Oldest domestic dimmer switch

[Lucien Nunes]

What is the oldest dimmer switch you have seen in use recently?
When did you install your first dimmer? (Veteran sparks only)

I ask because a friend recently sent me a link to a 'news' story about an old toaster still in use, which I don't even really see as an old toaster (OK, yes, it's older than my daily use toaster but I have much older working toasters.) There are quite a few of these stories around, where people find something from the 1950s still giving service and think that's remarkable. But what about the humble, unobtrusive knob on the wall?

I have one old dimmer still in use at home, it's a 'Crescent 2' 300W model which probably dates from the late 1960s. I haven't been able to pin it down any tighter from the components, but if I trawl some magazines of the era I think there are ads for this specific dimmer. Mounted on MK rounded corner face plates, they use an old circuit based on a stud thyristor inside a bridge rectifier, triggered by a germanium transistor, instead of a triac triggered by a diac which has been standard since the 1970s. This has the disadvantage of two extra diode-drops so higher heat dissipation, but the advantage of absolute firing symmetry. I see flicker very easily and can spot a diac-triggered triac dimmer that has a 50Hz component in its output due to slightly different triggering voltages on the two half-waves.

We had five of these at one time, some were retired with the coming of the CFL, another replaced with an IR remote-controlled one, etc. One did fail with a leaky AC128. The last one is likely to get retired soon as I want to try LED in this fitting that is on for many hours a day and will need a compatible dimmer. At the moment the Crescent 2 is dimming a 150W GLS lamp, it has had up to 200W (often 2x 100W in a bayonet Y-adaptor) as it's a key light source in the lounge. The main reason the Crescent has survived so long is the high quality Allen-Bradley pots they used, which never seem to wear out.

Let's see your old dimmers. I have a soft spot for the edge-thumbwheel-controlled MKs of the 70s-80s with the separate switch. I don't think we have any in the collection though.

 

[littlespark]

Now you’ve taken it off the wall, I bet it doesn’t work when you put it back…

Can’t say I’ve seen anything like that. Even when I started late 80’s, existing dimmers were exactly as we have them now.

 

[Dustydazzler]

I have seen roller ball & slide type dimmers in houses that were wired in the late 1960s /early 1970s

If you had a dimmer switch back then you were pretty posh

 

[Dustydazzler]

Also back in the day most dimmer knobs were gold or silver , which again made them look a bit posh

 

[marconi]

My uncle, an electronics buff, gave me several boxes of Radio Constructor magazines as a young teenager. In one was a project to build a thyristor dimmer using the leading edge phase trigger control technology you mentioned of a thyristor inside a bridge rectifier with a pot, capacitor and transistor time delay. I made it and it did indeed dim the only light in the living room, a pendant with a 100W lamp. It overheated/behaved erratically if I used a 150W lamp. It had to be consigned to history because it produced terrible electromagnetic interference to the radios in our home and our neighbours which I as a 12 year old in 1973 was not able to eradicate or suppress despite one or two attempts with chokes and shielding. The radios would have been AM. We did not have a hifi then.

Does not your Crescent 2 (300W) do the same to your radio sets or has FM and DAB come to your rescue?

You will like to read Spira(SCR)and Alessio(triac):

History of the Dimmer Switch - https://kbelectricpa.com/history-dimmer-switch/

Solving the Phase-Cut Dimming Challenge — LED professional - LED Lighting Technology, Application Magazine - https://www.led-professional.com/resources-1/articles/solving-the-phase-cut-dimming-challenge

 

[Lucien Nunes]

I've not been aware of any RFI issues with the Crescent dimmers but I just ran a simple practical test using a Bush DAC10 held up close to the dimmer and wiring (T+E in stud wall.) On MW with no station tuned, there is a small amount of buzz audible in the middle of the dimming range although not at full. With even a weak station tuned, by the point the AVC starts to take effect, no interference is audible nor is the tuned RF level affected.

I tried the same test with a modern MK 500W leading-edge dimmer in another room, loaded with 100W, and the buzz was very much louder and also marginally present with a weak station tuned. Not an accurately controlled comparison but it suggests the Crescent dimmer is no worse for RFI, and possibly quite a lot better, than the MK.

