Help identifying unknown component in glass package with axial leads.

[eas]

Now that we have warmer weather, I'm trying to diagnose and fix a triac-based speed controller board for a Vornado fan that expired last year in a puff of phenolic vapor and I could use some help identifying a component.

I was expected to find a blown electrolytic capacitors, but there are no electrolytic capacitors, and I'm unable to spot another failed component, so I've started pulling off components to test. The Triac itself may or may not work, but I also noticed that the pictured (attached) component registers as an open circuit (in either orientation) when tested with the resistance and diode functions of my DMM.

My googling and other attempts to identify the thing haven't yielded an answer yet, so I thought I'd ask here. I don't have the circuit or a diagram thereof on hand to help with identification right now, but I can add one later if needed.

Thanks!

PS. WTF .jpeg isn't an allowed file extension, but jpg (and aiff) is?  It's 2017!

[james_s]

That's a diac for triggering the triac. It's a bit like a zener diode, it will appear open-ish circuit until a threshold of around 30V.

[Cerebus]

Clearly something diode-ish, and you mentioning Triacs makes me suspect it might be a diac, at one time very commonly commonly used in triac triggering circuits. Diacs behave a bit like a couple of back-to-back zeners, and you have to get over a certain threshold voltage before they conduct and it's highly likely that your meter doesn't have a high enough voltage on its diode setting to turn one on.

[David Hess]

If it is in series with the TRIAC's gate, then it is a DIAC.

A DIAC is another type of thyristor like a TRIAC but it has no gate lead.  Instead it triggers above a specific voltage, usually about 30 volts, and then conducts with a low forward voltage drop until the current drops to zero.

If you want to test it, use like a 4.7k resistor in series and ramp up the voltage to 40 volts DC while watching the voltage across the DIAC.

In older designs, a neon bulb which operates somewhat like a 90 volt DIAC was used.

[eas]

Thanks, all, and particularly David Hess for leading me through testing the DIAC without smoking it.

The triac itself seems to work properly, too, which means I have more digging to do...

[Gyro]

If it expired in a "puff of phenolic vapour" then you might want to check the speed control pot, those tend to be cheap carbon types which smell exactly like that when they die. Whether it just committed suicide or was pushed would be another matter. There's usually a film capacitor in the trigger circuit too, which would toast the pot if it shorted.

[eas]

Thanks for the tip on the potentiometer. I should have checked back here before finishing out the repair, but it may come in handy in the future.

The only dead component I found was a thermal fuse in the motor windings. I couldn't see any physical issues with windings or motor, and the bearings seemed to turn Ok, so I ordered a new thermal fuse of the appropriate rating and put everything back together.

While I was at it, I added some SAE20 oil to the resevoirs for the sintered brass bearings. Vornado claims the motors don't need service over their "lifetime." Maybe they've derated the motors enough that that's true over 10y or so, but the motors themselves have little pop-outs in the stamped sheetmetal housings to allow the addition of oil.

The front bearing is easy to reach just by taking off the grill and pulling off the molded fan blade. The back bearing is a hassle. You have to disassemble the motor to get to the fasteners that hold the motor housing to the fan to get at the oiling inlet. If you are doing that, it's easier just to leave the back of the housing in place and oil the pads from the exposed inside surface.

But enough about the mechanics, I just thought I'd add a helpful hint for anyone repairing a Vornado fan that comes across this thread. Everything works, for now. I did a full-speed test for an hour or so without I didn't notice any glitches as I swung through the range of the potentiometer.

I still don't know what caused the thing to overheat. There was a little bit of pet hair and dust flug in the path airflow takes through the motor housing, but seemingly not enough to cause a significant restriction. The bearings seemed to turn freely. I'd oiled them a few years ago, and while the felt resevoir pads looked a little depleted, they still had oil. Adding oil didn't cause an obvious reduction in resistance either.

I guess time will tell whether it goes for another several summers, or if it craps out again soon.

Thanks again for the help.