Location of TVS & clamping voltage

I have TVS diodes in six or seven MTH locomotives, and I've seen no degradation of the signal strength or loss of function.  I'm not sure what happened in your locomotive, but I haven't seen that effect.

 

As far as the unprotected half cycle that Dale speaks of, the TVS has no effect, positive or negative.  The point is that the overload protection does it's sensing on one half of the AC waveform, not on both halves.  Since each FET passes either the positive or negative side of the waveform, one of them can be subject to the full overload.  I don't know that this is a significant thing, never actually thought about it.  That overload shouldn't last long, as the circuit protection should kick in very quickly on the other half cycle.

 

I read things in a different manner I guess.

 

Although the silicon avalanche junction Transient Voltage Suppressor (SAJTVS) will first fail short in most applications, there is always one transient event that will cause it to open initially. In this case, the transient energy is large and of short duration that the silicon chip itself explodes.

That's a direct quite from Failure Modes and Fusing of TVS Devices by Vishay General Semiconductor Corp, one of the references you refute.  Considering the sources of transients we deal with, having a transient large enough to blow the 1500W device seems unlikely to me.

 

Note that as Dale points out, it's very unlikely that the TVS has any part in the failure of your FETs, nor would it be likely to have any part in preventing it.

 

Dave

The TIU is reported to already be TVS equipped.

 

From a really old Navy "sparktrician" I submit that Gun Runner John's successful hands on experience with TVS and that of us who have used them over the years gives me reason to counter your support for the opposition[you must have been a port-side Sailor?]. In my opinion GGG, appears to enjoy debating with quickly applied homemade facts and reporting on his credentials and [quote] "I am post fact" ...LMAO.

Well, Dale H has generously posted the experienced-based facts for you.

 

John has only advocated installing a TVS in engines if it happens to be opened up for some reason and only if one has the skill to do so. I am now past that point and based on results perhaps you are also. 

Hi for knowing where the tvs diode shorts just put a led with resister for the voltage drop and the tvs shorts the light will go out that is connected to the tvs diode that is  in parallel, that would save the hunt for the bad shorted tvs diode!

Alan

If you don't want to use led's just use a light bulb for the appropriate voltage probably 18 volts. you might have to isolate one power feed at a time but the one that does not light will be the bad TVS!

If you were to add a light in parallel with the TVS and the TVS shorts, I would think that  the light in parallel with the TVS would not light due to the dead short and the transformer overload protection kicking in and closing down the circuit right away.

 

Rad400

I believe it was stated previously in this thread that adding a light bulb to indicate TVS failure would also defeat the purpose of the TVS.

 

Also thanks for the comments on the device I posted about and thanks to Dals M. for the schematic, I will study it further (may not get too far, but a fun challenge anyway). Maybe something will stick to the old brain cells?

 

The earlier response that you are referring to, was for a light bulb in series with a TVS which would affect the operation of the TVS.

 

RAD400

Dewey, you don't know me but you make it personnel and accuse me of making stuff up, yet some how you know how much experience John has and how little I have.  How is that and who is making up facts now.

 

I got snotty a little with John because he got snarky with the google link.  But he took stuff out of context from one article,  I read several of them and then posted the pertinent info.

 

Instead of being a smarty go read yourself, then tell me where I am wrong, rather than I am wrong because you believe John.  Poor argument.

 

Go back and read what I have stated.  I support TVS, I don't support placing then in the engine.  I also state they can fail undetected.  NO one has disputed that though some may want to say it is rare.  I recommend replacement, because I am seeing signs that they may not work all the time, or they have worked and failed, and the next surge is taking out the boards.

 

So please don't make it so personnel,  go ask around and read my post, I DO NOT make stuff up and I DONOT post stuff as my own when SOME ONE else identified it.

 

By the way you might be surprised I know more about Navy Electrical than you do.  But hey you know me from the internet.  Hit me up any time you want to discuss Navy Steam, Gas Turbine or Nuclear power plants.  G  

 

 

Of course you know more about Navy electrical than I do--I assume because you just said so. I guess its hard to be humble when you are perfect in every way?

