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Friday, February 12, 2016

No Clipping

As the exciting future of solid state lighting increasingly becomes the present, some of you may have noticed that LED lamps are a bit more, sterile, for lack of a better term, than their incandescent predecessors.  Now I am not talking about the narrow spectrum or the supposed "coldness" of the light, but specifically the way that LED's behave when flashing.

When flashing LEDs are very binary in operation.  On, off, on, off.  As soon as the voltage crosses some threshold the LED lights up, as opposed to an incandescent bulb that has distinct rising and falling edges as evidenced in the following montage.

But wait, perhaps LED's are not to blame.  Amtrak's FAIR interlocking was an early test for retrofitting incandescent position light lamps with LED light sources and its flashing indication aren't quite so harsh.

If you are wondering if this effect is caused by the flasher relay or other interlocking hardware I actually came upon a natural experiment that is only possible in Position Light territory.

As Amtrak's Train 19 rolls past the 6AE signal at FAIR interlocking, the two green PL lamps appear to be flashing out of sync, but what is actually occurring is that the top lamp is set to flash in a clipped LED style and the bottom is flashing in a ramped incandescent style.

I asked some electrical engineers and they suspected that the magic is all with the details of the LED's local power supply.  It appears that the early units at FAIR have a capacitor tied into the circuit, slightly delaying the flash cycle as it charges and then delaying the cycle as it discharges. The other (cheaper?) supplies appear to only supply a fixed voltage or nothing.

I don't know what is driving the clipped style LED signal lamps appearing all over the world.  It might be cost, or simple ignorance or even a belief by the signal vendors (or component purchasers) that the more binary behavior is desirable.  Hopefully, as was the case with Amtrak's initial LED tests, some railroads will request components that behave in a warmer, more analogie style.

PS: If anyone has EE experience and knows how the power supply circuits are constructed, please share in the comments.


  1. I've noticed that the yellow runway guard lights (on the taxiway before crossing/entering a runway) are now being converted to LEDs with some active electronics to make them fade in and out like the incandescents would. This is particularly clear since on the ones I've seen the electronics are not particularly sophisticated and you can just tell that they are stepping up and down in brightness. They also seem to start their "fade" in at at least half brightness.

    Here's a video that shows it rather clearly:

    I'd bet the FAA requires this so they're hard to confuse with, say, visibility strobes on vehicles?

    1. Good catch. I believe that LED intensity does vary by voltage as I can recall many electronic devices with an LED power light having the light fade out after I pull the power plug.

  2. Yes, the power supply inside the bulb is likely responsible for the different behavior.

    Often a design called switched-mode is used. These supplies are physically compact, waste little energy, and can easily transform large input voltages into small output voltages. Unlike linear supplies, a switching supply "snaps on" as the minimum input voltage is reached. This is also how computer and gadget makers can use one design to handle the wide variation in AC mains voltages around the world.

    1. True, but don't the smoothing caps in a switching power supply result in the voltage ramp down (as they discharge) after the power is cut?