any replacement adapter would have to come from the manufacturer.

Which explanation is correct, I can’t say, and perhaps there’s truth to both. Still, I’m sure you have had the frustration of needing to improvise an adapter unit and connector for some product, only to find you don’t have the needed adapter or connector. While the adapter ratings are usually marked and hopefully still legible, fitting that connector is tricky. It’s especially difficult for the diameter of the inner pin, as some differ by only a fraction of a millimeter. Even if you have a digital caliper, it’s hard to measure the pin and mating opening with adequate accuracy. Among the suggested ways to do it is to try to fit different drill bits or rods in the opening and find a good fit, but there are still issues of measurement granularity and errors.

Sometimes frustration with these connectors is such that we decide we will not be defeated or humbled by this low-cost, low-performance power connector. I have an old but still functioning Brother PT-2300 P-touch label-maker unit that I use a few times each year (Figure 1). It uses eight AA batteries, and I don’t like leaving batteries in there unused for a long time (corrosion, waste of money, the usual misgivings). There have been a few times when I needed it and didn’t have eight fresh batteries, so I resorted to “borrowing” the needed batteries from other items, then put them back, which is a nuisance. Also, the user manual says that the batteries may be unable to provide the needed power when printing larger fonts, and so strongly recommends the use of the non-supplied AC/DC converter in those cases.

Brother label makerFigure 1 This old but still working Brother label maker reminded me of the headaches that barrel-connector compatibility can induce. Source: Newegg, Inc.

This would be no problem, I naively assumed. I went into my collection of at least 30 AC/DC units, found one with the needed voltage and current ratings, and figured all I had to do was find an appropriate barrel connector to match, do a little splicing, and I’d be all set.

The reality hit when I could not find a connector that fit. Some were too big – that was easily determined – but some seemed to be slightly too small or have a connection on the inner pin that seemed loose. Yes, you can get “universal” adapter kits, but I didn’t want to wait, and I didn’t want to use a whole kit for this occasional use. Also annoying is that this label-maker unit has that “squarish” Type B USB connector that I thought I could use for delivering power (Figure 2), but it is only for data connection to a PC.

Examples of USB Type B connectionsFigure 2 The Brother unit has a USB connection for data only, and it’s the less-common Type B version. Source: Science ABC

I decided I would not let this label maker tell me what I could and could not do for providing power. Fortunately, unlike many products where the two parts are snapped together with tabs that both inhibit opening but break when you force them, or the two parts are glued together, this label maker had five easy-to-reach screws. I soon had the unit open, found and removed the small, separate PC board with the adapter receptacle and power subsystem (Figure 3), and soldered two wires to the board. Now I can connect any suitable power adapter using basic clip leads and ignore the barrel-connector issue entirely.

PCB from a Brother label makerFigure 3 Once the decision was made to open the unit and directly access the power and ground, the rest was easy; the barrel receptacle is the black component at the top, and the USB connector is the partially shielded white one below it.

Coincidentally, there’s been a big push now to get vendors to standardize on a USB connector and power source rather than a barrel connector for AC adapters for charging cell phones and other smaller products, as long as their voltage and current requirements can be satisfied. That sounds like a good idea, but I wonder if we’ll just have a more-complicated version of the barrel-connector conundrum, given the many standard USB connectors? Also, if the device being powered or charged needs more voltage than the USB connection can deliver, will manufacturers add a DC/DC boost circuit to their product to allow use of that lower voltage?

Did I waste too much time on this, when I could have just gone with the $10 worth of AA batteries? What’s been your level of frustration with barrel power connectors? How did you resolve it? Do you think the push for a USB connector for charging (or powering) small devices will simplify the issue, or just add a new layer of confusion?


  1. Coaxial power connector, Wikipedia
  2. How to Select a Dc Power Connector,” CUI Devices
  3. Measuring Power Supply Barrel Plug ID 2.1mm vs 2.5mm,” Digi-Key
  4. How can I tell the size of a barrel power connector?” Electronics Stack Exchange

Bill Schweber is an EE who has written three textbooks, hundreds of technical articles, opinion columns, and product features.

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