In the Crescent, the incoming connections pass through a pair of ferrite-rod chokes of perhaps 100 μH, with 1kΩ + 0.1 μF inboard of them across the bridge input. Nothing remarkable there.

 

[Avo Mk8]

Apologies for sligtly off topic, but as a young teenager, my first 'dimmer' (actually "rheostat" I guess) came with another larger one on a black bakelite panel from Proops in Tottenham Court Road in the early 1960's, surplus wartime stock, as used in Lancaster bombers etc. Cost probably a shilling or two. I used it on my model railway to control the LV lighting.

The ultimate trip to London would comprise visiting the large number of shops in Lisle Street and Tottenham Court Road selling the most amazing array of wartime surplus electronics, for not much money, receivers, transmitters, parts thereof, telephony, RTTY, aircraft and vehicle based components of every sort. Boxes of stuff spilling out onto the pavements. Everywhere was an Aladdins Cave!

My first foray into 'solid state' dimming was to construct a circuit (which I imagine I found in "Practical Wireless") inside a 3 pin adapter, to control the speed of a mains powered drill, before the days they were designed in. Circuit used a neon bulb to trigger the thyristor, similar to below (but UK version!)
I don't think it created RF interference, at least none that I was aware of!

Happy days!

 

[Dave OCD]

Lucien, I can remember installing quite a few of the MK thumbwheel dimmers with separate switch , expensive at the time !
Strangely to me there was a 'fixed load' and 'variable load' terminal and I misinterpreted this when fitting my first one, connecting the light to fixed load.... Could never really see the point in why you'd want to have more than one light in the same room switched together with one dimming and the other/s staying at full wallop ?

 

[Lucien Nunes]

Two possibilities come to mind. One is that before dimmers, there were fixtures with multiple lamps split between two feeds, e.g. a 6-lamp electrolier with two lamps on one feed and four on the other, which would be controlled from two switches for 1/3, 2/3 and full options. Replacing the double switch with the MK dimmer would allow you to put 1/3 on fixed and 2/3 on the dimmer, so you could vary from 1/3 to full without losing the colour temperature and efficiency at the 1/3 setting because there's still the two lamps at full brightness. The other which is a bit far-fetched is whether they were compatible with dimmable fluorescents, which need a fixed feed for the heater transformer as well as the dimmable feed. But they also need a high / adjustable minimum level setting to stop them flickering and going out, and AFAIK the MK dimmer didn't have this, plus, fluo dimmers normally need hard firing.

But I expect the reason is that the switch was made from standard parts and the common terminal was already there so they let it show through the housing in case it came in useful some time. Dimmers with 1-way switching like the Crescent, could use generic SPST or DPST switched pots as widely used in electronic goods for on/off/volume. But for 2-way switching there were no generic 2-way (SPDT) switched pots, so using a separate conventional 2-way switch was the solution until the pot manufacturers caught up.

IIRC there was another brand, possibly WG, that had the fixed / common terminal accessible, as well as the separate switch & control knob layout.

 

[brianmoooore]

The ultimate trip to London would comprise visiting the large number of shops in Lisle Street and Tottenham Court Road selling the most amazing array of wartime surplus electronics, for not much money, receivers, transmitters, parts thereof, telephony, RTTY, aircraft and vehicle based components of every sort. Boxes of stuff spilling out onto the pavements. Everywhere was an Aladdins Cave!

My first foray into 'solid state' dimming was to construct a circuit (which I imagine I found in "Practical Wireless") inside a 3 pin adapter, to control the speed of a mains powered drill, before the days they were designed in. Circuit used a neon bulb to trigger the thyristor, similar to below (but UK version!)
I don't think it created RF interference, at least none that I was aware of!
View attachment 94213
Happy days!

Still lots of bits from Tottenham Court Road in one of my attics, including a couple of modified WS19s.
I also made up a Practical Wireless or might have been Radio Constructor drill speed controller into a die cast aluminium box, and it's still going strong today.
Hadn't been used since the advent of drills with built in speed control, but last year I needed to get a circular saw into a tight space, so fitted a saw blade into my 9" grinder. Far too fast of course, so I dug out and dusted off the old box, plugged it in and it controlled the grinder speed perfectly.