It seems like an age since I last updated my blog – and indeed it has. Rarely has there been such an extended silence, although, I’m afraid that this might become the norm. Instead, posts will end up coming out in “batches” – while I may progressively work on them in my free time, this is starting to dwindle due to the increasing workload involved with my PhD … sometimes overwhelming. But don’t be overwhelmed by a “flood” of posts here and there – it’s just my way of making sure information “gets out there”.
While I may have been too busy to write, I haven’t been too busy to salvage. It’s one of the rarely had pleasures where one can just pick up something which has been discarded, something which is obviously no longer worth anything to its original owner, and feel a sentimentality towards it – a nostalgia, mixed in with a bit of luck and “being in the right place at the right time.” Or sometimes it’s just a case of everyone else “walking by”, blissfully unaware what lays on the “cold-hard ground”.
As a result, my room is starting to pile with salvaged and new stuff awaiting a blog post – such as this:
It says Advance Volstat on the front, with its specifications marked on the back:
If you guessed that this was just a “glorified power brick”, you’re halfway there. Looking at the name, it looks to be an amalgamation of the words “volt” and “static” – meaning regulated voltage supply. It’s listed with a serial of 1068 (quite low!) and a wide input voltage range of 190-260v. The frequency is 50Hz – this gives us a hint to what it is. The output is 12v RMS (AC) at 15W, with a power factor of 1.0. Made in England.
A quick search doesn’t reveal much – but thanks to Grace’s Guide to British Industrial History, we can find that Advance Electronics started as Advance Components. It also dates this unit as being between 1964 to 1974 – so roughly 39-49 years old!
Thinking of voltage regulation, one might be inclined to think of something fancy like a tap-changing transformer under servo control – but this is a much more simple device with no moving parts – a ferroresonant transformer. The diagram makes it clear just how this thing works. Sure, operating at saturation is inefficient, and heat producing, but it’s quite neat to see this electromagnetic property used as a regulation device in practice. No need for moving parts!
Why might one need such a device? Early devices may have operated on AC and needed a stable source of AC for oscilloscopes to drive their screen deflection coils, or other sensitive measurement circuits to reduce any sources of errors from mains. The other thing ferroresonant transformers are good at handling is harmonic noise, and notching – using this would isolate the secondary from the primary and protect it from some power quality problems.
This one belonged to the Sydney City Council Measurements Laboratory and UNSW Electrical Engineering as evidenced by the glued-on placards:
We can see what appears to be the primary and secondary coils through the vent holes. Do I dare plug it in? Well, in short, probably not … lets look inside first …
Removing the rear cover (the one with the specifications) reveals a sizable mains capacitor with wires soldered on. The capacitor is in an aluminium can, and is likely to be an oil/paper type capacitor. Some of the older oil/paper capacitors have known to smoke and catch fire due to degradation of their insulation over time – one of the key reasons I will not be plugging this unit in. A handwritten mark suggests it was measured at 2.67uF. It looks like the exact value is highly important to ensure the resonant frequency of the system and the effectiveness of “distortion” correction.
As a bit of a side note – the screws are all slot type – which also suggests its age and origin. Slotted screws really drive me crazy as they do not “centre” like the Phillips head screws do, making slipping and stripping the head of the screw a possibility. Extra care is required …
The capacitor is mounted with a clip on the top that is screw secured. The markings are on the other side, necessitating unmounting the capacitor –
Yum. It’s a Plessey/TCC capacutir rated at 2.5uF +10/-5% at 360V RMS. British made – and proud of it!
As another aside, we had a Plessey plant in Leightonfield (not too far from my place) – you can still see the logo in the Google StreetView today, although the facility has been taken over by some wholesalers. Just in case it gets plastered over some more, here is what it looks like now:
Australia had a fairly advanced electronics industry – where did it all go? There was a documentary called State of Electronics (whose trailer is here – good viewing!), although I haven’t seen it released yet (and this was in 2008, last heard from in 2010). It would be interesting to watch – mainly for the nostalgia …
Anyhow, continuing on with the teardown of the Volstat … lets take it apart from the front now –
Removing the front plate reveals the solder lugs for connection on what appears to be a Bakelite board strip. Terminals 1 and 2 are for primary connection, Terminals 5 and 6 are the secondary output, and Terminals 9 and 10 are for the capacitor (wires go through the core of the Volstat into the back).
The ground wire (green) is attached to a lug which is wedged between the front plate and the body by the screws – look out for it in the next picture.
There’s the front entirely removed – the windings on the grey-painted laminated core are visible – they are wound in a side-by-side configuration which appears to offer isolation. The whole thing appears to be coated in varnish of some sort.
So there we are – another piece of “ancient” technology – nowadays, ferroresonant transformers are only used in very specific circumstances and switch-mode inverter technology has advanced to become very flexible and efficient making these units relatively obsolete.
On a side note, once the connection wires are desoldered, these units make excellent paperweight or doorstop – possessing the properties of a decent amount of weight and a bewildering appearance that’s sure to catch the attention of passers by!