Identify if its a GDT or TVS

[joeqsmith]

If you are not aware, we had a tree hit by lightning which damaged a few things.  One casualty was the cable modem.  I took it apart and showed some of the damage.  On the front end there is a device marked as a diode.  At first I thought it may be a TVS but after looking at it with a meter I thought its most likely a GDT.   

One viewer stated that the device is a TVS.  I'm going with some sort of gas discharge tube based on my own personal experiences.   I have attached the comments.    What's your guess?  GDT, TVS or some other device? 

https://youtu.be/40TaqXOIrHo

[joeqsmith]

Picture of the cable modem and the part we are interested in looking at. 

[joeqsmith]

Close up views of the unknown device and the damage around the transformer.

[joeqsmith]

Could be a GDT.  I have not found any glass TVSs yet.   

http://www.alibaba.com/product-detail/2018-UL-ROHS-Surge-Absorbers-Spark_60731858896.html

[joeqsmith]

Found one.  They don't appear to be as common as the GDT.   

https://www.indiamart.com/proddetail/1-5kexxx-tvs-diode-13341138330.html

[joeqsmith]

Under the microscope it looks like a GDT to me.

[joeqsmith]

There appears to be no signs of damage to the part.  The next step is to remove it and mount it onto a test board so we can take a few measurements.   

One day I am going to buy a new Pace with some micro tweezers. 

[joeqsmith]

My 1970s era VNA is all warmed up and ready.  This is an 8754A that I rebuilt many years ago.  It has the frequency doubler option and the S-parameter test set to go with it.   It would be useless without Labview. 

Shown after SOLT and w/ 50ohm standard attached, sweeping to 2.7GHz.

[joeqsmith]

Not sure about consumer market availability, but melf TVS is not rare in defensive/aerospace applications where hermetic seal is a must.

I just Googled "transient TVS glass" and "gas GDT glass".  Then looked at images.  Too lazy to search any other way.    Do you think it's a TVS, GDT or something else?

[joeqsmith]

I suspect if it is a GDT, it will look like a very small capacitor.   Here is a 10pF. 

[joeqsmith]

1pF.  VNA is still warming up.   

[wraper]

It's certainly not GDT as it has D... designator. Should be bidirectional TVS or zener if it measures open both ways. As it measures open, it should be fine. Very unlikely for TVS to fail open without transient energy which would cause explosion or hole in the package. You could attach it to a laboratory PCU through resistor and check for clamping voltage.
Also it looks just like a diode/zener in melf package.

[joeqsmith]

It's certainly not GDT as it has D... designator. Should be bidirectional TVS if it measures open both ways. As it measures open, it should be fine. Very unlikely for TVS to fail open without transient energy which would cause explosion or hole in the package.

I knew someone would not be afraid to voice their opinion.   During the video, I show the meter in conductance both directions.  Let me try it on the electrometer.

[joeqsmith]

Not sure about consumer market availability, but melf TVS is not rare in defensive/aerospace applications where hermetic seal is a must.

I just Googled "transient TVS glass" and "gas GDT glass".  Then looked at images.  Too lazy to search any other way.    Do you think it's a TVS, GDT or something else?

I think it could be a GDT, but I just want to say glass TVS does exist.

Well we knew that as I found one.      Are you leaning towards a GDT? 

[wraper]

Yes. Two smooth parallel plates encapsulated in a hermetically sealed package.

And what is between the plates is exactly like in MELF diode/zener.

[joeqsmith]

Over ranges the electrometer.  Yes, it's connected.   

[wraper]

Yes. Two smooth parallel plates encapsulated in a hermetically sealed package.

And what is between the plates is exactly like in MELF diode/zener.

I didn't see the silicone or contact.

If you have any and dare to use microscope, you'll usually see round thing (which actually is an optical illusion due to the package) in between of the electrodes. I already posted a pic of a zener which looks very similar. Electrodes of GDT look differently to begin with and there is nothing in between of them.

[wraper]

It may look round on the first glance but if you look closer there is a rectangular die.

[joeqsmith]

I measured 395pA of leakage at 50 Volts with this setup.   May need to clean things a bit more...

[joeqsmith]

I can imagine they may have an insulator inside to set the gap.  It may all be bonded as one part prior to encapsulation. 

[joeqsmith]

Notice on this GDT (yes it's a GDT) how the gap is formed. 

[joeqsmith]

Quacks like a duck.   I'm sticking with some sort of GDT.   Next, I need to see where it starts to conduct and how it behaves.   

[CopperCone]

Eh, I don't know if I would bother. You get the best GDT that will fit in the circuit. There is no good reason you can't solder in a better one. Get one with the lowest working voltage you can unless there is some kind of layered protection, which I doubt.

The only GDT parameter would be if it is used in combination with another shunt device with similar voltage, where you would have some kind of impedance (inductor) between the GDT and the other element.

Protection systems = cost. You can always do better.

You could have GDT - > inductive impedance for firing -> Thyristor -> blocking/opening -> fast diodes

If you don't have a thyristor and a TISP or such, you can probably put anything you want there. Clearly the one they put was not up to the job if it exploded. Bourns probably sells faster GDT then what they decided to put in there. You want to minimize the amount of energy going into the semiconductor stuff, thats the only job of the GDT.

[floobydust]

I see them as GDT used in car radios. Very hard to find.
Pic is a Ford, also seen in Kia car radios.

[wraper]

Quacks like a duck.   I'm sticking with some sort of GDT.   Next, I need to see where it starts to conduct and how it behaves.

GDT with D... designator,  silicon die inside, does not look anything like GDT and has leakage current. Yeah, . I'd say quacks like a horse.