Identify if its a GDT or TVS - Page 1
Forum section: electronics / projects

author: joeqsmith
author_rank: Super Contributor
timestamp: July 10, 2018, 09:48:11 pm
content: 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

author: joeqsmith
author_rank: Super Contributor
timestamp: July 11, 2018, 12:02:31 am
content: Picture of the cable modem and the part we are interested in looking at.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 11, 2018, 12:03:58 am
content: Close up views of the unknown device and the damage around the transformer.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 11, 2018, 12:10:14 am
content: 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

author: joeqsmith
author_rank: Super Contributor
timestamp: July 11, 2018, 12:13:47 am
content: Found one. They don't appear to be as common as the GDT.https://www.indiamart.com/proddetail/1-5kexxx-tvs-diode-13341138330.html

author: joeqsmith
author_rank: Super Contributor
timestamp: July 11, 2018, 12:37:50 am
content: Under the microscope it looks like a GDT to me.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 11, 2018, 12:43:13 am
content: 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.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 11, 2018, 12:48:57 am
content: 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.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 11, 2018, 12:51:06 am
content: Quote from: blueskull on July 11, 2018, 12:46:23 amI 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?

author: joeqsmith
author_rank: Super Contributor
timestamp: July 11, 2018, 12:53:10 am
content: I suspect if it is a GDT, it will look like a very small capacitor. Here is a 10pF.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 11, 2018, 12:55:34 am
content: 1pF. VNA is still warming up.

author: wraper
author_rank: Unknown
timestamp: July 11, 2018, 12:56:50 am
content: 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.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 11, 2018, 01:04:52 am
content: Quote from: wraper on July 11, 2018, 12:56:50 amI 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.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 11, 2018, 01:06:50 am
content: Quote from: blueskull on July 11, 2018, 12:59:13 amWell we knew that as I found one.Are you leaning towards a GDT?

author: wraper
author_rank: Unknown
timestamp: July 11, 2018, 01:11:02 am
content: Quote from: blueskull on July 11, 2018, 01:08:53 amAnd what is between the plates is exactly like in MELF diode/zener.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 11, 2018, 01:21:15 am
content: Over ranges the electrometer. Yes, it's connected.

author: wraper
author_rank: Unknown
timestamp: July 11, 2018, 01:22:41 am
content: Quote from: blueskull on July 11, 2018, 01:16:18 amIf 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.

author: wraper
author_rank: Unknown
timestamp: July 11, 2018, 01:31:18 am
content: It may look round on the first glance but if you look closer there is a rectangular die.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 11, 2018, 01:58:54 am
content: I measured 395pA of leakage at 50 Volts with this setup. May need to clean things a bit more...

author: joeqsmith
author_rank: Super Contributor
timestamp: July 11, 2018, 02:02:02 am
content: I can imagine they may have an insulator inside to set the gap. It may all be bonded as one part prior to encapsulation.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 11, 2018, 02:03:58 am
content: Notice on this GDT (yes it's a GDT) how the gap is formed.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 11, 2018, 02:13:09 am
content: 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.

author: CopperCone
author_rank: Super Contributor
timestamp: July 11, 2018, 02:18:26 am
content: 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 diodesIf 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.

author: floobydust
author_rank: Super Contributor
timestamp: July 11, 2018, 03:06:48 am
content: I see them as GDT used in car radios. Very hard to find.Pic is a Ford, also seen in Kia car radios.

author: wraper
author_rank: Unknown
timestamp: July 11, 2018, 08:05:35 am
content: Quote from: joeqsmith on July 11, 2018, 02:13:09 amGDT with D... designator, silicon die inside, does not look anything like GDT and has leakage current. Yeah,. I'd say quacks like a horse.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 11, 2018, 09:00:53 am
content: Quote from: wraper on July 11, 2018, 08:05:35 amWell at least no one thinks its a MOV.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 11, 2018, 09:32:06 am
content: Quote from: CopperCone on July 11, 2018, 02:18:26 amI have no plans to attempt to repair this modem or modify the new one. The goal for this thread is only to identify what technology was in use. What you you think this device is?

