OK, so here’s one that should be simple, but apparently not so much.

I’m troubleshooting a halogen torchiere floor lamp, which recently stopped lighting up. Removed the halogen bulb, and tested continuity, and visually inspected. Continuity across the bulb, and no obvious break in the filament.

Next tested voltage at the socket. 122v or so. Then tested across the contacts of the lamp. Voltage looks normal there, too.

Inspected both lamp and bulb for any signs of arcing or corrosion. Nothing obvious.

OK…maybe something just wasn’t making good contact. Re-assemble, plug the lamp in, turn it on and…nothing.

I’m stumped. This is really basic. Voltage + continuity across the bulb should equal photons and heat.

What am I missing here?

My next step would be to go ahead and try a new bulb.

Fair enough…but what would prevent a bulb with good continuity from working?

Total guess here, but don’t those things often run with a TRIAC-driven dimmer? It might be providing voltage, but at a very low duty cycle (too little to light the lamp) or the dimmer could be failing under load. When the VOM reports the impossible, it’s time to break out the O-Scope.

@ mcalsyn - Makes sense. I need to check if my DSO Nano can handle 120v before I try that, though.

Any recommendations for online resources for what kind of signal I should be looking for?

@ devhammer - You should see something like the green waveform here : TRIAC - Triode for Alternating Current - AC power control - Electronics Area

It should reach full + and - amplitude, but the sin wave should be clipped on the time scale, reducing the duty cycle. At full brightness setting, you should see a full or very nearly full sin wave. I would test this with the lamp in place so that the triac is under load.

Also, just in case it is the connector, have you tried jumpers between the socket and the lamp, just to be sure the lamp is really getting juice?

And yeah, all standard warnings about line voltages and safety apply. And you are getting hardware advice now from a mostly software guy. Rarely a recipe for success.

Regarding that last part:


Jumpers aren’t really practical, as the bulb is the tube style that fits into a spring-loaded pair of terminals, so pretty difficult to get jumpers attached. Similarly, the terminals on the bulb are inverted dome shape.

On the waveform, then, it looks like it should be a sine wave that’s typical of AC, but with part of the waveform cut off, and how much dependent on the position of the dimmer knob, is that correct?

@ devhammer - yup

@ mcalsyn - Well, sadly, it appears that my scope (DSO Nano) doesn’t have the range to represent the full waveform on the screen.

But…even with that limitation, I’m not seeing any evidence of a cutoff point when moving the dimmer knob, so I’m guessing that the dimmer is shot.