What I Learned Today

I have a bedside lamp that does the cool off-low-medium-high cycle thing. Or did, that is. Recently, the bulb in my lamp burnt out, so I bought a replacement. When I put it in, though, it wouldn’t cycle. I could feel the faint fuzziness of the lamp registering my body’s capacitance, but the light was unmoved; it just stayed permanently on. Perplexed, I thought maybe I’d bought the wrong kind of bulb, so I bought some more. Same result. It seemed too much of a coincidence that it would stop working right when I needed a new bulb, but then I found this interesting tidbit, from a FAQ on touch lamp problems:

These are susceptible to damage from voltage surges or just plain old random failures. In addition, the current surge that often results at the instant an incandescent bulb burns out (the bright flash) may blow the thyristor in the electronics module. If the lamp is stuck on, the thyristor is probably shorted.

My thyristor is shorted!

So, what’s a thyristor? One definition:

A semiconductor used as a gate circuit. The solid state equivalent of a thyratron.

But what’s a thyratron? Wikipedia to the rescue:

A thyratron is a type of gas filled tube used as a high energy electrical switch. Triode, Tetrode and Pentode variations of the thyratron have been manufactured in the past, though most are of the triode design. Gases used include mercury vapor, xenon, neon, and (in special high-voltage applications or applications requiring very short switching times) hydrogen.

So a thyristor does the same thing, without the gases. What’s actually in a touch lamp is a triac, composed of two thyristors.

But anyway, the unfortunate upshot is that the lamp is now non-touch-sensitive. Apparently I can buy a plugin touch-sensitive box to restore that functionality, or, of course, I could just buy a new lamp. And track its usage so I can estimate when the bulb is about to die and swap it before the cataclysmic “bright flash” kills that lamp, too. Maybe a lamp with a manual switch isn’t looking so bad now.

Update (Feb. 13, 2011): I have successfully fixed the lamp! Thanks to Cakky_blue, below, who suggested exactly the right fix.