    

Dale, I take no offense to this, and I think we are on the same page.  I don't dispute TVS use, I do think it needs a sensible approach.  I do think we sometimes get to enthusiastic with a piece of technical information and don't necessarily apply it prudently.  Nothing wrong with a heated professional debate. 

 

My MTH experience show me that unprotected failures of Audio amps, speed control amps, and occasional battery charging circuits are the only fault that I can't attribute to pinched wires and other operator caused shorts.

 

I have fixed one audio amp on an engine protected TVS.

 

I have seen all sorts of failures in TIU that have 3 types of TVS in them for Rev L. 

 

All I have questioned is why have these failure still occurred with TVS protection, of course I recognize there could be more if no TVS at all.  But it leads me to believe you need to routinely change them if you think they have been handling some surges.  Especially since we don't measure the magnitude, nor numbers that occur during daily operations of the layout.   G

 

So did you, Why don't you do some of the fact finding you seem to be an expert on.  G

Good edit, But I read the first post.  What value have you added here?  Isn't there a Sports forum you should be posting on.  Your comments are more appropriate there.

By putting anything in parallel with the TVS aren't you providing a path for a surge to bypass the TVS and defeating it's purpose?

I hereby offer to test any TVS devices sent to me for free.  That way we can find out if they really do degrade or fail open.  If you want your devices back after testing, I will charge for shipping and a small handling fee.  After all, my time is worth something.

 

I propose to use a triangle wave from a function generator to drive an audio power amplifier to get the necessary peak voltage.

I'd think that some TVS diodes from a TIU that has been in use for a few years would be a good place to start.  They've had a chance to fail, though I'll bet money that you'll find none have failed.

 

You guys really should bury the hatchet. Funny thing is I was in my early 20s when someone gave me a juke box,a 1955 Seeburg V200,a very complex machine with torroid magnet switching. Way ahead of its time designed by a Swedish engineer. It did not work so I wanted to fix it. I knew zero about electronics and suffer from mild dyslexia. An electrician who served in the navy lent me a book, a US Navy manual of electronics which I studied and learned. In a few weeks I fixed the juke box which I still have today. I found a bad capacitor with a volt meter.  It needs more fixing now when i get around to it.  What it did for me is it gave me confidence that I could learn and do something on my own. Something I never had before in my youth. I was partially deaf and lipread a lot of sounds until I was 13.Because of this I was diagnosed once as retarded.So learning electronics for me was a lot more than just a career,it was about conquering adversity and acquiring self confidence. .  My grandkids think I can fix anything and sometimes I can sometimes. 

 

I just trouble shot a board on my wife's 18 year old Sundance spa. It had a bad solder joint in the board where 220 volts was jumping across the joint. This caused a surge/spike which tripped the GFI 50 amp breaker. When it did the varistor across the 220v input blew apart also. Looking at the board I knew what everything did and was. Immediately I noticed a design flaw. A weak solder joint design which had to carry 30 amps instead of just using the relay leads. Immediately when I looked at it I told my wife it was a bad design. As an electronics technician,I was so used to seeing badly designed systems and boards.  I could recognize the defect immediately.  I was able to fix it and save $350 for a new board by simply making a jumper wire. The board was actually revised later and used the same type  jumper wire I made.  Instead of buying the varistor from a pool supply I purchased 10 of them cheaper from a electronics supplier. It is nice to learn and understand things and even at my age I still want to learn. I am also happy to share what I know with others. I have learned a lot by making mistakes.  I am a college dropout without credentials,other than my experiences. 

 

I hope you 2 will refrain from personal attacks. It is just about toy trains after all.You probably can learn from each other. No one knows everything.