author: joeqsmith
author_rank: Super Contributor
timestamp: July 11, 2018, 10:41:30 am
content: https://youtu.be/QWDTsfXJHyw

author: joeqsmith
author_rank: Super Contributor
timestamp: July 11, 2018, 10:17:23 pm
content: Quote from: wraper on July 11, 2018, 08:05:35 amLooks like a duck and swims like a duck.The TVS believers go silent.....It appears the GDT starts to conduct at roughly 160 volts.

author: wraper
author_rank: Unknown
timestamp: July 11, 2018, 10:35:00 pm
content: Quote from: joeqsmith on July 11, 2018, 10:17:23 pmQuoteIn a rocket.QuoteIt's either 160V zener or TVS. GDS cannot "start conducting". It either does not conduct or breakdown occurs and voltage across it drops dramatically.http://www.citel.us/gas_discharge_tubes_overview.htmlQuoteEDIT: I watched your video. Dunno maybe it's just Zener which was broken open as it was glowing, probably because of your generator. But there is obviously a rectangular die inside it. Although after your experiments it might look different.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 11, 2018, 10:50:43 pm
content: Quote from: wraper on July 11, 2018, 10:35:00 pmSure, there is an area where it will start to conduct. A GDT it not a perfect switch.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 11, 2018, 11:05:28 pm
content: Scott asked to see it under a microscope. Same setup as before.https://youtu.be/A7sEDzuB_gU

author: wraper
author_rank: Unknown
timestamp: July 11, 2018, 11:06:00 pm
content: Quote from: joeqsmith on July 11, 2018, 02:03:58 amYeah, and every of them look completely different. 3 pictures, which of them look more similar?

author: wraper
author_rank: Unknown
timestamp: July 11, 2018, 11:09:46 pm
content: Die looks like it's glowing with occasional discharge occurring on the side. I certainly have seen glowing BJT dies.Quote from: joeqsmith on July 11, 2018, 11:05:28 pm

author: joeqsmith
author_rank: Super Contributor
timestamp: July 11, 2018, 11:12:54 pm
content: I assume you TD the video when you just watched it.The glow upsets you?Quote from: joeqsmith on July 11, 2018, 02:02:02 am

author: wraper
author_rank: Unknown
timestamp: July 11, 2018, 11:18:37 pm
content: Quote from: joeqsmith on July 11, 2018, 11:12:54 pmI watched it embedded here, didn't visit youtube. Why the hell GDT would have insulator in the middle? To destroy it's performance and ensure that arc runs across the insulator and outer glass thus damaging them? If you look on the actual GDT picture you provided, it's specially made to ensure that there is a gap between electrodes and outer glass.

author: wraper
author_rank: Unknown
timestamp: July 11, 2018, 11:21:21 pm
content: Some larger examples

author: joeqsmith
author_rank: Super Contributor
timestamp: July 11, 2018, 11:51:09 pm
content: Quote from: wraper on July 11, 2018, 11:18:37 pmYes that's another style. The one GDT I show also has material between the gap. I suspect its a ceramic rod that is coated and the gap was laser etched. Pure guess on my part.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 11, 2018, 11:56:48 pm
content: Finally, a little fun. How do they compare with some sort of transient. So, to be clear this is not an IEC combo generator. It loosely follows the voltage profile. It has a 2 ohm source and can deliver about 20J into a dead short is all.This the axial through hole GDT. Scale is 500V/div. Notice how the GDT is very slow. I've show this before in a video. It basically allows the voltage to reach some pretty big numbers. No surprise.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 12, 2018, 12:00:37 am
content: Now let's have a look at the GDT from the modem. Again we can see how slow it is . Even slower than the axial part. I've shown how fast TVSs switch before.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 12, 2018, 12:02:16 am
content: Quote from: wraper on July 11, 2018, 11:18:37 pmI need to apologize for calling you out but the timing was perfect.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 12, 2018, 12:48:55 am
content: I checked what higher voltage TVSs I had on hand. This is a 1V5KE150A. IPPm for this device is 7.2A and the clamping voltage at IPPm is 207V. I'm exceeding the peak current by a fair amount but the pulse is short. Notice this is 2us/div where the other two were at 50us.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 12, 2018, 02:03:52 am
content: Old paper on GDTs. I had asked someone if theirs were radioactive. This paper seems to have been written about the time they were experimenting with it.http://www.dtic.mil/dtic/tr/fulltext/u2/a214199.pdf

