TVS questions

More often than an open or degraded failure, a TVS will fail as a short. As long as you have a good breaker or fuse on your power source, you will be in good shape.

Every one of my lock on’s connected to a track running under DCS has a TVS diode connected to it. I use a lot of lock on’s.

Since Bruce said they fail as a short that worries me. I cannot get to all my lock on once I have the layout scenicked.when I take the Christmas layout all down is there a way for me to test which one is bad. I don’t have sophisticated electronic testing devices. I have a multi meter and that’s about it. The only other thing I can think of is when I take the layout down take each track that has a lock on connected to it hook power up to it and see if I get a short.  

as always thanks

walt

Walt,

As @gunrunnerjohn frequently mentions, the TVS has capacitance associated with it. Too many TVS's may start degrading the DCS signal.  If you fear a shorted TVS, you can simply set your multimeter to the "Ohms" or "resistance" mode in the "R x 1" setting with the two probes across the two TVS leads. If the resistance reads "0", then the TVS is shorted.

Apologies for a longer than my usual comment.

thanks Bruce.  much simpler than my idea .  I've NEVER EVER used that function so I'm glad that you explained what to look for.

I should have started off saying this:  The reason that I thought it COULD be a TVS problem occurred yesterday.  I've been running my trains fine for 3 weeks or so.  Yesterday on the track that housed my PS3 Blue Comet as soon as I applied power the ammeter on my transformer shot all the way up to max.  The train derailed the day before and I THOUGHT that I put it back on the track correctly but shut things down at that point.  I doubled checked and it was OK.

I took the entire train OFF of the track.  Applied power and once again the ammeter shot all the way up.  I have 6 lockons on that track, 3 with a TVS device.  I didn't know what to look for.  I "jiggled" each lockon and twisted, sort of, each TVS.  Applied power again and all was well.

Put the Blue Comet back on the track, applied power, and it ran fine again.

So in a way I'm still VERY stumped about why the ammeter shot up right away.  My only thought was a bad TVS but....  would a shorted one cause my ammeter to react like that?????  And do they somehow reset?

I'm hesitant to use them again if the answer is "YES" a shorted one will short out the track too.  But from everything that I've read they are a great protective devices to use.

As always, thanks

walt

A shorted TVS will never reset, it's shorted for the rest of it's dead life.

@walt rapp posted:

I took the entire train OFF of the track.  Applied power and once again the ammeter shot all the way up.  I have 6 lockons on that track, 3 with a TVS device.  I didn't know what to look for.  I "jiggled" each lockon and twisted, sort of, each TVS.  Applied power again and all was well.

Just an idea....

Depending on how you have the TVS installed on the lock on, is there any chance that one side was shorted to the other side of the lock on?  I used to curve my TVS units into a semicircle and have been guilty of not paying attention to the fact I should have made the wires shorter to avoid any short possibility.  Wish I had taken a picture back then to better explain.  You may have jiggled the short.

@gunrunnerjohn posted:

A shorted TVS will never reset, it's shorted for the rest of it's dead life.

Could it not “open” after being shorted for awhile?

Are you using tubular track?  If so, maybe an insulator worked loose and the track (not the TVS) is shorting out?

Also, what happens if you remove the TVS?

@rplst8 posted:

Could it not “open” after being shorted for awhile?

10KV would open it😜

I have use these lock-ins with theTVS For gosh I don’t know 5 to 9 years. It’s not like I install them new every year I put them away with the device still connected.

I think Hokie 71 might have come close to how somehow things got corrected. Like I said the blue comet engine derailed real close to one of one of the lockons was located.  After my transformer sensed a short like I said I jiggled each lock on but the one closest to where the wreck was I thought maybe the one end of that TVS had touched the track somehow. So I bent it away from the track. I don’t know how it could have changed its position but now I am wondering after what hokie71 said if that wasn’t the actual short

Thank all of you for your comments. Gunrunner it’s good to know they can’t reset so thanks for that comment

As always, thanks.- walt

@SteveH posted:

10KV would open it😜

That much aye? Well, I guess that’s good to know!

@rplst8 posted:

Could it not “open” after being shorted for awhile?

Anything's possible, but my experience and general consensus is, when they short, they short.  It's a semi-conductor device, and when you melt the insides down, they normally stay that way.

I appreciate all of the comments.

but I still have what for me is an important question to get answered

if a TVS Device shorts does that also create a short on the track it’s self

As always, thanks

walt

@walt rapp posted:

if a TVS Device shorts does that also create a short on the track it’s self

Yes.

thanks for answering and confirming my hunch.

Does anyone worry about using these devices, especially in a hard to reach area if a short in one can disrupt everything?  Because I am.

walt

There's no good reason to put the TVS in a hard to reach area.

Walt, there are a few things that can be done to reduce the concern of having potentially failed shorted TVS diodes distributed around the layout.  

First would be protecting the transformers with some kind of modern circuit breakers.  Personally, I prefer Airpax Instant breakers for running a combination of Conventional and Command Locomotives from postwar transformers.  But there are others that will suffice as well.  Modern transformers already have adequate circuit breakers built in.  Understanding that as long as adequate over current protection is provided by circuit breakers, a shorted TVS diode will do no harm and is merely a temporary delay when compared to the damage that can occur in trains equipped with modern sensitive electronics.

