Hi all, I am a builder and moder. This was found in my bin while looking for a zener. Dimensions are 2mm x 4.5mm resistance is 8.5 K , FYI the measurements taken using a harbor freight caliper and Centech DMM. Thanks

 

Looks like a diode to me. Probably a floor sweep version.

 

Does it measure the same resistance both ways round, and when hot/cold?

 

This is why you keep an old Huntron Tracker.

 

1N914 or 1N4148 type. Small signal, 150 ma, or I might be entirely wrong.

 

If it's a diode, why is its directional band in the middle of it? could it possibly be some sort of small TVS?

 

It might also be a diac.

 

I just looked inside a Lutron brand dimmer and the diac looks exactly the same except the middle (black) band is narrower.

 

I just looked inside a Lutron brand dimmer and the diac looks exactly the same except the middle (black) band is narrower.

Is there a way to actually test it?

 

Is there a way to actually test it?

Something vaguely like a curve tracer. I would find an AC source, add a resistor to limit the current to 1 ma peak, and watch it on my oscilloscope.

 

A meter with diode check should confirm it by a conductance both ways.
Max.

 

Except a diac has a typical break over voltage of about 32 volts and a hand held meter will never get to 32 volts.

 

It might also be a diac.

That would be my guess - the ones in CFL start up circuits are usually blue and marked DB3.

The DB3 diac breaks down somewhere around 32 -34V, its essentially a triac without a gate - the only way to trigger it is cross the breakover voltage.

Motorola did oodles of trigger diodes of various types - an old data manual might provide a useful overview.

 

Except a diac has a typical break over voltage of about 32 volts and a hand held meter will never get to 32 volts.

Its not that precise - the DB3 diac can be anywhere from 32 to 34V.

A useful way of testing diacs is a blocking oscillator to charge a capacitor and discharge it periodically with the diac.

The DB3 can handle about 2A peak pulse - don't make the capacitor too big, and give the charge somewhere to go.

 

Its not that precise

What's so precise about, "about 32 volts" compared to, "32 volts to 34 volts"?
Too much coffee today?

Mmm...that' a good idea. I think I'll get me another coffee.

 

Back around 1980 or so I recall seeing .01uF decoupling caps that looked like this. Any idea how old this thing is?
You might try to measure capacitance.

 

Wait, sorry I didn't read very carefully. I bet this is a thermistor. Measure resistance while you heat it or cool it. I used thermistors that looked like this some years back.

 

You might try to measure capacitance.

You might, but an intentional capacitor with a leakage of 8.5k would be a pretty bad capacitor.

Oh...you already corrected.
OK.

 

Back around 1980 or so I recall seeing .01uF decoupling caps that looked like this. Any idea how old this thing is?
You might try to measure capacitance.

I've seen them too and bigger - you can usually see the capacitor inside the glass body.

AFAIK: they're usually encapsulated MLCC, but some might be Siemens stacked foil type.

 

Wait, sorry I didn't read very carefully. I bet this is a thermistor. Measure resistance while you heat it or cool it. I used thermistors that looked like this some years back.

The thermistor I found in a MSF/temperature display wall clock looked the same as that - you'd certainly get a resistance reading and you should be able to make it vary by gripping it between your fingers.

A diac is unlikely to read anything - some old analogue meters had a 30V resistance range battery. A fresh battery might just trip a diac on the extreme low edge of the tolerance spread.