 

Dale H 

Dale, Don't worry I am done with Dewey.  But I am sure if some one challenged your integrity out of the blue with no facts you would take it a little more seriously then if he called you a name.  G

John,  These are the statements I find hard to digest.  You just pointed me to several technical documents that point out 3 methods of failure, even if the most common is a short.

Then you make a statement you will bet money that you will never fine a degraded or open TVS in a TIU.  How can that statement be made or even true.  Given even a manufacturing defect. Like those capacitors of the early 2000s.  G

George, it's simple statistics.  While you see a lot about the failure modes of TVS protection, it's really not all that common that they actually fail, and as I still contend, the vast majority will fail shorted as is mine and industry experience.

 

If you collected 10,000 TIU's, I wouldn't be so confident, but if you collect 40-50 TVS diodes from TIU's, I'm betting on statistics.  Note that I didn't say failure was impossible, even an open or degraded function, only that it's going to be rare enough to be very unlikely in a small sample.

 

The FAA endorses the use of TVS protection in flight critical hardware, and I'm fairly confident that didn't come on a whim.  We used a lot of ESD protection on air data instrumentation and fuel controls because it's effective.  Aerospace provides a much harsher environment than model trains, yet the MTBF wasn't adversely affected by the use of TVS protection.  TVS diode protection is used extensively throughout industry for the same reason.  I'll bet money that the electronics in your car has a number of them, that's another environment that has a lot of potential for power surges.  Depending on the make of your car, that may not be my best example!

 

What's the spec of the TVS used in the TIU anyway? 

 

For the Bombardier corporate aircraft Challenger CL600, 601, 604; when ever there was a suspected lightning strike.  The box holding all the transzorbs were removed from the aircraft and routed to my lab for testing.  Rarely saw them shorted or blown open but their characteristics would begin to degrade. Turn on voltage(when they would start to conduct current) would change.  Just off of memory out of 100 boxes maybe ten would have bad transzorbs in them.  Just a guess but not many.

It takes a lot to actually blow one open, that's likely why they short.  The change in specification is probably the most insidious issue, I suspect that will happen over time if they're in a high EMI environment.

 

 

John,  I understand what you are saying, and I understand the use of TVS.

 

I also understand the human factors of folks willing to buy something that is presented as a solution to expensive train repairs.

 

What I questioned was placement and durability.  Presenting issue with engines and TIU that have TVS.

 

We have taken this TVS item, built into and used to protect specific equipment or more accurately specific chips on equipment in industries were equipment failure can't be tolerated.

 

Lots of engineering and also lots of trial and error.

 

So I just question what are we protecting inside the engine?  What component on the PS-2 board?

 

How can a TIU that has TVS continue to have failures.  Do the TVS fail? I know they short, I replaced enough of them.

 

So let me present some more facts.

 

What does your ASC notes tell you to replace if you have a DCS signal loss on a TIU?  U502 right.  When is the last time you had a bad U502 on a Rev I or newer TIU?  I have not had one, in fact it is a whole different series of chips that fail.  So much so that MTH went back and added SMD TVS to protect those 4 chips in the TIU.  One per channel.

 

What TVS did you have as a replacement for TVS in TIU in the repair kit?  They are 33s.  Guess what TVS is in the Rev I and L?  39s.  Did this shift in a TVS cause this change in TIU damage?

 

I brought this up to MTH this week as a matter of fact conversation.  This was after I had to work on some older REV H TIU that had 33s.  That was when the tech said, he wonder if that caused the shift in failure type.  I did not put 2 and 2 toegether on the mismatch of repair kit TVS rating, versus what I was seeing in newer TIU until I saw the older TIU.  He is going to bring it up to the engineer to see what he thinks.

 

So coming full circle, having seen one engine still have a PS-2 audio amp failure on a derailment with a TVS installed, with TVS protection on the TIU output.  What does it do? Have we picked the right size, can it cause other issues?  Should they be changed out periodically?

 

When you look at what QSI was doing, they were trying to protect capacitors and diodes that were made in the late 80s 90s with 35V ratings.  We already know they had underrated  components until later runs in the 90s.

