Brute Force Beats the Cannon Power Connector Barrier

Like most of you, I’ve accumulated a random collection of AC/DC and AC/AC power units – often referred to as “wall warts” – over the years. After all, what engineer is crazy enough to throw one away if it still works, even though the unit it was powering is no longer there?

Also, even though the adapter itself is outdated, there’s still that barrel (coaxial) connector that might come in handy in the future. At one point I even dreamed of taking the time to number each of my adapters and then create a database calling each one with the output type (AC or DC), voltage and current, and the connector type, polarity and dimensions. But I decided that 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 the ones pictured in Table 1, leaves me confused. (The Wikipedia reference lists even more). I researched online and found two explanations.

Table 1 This is only a subset of the many pairs of internal and external dimensions of defined barrel (coaxial) connectors used. Note the slight but still substantial differences. Source: CUI devices

The charitable justification is that sellers 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 faulty when he was not. In other words, having the many distinctive barrel connectors was a crude way to “key” the adapter to the product for what is now called “user experience”.

Of course, there is also a more cynical explanation. Using different connector sizes was a way to tie users to a specific PSU and connector combination, so any replacement adapters had to come from the manufacturer.

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

Sometimes the frustration with these connectors is such that we decide we won’t be defeated or humiliated by this low cost, low performance power connector. I have an old but still working Brother PT-2300 P-touch label maker which I use several times a year (Figure 1). It uses eight AA batteries, and I don’t like leaving batteries unused for long (corrosion, waste of money, usual apprehensions). There were a few times when I needed them and didn’t have eight fresh batteries, so I resorted to “borrowing” the batteries needed from other items and then put them back in place, which is annoying. Additionally, the user manual states that the batteries may not be able to supply the power needed when printing larger fonts, and therefore strongly recommends the use of the AC/DC converter not supplied in these case.

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

That wouldn’t be a problem, I thought naively. I went into my collection of at least 30 AC/DC units, found one with the necessary voltage and current specifications, and figured all I had to do was find one proper barrel connector to match, do some splicing, and I’d be all set.

Reality hit when I couldn’t find a connector that worked for me. Some were too big – it was easy to tell – but some seemed slightly too small or had a connection on the inner pin that felt loose. Yes, you can get “universal” adapter kits, but I didn’t want to wait, and I didn’t want to use a full kit for this one-time use. What’s also annoying is that this labeling unit has that “square” USB type B connector that I thought I could use to supply power (Figure 2), but only for the data connection to a PC.

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

I decided not to let this label maker tell me what I could and could not do to provide power. Luckily, unlike many products where the two parts are snapped together with tabs that prevent opening but break when you force them, or the two parts are glued together, this label maker had five easy-to-reach screws. I quickly opened the unit, found and removed the small separate PC board with the adapter plug and power subsystem (picture 3), and soldered two wires to the board. Now I can connect any suitable AC adapter using basic clamp cables and completely ignore the barrel connector issue.

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

Coincidentally, there’s been a big push now to get vendors to standardize a USB connector and power source rather than a barrel connector for power adapters to charge cell phones and other smaller products, as long as their voltage and current requirements can be met. Sounds like a good idea, but I wonder if we’ll just have a more complicated version of the barrel connector puzzle, given the many standard USB connectors? Also, if the device being powered or charged needs more voltage than the USB connection can provide, will manufacturers add DC/DC boost circuitry to their product to allow this lower voltage to be used?

Did I waste too much time on this, when I could have used the $10 AA batteries? How frustrating have you been with the cylindrical power connectors? How did you solve this? Do you think pushing a USB connector to charge (or power) small devices will simplify the problem, or just add another layer of confusion?

The references

  1. Coaxial power connector, Wikipedia
  2. “How to Select a DC Power Connector”, CUI Devices
  3. “Measurement of 2.1mm vs. 2.5mm Power Barrel Jack ID”, Digi-Key
  4. “How do I know the size of a cylindrical power connector?” Electronic battery exchange

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

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