Weird Extension Lead

Quick blog post… Go!

I found this weird extension lead dumped at the side of the road this evening.

It looks like a normal 4-way extension lead but for the bit in the middle with the “GreenPlanet” sticker and a curious “SMALL/BIG” switch on the side. There seems to be no information on the exact model anywhere on the internet, but the model number is “TL0893INFOB”. I wasn’t sure what this was about at first but I know there’s a lot of snake oil sold around magic “energy saving plugs” so I knew the possibilities were endless and it was certainly going to be something stupid. From the “master” and “slave” labels I figured out eventually what its purpose must be: to turn off the phantom load on the “slave” sockets when the “master” is switched off. When I got home I googled it and my suspicions were confirmed. The reviews I found for this thing (or similar devices under the same brand) were almost universally negative, with complaints including:

  • Doesn’t switch off devices when the master is switched off
  • Randomly switches off “slave” devices while the master device is on
  • A loud hum while it’s working

I’ll admit: I didn’t fully test it before I took it apart. I meant to but I tested it with a laptop charger and it turns out my laptop isn’t great at telling you when it’s plugged in so I got entirely false negative results and assumed there was an internal short blowing the fuse. I did notice though that plugging my laptop into the master socket didn’t do anything (no indicator light, no sound of a relay closing), which is probably why it got thrown away.

Taking it apart, it’s exactly what you’d expect: a board with some crude load detection circuitry and a relay.

I had absolutely no interest in reviving the magic power saving circuitry so I just ripped it out. It didn’t look interesting enough to fully reverse engineer but it looked like a cement resistor (which I ripped off while removing it) measured the current to the master socket, the circuit would detect this above a threshold and switch on the relay to enable current to the “slave” sockets. I’m not sure what the SMALL/BIG switch does but I wouldn’t be surprised if it’s a threshold selection. The power supply seems to involve a capacitive dropper. The IC in the middle is an LM324 quad op amp and there’s a 2N5551 which I suspect is activating the 24V relay. I wonder if the mains frequency ripple across the relay coil is the cause of the noise which some reviewers complained about.

With a couple of terminal blocks, the circuit is bypassed and it’s just a normal extension lead. If I had the desire to, this could have been a fun project for making a “smart plug” with the convenient place to install the circuitry.

I’m always looking for an extension lead so this was a handy find, and interesting to take apart too. It was always a stupid idea because it’s overcomplicated. Instead of something like this, it’s more important for new equipment to be designed with low standby power, eliminating the need for this kind of trickery. The drawbacks are too many: essentially there’s no reliable way for it to tell if equipment is “on” or “off” and the inclusion of the “SMALL/BIG” switch makes it user unfriendly by giving you another hurdle to cross before you’re able to use it. I’m not surprised it ended up dumped at the side of the road.

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