I recently bought a replacement laptop PC from Dell and was happy to see that its AC/DC power adapter had the same ratings as the ones I had from two now-defunct Dell laptops (19.5V, 2.3A/45W). This would be handy, I thought, since I could now leave one adapter plugged in at my office and put the other one in my travel bag. No need to unplug and pack the first one when heading out, and do the reverse when coming back. It's not a big deal, but it is a nice little enhancement and time saver (and avoids the "oops, I forgot to pack it" syndrome).

Slight problem: the DC plug connector was not quite the same size. I rummaged through my vast collection of over 30 such adapters and–no surprise–could not find one that fit. No big deal: I used it as an excuse to go to the You-Do-It electronics centre nearby and get a suitable plug. In fact, if I was lucky, I would buy a socket that matched the old adapter plug, make up an adapter cable with that socket and the plug for the new laptop. That would be easier to build, more reliable and leave the older adapter cord intact in case I needed it for another use; why ruin a good one if you don’t have to?

One of the nice things about the DC plug/socket section at You-Do-It is that they have a handy board on which they have mounted one of every plug and adapter, so you can do a physical match-up rather than try to "eyeball" the fit, or hope the measurements you made with callipers are accurate (and you really can't measure the inner diameter of the plug, or pin diameter of the sockets accurately enough, as some of these differ by only a fraction of a millimetre).

I was disappointed when the matching 4.5mm × 3.0mm plug I needed was out of stock. I explained my situation to the clerk, and he informed me that even if they had the right one in stock, it wouldn't have mattered and would not have worked. I was bewildered, until he explained that the newer Dell units have an internal third-wire connection which allows Dell (and the laptop) to verify that the adapter is a genuine Dell unit.

I hadn’t heard of this, but a few minutes of searching on the web gave me what I needed to know. Apparently, Dell (and perhaps others) now embed a Maxim/Dallas Semiconductor 1-Wire EEPROM IC in the adapter and it reports to the laptop via that third wire. If the laptop doesn’t see the right code, it assumes that the adapter is not genuine, even if the voltage and current ratings are OK, and it disables charging.

Why do they do this? The answer depends on where you are coming from, as they say. Cynics argue–with some justification–that this is a tactic to get you to buy a replacement adapter (either as a spare or to replace a lost one) only from Dell, not via a third party. Dell would argue–again, with some validity–that there are a lot of substandard, counterfeit and very low-quality adapters on the market. They don’t want your laptop to be damaged, which (incorrectly) would reflect badly on their product. Each side has a legitimate point.

There's another potential problem here, even if you are using the Dell-purchased charger. The IC within the adapter can fail or be damaged by ESD picked up by the long third wire (the article cited maintains there is no ESD protection), or that extra wire in the already-fragile DC plug can break (these plugs and their wires are not very rugged, despite the abuse they get). In that case, your legitimate Dell charger is now dead, and you can't improvise a replacement. After finding out about this situation, I immediately put a sleeve of heat-shrink tubing on my new adapter's plug for reinforcement.

What's your view on keying the AC/DC adapter to the laptop vendor? Does it make sense, or is it an sleazy marketing trick–or maybe both?

This article first appeared on EDN.