 

PS-2 5V had several components operating near the max rating according to an engineer I talked too.  Probably the reason for so many revisions to that board in a short 4 year life span.

 

So my only thoughts on this were not to place them in the engine.  Too much risk for unknown benefit.  Looks at easier access placement on your layout, and consider periodic replacement.   G

Certainly all valid points George.  The only way we'll have have a valid statistical sample is if I convince enough people to put them in their locomotives.

 

When I was sprinkling them around generously in avionics, the basic rule was use the lowest value that would never trigger in any normal operation.  However, there were some significant discussions about what and where the protection should go at times.  I'm sure we also made some wrong decisions in the designs, but the net result is we usually were successful in getting through the EMI testing and having an MTBF that exceeded the requirements for the product.  I'll also say that much of our TVS protection was on input and output signals from boxes, that's where the delicate components are usually found.

 

We also had specialists in EMI protection that basically did that their whole careers, so there's a lot to the topic! 

 

Lots of engineering and also lots of trial and error.

 

On that we are certainly in agreement, I'm working on some trial and error right now.

 

So I just question what are we protecting inside the engine?  What component on the PS-2 board?

 

A sufficiently high amplitude voltage spike can travel far beyond the first component it encounters, hopefully we're protecting whatever is the weak link in the chain.

 

Don't get me started on failing 5V PS/2 boards, I have another one of mine that just bit the dust!

 

I don't think there is just one problem here.  I think the nature of the source is important - PW ZW, Lionel 135W brick, 180W brick, Z4000?  Using whistle boost a la Lionel "Multi-Control" transformers?  What is the AC line voltage at the site - 110V, 117V, 125V or more?  Do you see the house lights flash when an air conditioner turns off?  All of these parameters would enter into the proper selection of a TVS voltage.

 

The best way to explode a TVS is not with a 1500W transient, but rather with a sustained voltage that causes partial conduction to the point of exceeding a couple of watts dissipation in the TVS.

 

Does either of you snarling dogs have a schematic of the TIU that shows exactly where the TVSs are located with respect to the FETs?

 

Nope Dale, I don't know of a schematic for the TIU.

From numerous threads, the recommended TVS seems to be the 1.5KE36CA where the break down starts around 34V and completely clamps at around 50V. The question I have is, why do the manufactures like Lionel & MTH not use 36V TVS.  As per previous comments, manufacturers seen to using 39V & 51V TVSs in their newer equipment.  I would think the 36V or even a 33V TVS would be what we need, as we are dealing with a 18-20V power source and all of the electronics is designed around that voltage range.  The further away the TVS break down starts from the desired equipment operating voltage such as with the 39V & 51V TVS, more harmful overload voltage is being subjected to the components being protected.  It will be interesting to hear what MTH has to say in response to GGG question.

 

Listed below are the specs for Littelfuse TVS which I found on the Digi-Key web site showing where the voltage break down start and completely clamps off all.  Hopefully this charts helps in the discussion.  

 

<colgroup><col width="71" /><col width="81" /><col width="69" /><col width="72" /><col width="69" /><col width="65" /></colgroup>

	Operating	Break down	Clamp	Peak	Peak
LITTELFUSE	volts	volts	volts	amps	Watts
1.5KE33CA	28.2	31.4	45.7	33	1500
1.5KE36CA	30.8	34.2	49.9	30.5	1500
1.5KE39CA	33.3	37.1	53.9	28.2	1500
1.5KE51CA	43.6	48.5	70.1	21.7	1500

 

Rad400

  

I believe that these are 5% tolerance parts, which means that all those voltages are +/-5%.

Dale-That is probably true, but I don't think that any of the components that we a talking about are  precision 1% or less parts.  

 

Thanks for offering to test some TVSs.  I currently don't have any TVS to send you, but may purchase a verity and send them to you.  I want to install TVS's at my club and I started this thread to get a better understanding of TVS placement and the best one to use.

