Modern trains and modern lighting

[satisnek]

And there's nothing more incredibly boring than riding in a 220 diverted via Lichfield at night...

 

[Pigeon]

Pulse Width Modulation (PWM) LED dimming, which is what I was referring to, is used a lot more often than you think- not just in low-cost devices.

High cost of a device does not, unfortunately, give any guarantee that it doesn't use cheap-arsed crappy techniques and construction internally.

Having the LEDs either fully on or off, switching between these states many hundreds or thousands of times a second, has the advantage that the colour of the LED does not change as it is dimmed.

They don't change colour anyway. The colour of an LED is a fixed property, set by the band gap of the material. You seem to be getting confused with incandescent bulbs, which emit a black body spectrum corresponding to the filament temperature. LEDs do not change colour as they are dimmed, while incandescent bulbs do; and this remains true regardless of what method of dimming is used.

The advantage of bare PWM is that it may be more efficient than resistive dimming (under which heading I include things like linear constant-current sources, which are essentially self-adjusting resistors), and is usually easier to control when the control source is digital (for example many microcontrollers have hardware PWM generators built in).

But it does not remove the need for some additional device to define the current corresponding to "fully on" - exactly as in a fixed-brightness arrangement. In a cheap battery-powered device or one that doesn't care about efficiency, that defining of current is very nearly always done by running the LED from a voltage source through a resistor (and then PWMing it if desired).

For any kind of mains-powered general illuminator, and certainly for one powered by the kind of "mains" you are likely to get as a railway carriage lighting supply, such use of a resistor is out of the question because of the inefficiency (though again that doesn't stop it cropping up sometimes), and the appropriate method is to run the LEDs from a switched-mode current source. In that case, the circuitry both for fixed brightness and for variable brightness is nearly identical; the only difference is the latter allows you access to tweak the feedback ratio to adjust the output current.

To use a fixed-output current source and then follow it with a PWM stage for dimming is pointless to start with, since it is so trivial to simply make the current source itself variable; it adds unnecessary complication; and it makes the current source itself much harder to design - a switched mode current source that can handle the load impedance switching between zero and infinity hundreds or thousands of times a second and still play nicely under all conditions is a much tougher problem than one that only has to drive a benign constant load like a set of LEDs and can treat infinite load impedance as a fault condition.

Theatre lighting is one of those weird applications (like guitar amps) where specific requirements are so out of the ordinary that you are permitted or even constrained to do things which in more conventional applications you avoid doing or even take steps to prevent. Theatre lighting practices are not a standard from which to judge conventional applications that are concerned with simply letting people see what they're doing. I would hazard a guess that if they use PWM it's because they are concerned with maintaining linearity and fine control right down at the very bottom end where the lights are nearly out, which is less straightforward with a switched mode current source, but is irrelevant for train lighting.

In the late 90s/early 2000s the ageing WCML Mk3s had a habit of the lights tripping out at every neutral section - switching to dim seemed to make this less likely to happen, too, as a bonus.

Haha... I remember looong waits for long single line sections to become clear in old Mk 1 compartments. The lights would gradually get dimmer and dimmer until there was nothing but a faint red glow, then once the train got moving again and got over a couple of mph they would suddenly come back to full. Impressive low speed performance from the axle-driven generator, but not so impressive battery maintenance...

 

[bramling]

(Re mk3s as originally built).

I /think/ that earlier mk3s (in HSTs) didn't have these louvres or whatever they're called, but instead had a plastic diffuser filling the whole aperture below the tube. That was definitely a bit nicer than the fins where you can see the tube directly … and I think the same was (is?) true of the first type of 317, which resembled an ordinary mk3 carriage more than the later mk3 EMUs (window size/shape and lighting, maybe even seats?).

I couldn't find any photos showing this very clearly — there's this post https://www.railforums.co.uk/threads/gner-brown-and-grey-1st-interiors.135691/#post-2710376 but the lights are too saturated to be absolutely certain (though it looks as if there are no fins, just translucent plastic).

I also remember that the lights could be run in “dim” mode (originally with a switch that anybody could turn! … later changed to require a square key thing) which I think switched off 2 in every 3, leading to a very nice relaxed atmosphere, especially in 1st where the reading lamps provided more light if you needed it.

Having said that, and from the same thread, the brown interior in the first post with the big diffusers also looks quite decent. https://www.railforums.co.uk/threads/gner-brown-and-grey-1st-interiors.135691/#post-2709938

This is how I remember the GN 317s in the late 80s - translucent plastic diffusers, as was very much the fashion then. Whether this was the whole fleet or just some I’ve no idea, as I was very young at the time. Certainly by the early 1990s they had changed to the current type, which I always presumed was to avoid being a dust / dirt trap.

Clearly we never seem to learn the lessons on these things, as translucent diffusers have been used on many (or even most) trains since, and they all trap dirt, though of course this can be cleaned out from time to time, but instead tends to be left to a fleet “refresh”, where due to the lighting having been dimmed by the said dirt they then go for brighter tubes, so things go from subdued to blinding!