author: joeqsmith
author_rank: Super Contributor
timestamp: July 12, 2018, 03:21:00 am
content: Showing both GDTs on the generator running some higher currents than a few uA previously shown. I turned it all the way up on the last transient I applied to the modem's GDT.https://youtu.be/goiBhEn4xL4

author: floobydust
author_rank: Super Contributor
timestamp: July 12, 2018, 05:42:28 am
content: I'd treat it like a neon lamp, give it 100VAC at 1mA and see if she glows

author: joeqsmith
author_rank: Super Contributor
timestamp: July 12, 2018, 11:30:54 am
content: Quote from: floobydust on July 12, 2018, 05:42:28 amI fed it with an unfiltered, full rectified 240V signal through a 2Meg. The both drew about 50uA and glow. That's a couple of videos back. Also when it was under the microscope, it was the same setup and you can see the glow as well.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 12, 2018, 10:12:43 pm
content: Interesting old patent on a GDT.QuoteQuote

author: floobydust
author_rank: Super Contributor
timestamp: July 12, 2018, 10:52:36 pm
content: From that patent, using the stepped electrode shape :"... 0.0005" gap, a DC breakdown of about 400 volts and an impulse breakdown of about 460 volts have been obtained.For a 0.001" gap, breakdown voltages of about 550 and 650 respectively have been obtained.For a conventional gap of 0.002", the impulse breakdown voltage can be from 1200 volts to 2500 volts, or higher"At that tiny a gap, no way you can make glass that precise, heed a spacer. I think it's easier to use a larger gap and radioactive elements.I worked on lightning protection, tested atHydro-Québec testing laboratories,for the pipeline industry. They usually test electric utility stuff like breakers and transformers, bushings etc. to impulse voltage of 5.4 MV, a switching impulse of 2.7 MV and a peak test voltage of 1,800 kV AC or 1,200 kV DC.GDT's are unfortunately slow, the transient pass-through before they conduct still damages things.It's strange to see them used in some multimeter front-ends.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 13, 2018, 11:57:09 pm
content: The GDTs like the MOVs, sit behind a PTC and surge rated resistor/s and you will normally have a faster clamp for the lower impedance legs. I've looked at three meters that used GDTs. The HIOKI and Gossen held up very well against my tests. Obviously, that Gossen has a lot of issues but purely from my transient testing, it was never damaged. I wasn't impressed with the Keysight at all.My old HP 34401A bench meter also utilizes GDTs on the front end. When I pulled it apart for repairs, was the first time I have had it apart and just noticed them.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 14, 2018, 02:12:26 am
content: Before tossing out both parts, I made an attempt to do a direct discharge with my half cycle generator on each part.https://youtu.be/Yca42Ih-FPc

author: joeqsmith
author_rank: Super Contributor
timestamp: July 22, 2018, 05:20:17 pm
content: Glows like a GDT, arcs like a GDT, clamps very slow like a GDT.. Did finally get it to come apart.https://youtu.be/0thOfk4I3qs?t=2278

author: joeqsmith
author_rank: Super Contributor
timestamp: August 05, 2018, 10:45:32 pm
content: I did write the person who commented on the YT video about it being a TVS. Basically, I am looking for a suggested part number. I have no doubts that a TVS could be found in this type of package but they do appear rare. Not something I would expect to see in a commercial grade modem when plastic packages are widely available.

author: wraper
author_rank: Unknown
timestamp: August 08, 2018, 10:13:17 am
content: Quote from: joeqsmith on August 05, 2018, 10:45:32 pmTVS/zener at least do come in this package. Find at least one GDT part number that comes in such package as well.

author: floobydust
author_rank: Super Contributor
timestamp: August 08, 2018, 07:50:12 pm
content: I don't see a need for glass-packaged TVS as there is no gas envelope, like in a GDT.Hermetic-packaged TVS are MIL spec.by Sensitron.