 

Thanks for everyones input/comments.

 

Rad400

 

 

 

Dale,

 

I have a few extra of the 1.5KE33Ca size if you want them.  Do you still live on C. Zocalo?

Whoa!  I was offering to test suspected-bad units to see if they indeed show degradation.

 

Thanks, Ron, but I still have samples of both 33 and 36V units.

 

An interesting side note:  Apparently Lionel included the smaller TVS in some of the early PowerMasters.  I did find one unit that had a badly burned smaller unit.  The holes for the TVS aren't quite large enough for the 1500W unit, but with a bit of coaxing they can be enlarged.  The later PC boards no longer have both of the holes, but a small drill fixes that problem.... 

That TVS connects as a bridge across the two power FETs, not as a clamp on the input or output to ground.  It doesn't protect anything from overcurrents, just an overvoltage transient across the FETs.

The PM also has a small MOV device across the output terminals.  I have never checked the characteristics of that device, but maybe if I set up to run some TVSs, I should also run a couple of the MOVs.

No official response, but a recognition at some point a change was made.  I just repaired a newer REV L that is a factory MOD has the extra SMD TVS for the chips.  Turns out the outputs are back to 33.

 

As far as 51 in transformers, I can only assume they are there to protect the FET and the transformer components without trying to limit output by clamping too early.

 

If the FETs have higher voltage ratings you don't need the lower TVS, if that is the component you are trying to protect.

 

As mild mannered Dale pointed out, so many other factory can effect this selection.

 

PS-2 5V boards had TVS on the board.  They were not directional. So I assume they were after the rectification.

 

I have not seen a TVS in MTH equipment that catastrophically failed.  Shorted yes but no visible sign of damage.  I think that example in the technical papers is just an example of how bad it can be.  The current rating is high and the transformers are tripping or folding back well before those kinds of currents are reached.   G

The only TVS components I've ever seen that actually blew apart are in EMI testing with pin injection of high energy pulses directly into inputs of avionics.  It was a message that we didn't have a proper suppression design.  These were much smaller SMT devices that were obviously sized too small for the purpose.  The 1500W ones we use would take a lot to physically blow them.

The TVS we use is not really a 1500W device.  It is a 2 or 3 watt device with a surge rating of 1500W.

I think part of the problem for manufacturers is that they must size for worst case.  A relatively benign 18V Lionel 180W brick with good internal protection is very different from an old Z with a rating of 6-24V!  Peak voltage for 24V RMS is 34V!

Dale, I understand that, they're all rated for surge.  You're right about the Z, but I don't think I'd be tempted to use that for modern TMCC stuff, too much danger of releasing the magic smoke.

 

So from what I am gathering, if you have a Z4000 or ZWL, a 33V TVS would provide maximum surge protection for the components we want to protect in the TIU and engine electronics. A 33V TVS has an operating voltage of 28V and starts to break down at 31.4 which should be within the range we want.

 

Am I missing anything??

 

RAD400

 

 

Don't remember if it was already mentioned in this thread, but someone once posted that the 36's were needed for conventional for horn & bell operation and the 33's were fine for command control. However, I don't recall who posted this (it's been a long while) and I will defer to the other more knowledgeable folks that may have contradictory views or information.  

Weelllll maybe...

Just for clarity for everyone following this thread:

The 28V 'operating' rating of the 33V TVS is a peak voltage, which translates to 20V RMS for a sinewave.  This is good for a Lionel brick with 18V output, even on a high line voltage of 125V (about 7% above nominal 117V.)  But don't forget to also apply the -5% tolerance of the TVS to the 28V rating...

 

But what about a Z4000?  Isn't the max output voltage a bit higher than a brick - something like 20V RMS?  (I have never had an opportunity to play with a Z4000.)  If so, high line voltage and lower end tolerance on the TVS eat up 3 volts, leaving us at 3V over the TVS's operating voltage.