author: wraper
author_rank: Unknown
timestamp: August 09, 2018, 07:51:51 am
content: Quote from: floobydust on August 08, 2018, 07:50:12 pmThen why tons of diodes and zener diodes come in glass package?

author: floobydust
author_rank: Super Contributor
timestamp: August 09, 2018, 07:21:17 pm
content: Small switching diodes in glass hermetically-sealed packages are widely available, like DO-35, DO-41, MELF etc. I don't remember ever seeing TVS in glass packages.I think the difficulty is to gas-fill these with say, neon to make a GDT. Or to make these larger for more power handling, for a big die (TVS) or electrodes (GDT).joeqsmith's part, might have been:Mitsubishi DA53Ruilon chinaRL501 series 0.5kA 2mm dia. x 4mm.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 10, 2018, 02:13:32 pm
content: Quote from: floobydust on August 09, 2018, 07:21:17 pmThey are both axial, not a MELF. Rare parts in a commercial product would make no sense at all.

author: floobydust
author_rank: Super Contributor
timestamp: August 10, 2018, 10:02:27 pm
content: It makes sense as there is no impulse standard for cable modems that I know of. What size GDT is suitable?GDT's meet Telecom and Mains standards such as UL1449, UL497B, FCC Part 68 etc. so they are sized large.A tiny GDT is off the radar and has no approvals, maybe a 0.5-1kA rating.RLM501MELF 0.5kA 2.1mm dia.RLM102MELF 1kA 2.8mm dia.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 10, 2018, 11:03:07 pm
content: Yes, GDTs in a MELF package seem common. I have not heard back from the person who posted on the YT video. Waiting to see this glass MELF TVS they feel would go into a cheap modem.

author: floobydust
author_rank: Super Contributor
timestamp: August 11, 2018, 03:27:15 pm
content: First time to see small GDT; in Sanwa PM300 DMM?https://www.eevblog.com/forum/testgear/aneng-302-pocket-meter-by-voltlog/msg1737086/#msg1737086In your testing, I wasn't sure if it (small GDT) could handle a Cat. III impulse.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 12, 2018, 03:49:37 pm
content: Quote from: floobydust on August 11, 2018, 03:27:15 pmThe question mark does not appear to follow a question. I have never shown a category III impulse and have no way to create it. Someone had made a similar comment about my tests damaging MOVs. Perhaps you do not have an understanding how most of the meters I have looked at are designed. I suggest reading post #2411https://www.eevblog.com/forum/testgear/hear-kitty-kitty-kitty-nope-not-that-kind-of-cat/2400/Sounds like Wraper is not having any luck finding a TVS that they feel would have been a replacement. I'm not surprised as it seems it would be a special part. Looking at the YT metrics, it did get a DV from someone in Latvia.Perhaps someone was upset that I was showing too much data.

author: floobydust
author_rank: Super Contributor
timestamp: August 12, 2018, 08:22:19 pm
content: It's many questions that little GDT raises, RIP. I understand multimeter protection circuits and 61010 well enough to get equipment designs certified.The Sanwa PM300using a slightly larger GDTwith no MOV's, "The world's only CAT IV IEC61010 rated true pocket sized multimeter"Seems strange a little GDT can pull it off, no MOV's required? Zotek and ANENG jump on board.You and I know the let-through/follow-through current would be a catastrophe in this crappy scheme.Looking at thePM300 approvals test reportit appears fake- 61010 ESD tested to 4kV, but no mains transient testing. ThePM300 product manualstates that wonderful term "designed to" which is not certified to...You think a little GDT can achieve multimeter 61010 approvals?

author: joeqsmith
author_rank: Super Contributor
timestamp: September 03, 2025, 02:16:53 pm
content: For others that may come across this thread, I am working on a Hioki DMM that uses a similar style GDT with insert. It's not a diode.... You can find a few photos of it here:https://www.eevblog.com/forum/testgear/hear-kitty-kitty-kitty-nope-not-that-kind-of-cat/msg6028739/#msg6028739Quotehttps://www.unictron.com/protection/circuit-protection-components/spark-gap-protectors-spg/