The second measure to address the potential for shorted TVS diodes is to have a multi-meter that can be used to check the resistance of suspected shorts.  With the TVS attached to lock-ons like you have them, finding which is shorted is only a matter of disconnecting one end of each diode, one at a time, and checking to see if it measures near zero Ohms on the meter, until the source of the short is discovered.

Since TVS diodes do eventually fail from doing thier job of absorbing potentially damaging voltage spikes, and considering their relative low cost, periodically replacing them helps ensure that they continue serving their purpose.

I hope this is helpful.

@SteveH posted:

Since TVS diodes do eventually fail from doing thier job of absorbing potentially damaging voltage spikes, and considering their relative low cost, periodically replacing them helps ensure that they continue serving their purpose.

Actually, as a rule, if you don't exceed it's ratings, a TVS protection diode should last indefinitely.  Other than a random failure, a TVS diode should last as long as any other circuit component if it's not overloaded.

The surge protection that does wear out is the MOV, it's a totally different technology.  As an MOV absorbs surges, it also ages.  After a certainly number of protection cycles, it will no longer be effective.

@gunrunnerjohn posted:

Actually, as a rule, if you don't exceed it's ratings, a TVS protection diode should last indefinitely.  Other than a random failure, a TVS diode should last as long as any other circuit component if it's not overloaded.

The surge protection that does wear out is the MOV, it's a totally different technology.  As an MOV absorbs surges, it also ages.  After a certainly number of protection cycles, it will no longer be effective.

In a weird oxymoronic sort of way, the TVS is vulnerable to ESD which can destroy or weaken them. I never really understood why exactly, since they're supposed to protect against voltage transients - but I guess everything has it's limits.

Since our hands are on the rails from time to time, I wonder if that's how they die.

The bipolar TVS is really two Zener diodes back to back.  Just like a Zener diode, if you exceed it's current or surge power rating damping a transient, it can fail.  However, if the limits aren't exceeded, it'll also last as long as Zener diode.  Zener diodes last decades in service as long as they're limits aren't exceeded.  You could conceivably weaken them by going over their ratings, but not enough to totally kill them.

thanks for the varied replies.  lots for me to think about,

I don't know what terms like ESD, MOV and some of the concepts such as 'going over their rating' mean.  I bought the ones that were recommended here several years ago.

I use POWER GUARDS (connected directly to the transformer).  I use a MRC dual 270 trannie and put fast blow fuses between the trannie and the TIU.

From a discussion that I THINK I remember here on the forum, TVS devices catch spikes that the power guard and fuse do not catch.  That's why I added them.

I have had to have 5 PS2 and PS3 engines repaired for blown boards, at around $250 a pop.  So I tried to do everything that I read here relative to protecting them.

Since I have a floor layout that is heavily scenicked, MANY of the places are impossible to reach without removing a LOT of scenery.  I follow the recommended star wiring for the tracks that run under DCS so some years I use a lot of lockons.

I suppose that I could use tvs devices up front only but I have read numerous times here that the BEST place for them is as near as possible to where a spike might occur.  that's why I have them on every lockon.

as always, thanks

walt

Walt, it sounds like you have some good electrical protections in place.  The best places for TVS diodes are as close as possible to that which you want to protect.  A good case has been made for actually installing them inside the locomotives, wired across the track power inputs.  If doing this, it’s best to insulate the exposed parts of the diode’s leads so that they don’t short out any other circuitry inside the locomotive.

You’ve probably also read that too many TVS diodes can degrade the DCS signal.  If you haven’t had any DCS signal issues, you may be ok with the number of TVS diodes distributed around your layout.

Adding them to the inside of the locomotives may allow you to reduce the number connected to the track, especially in difficult to reach locations plus provide better protection against high frequency voltage spikes like those caused by derailments.

Acronyms:

ESD - Eletro-static Discharge (static electricity)

MOV - Metal Oxide Varistor (electrical device with varying resistance that’s dependent upon the voltage applied to it)

If you are using the TVS diodes recommended here on the forum, they are rated for the typical voltages present on an O gauge layout.

Here's by far the best description and graphic as to why the TVS diode should be very close to the protected circuit.  Ideally, you'd solder it on the motherboard or logic board at the power inputs.

@Adrian! posted:

This comes up on many threads over and over so here's a simple explanation all consolidated into a single thread.

A TVS goes in the system to prevent over-voltage stress from damaging electronics. This is different than a fuse or circuit breaker that protects from over-current stress. Unlike current at low frequency (60 Hz for most layouts) the transient voltage events that result from derailments, collisions, and general sparking are very broadband transient events, usually in the range of 1ns to 100ns.

That means they are a wave function, like a pulse of voltage bouncing up and down the wiring that has a fixed length, not like a continuous current flowing through it. At 1ns (so 1 GHz bandwidth) the wavelength in air is 11.8 inches and in RF we usually say 1/4 wave is where the wave effects start to matter.... so about 3 inches in air. However cables are not air and have a slower wave velocity depending on their dielectric constant (a property of the insulator) and their physical geometry (diameter, and how far apart the + and - are). All in all ... you can think of transient pulses as about a 1 inch wide event.

So if you put a TVS into a circuit and that TVS is more than an inch away from the thing you're trying to protect, chances are you're not actually protecting it because of the wave effects.

See the handy diagram below that explains what's going on. The bird is your electronics and the guy waving the rope is the train, making angry waves. If you pinch the cable in the wrong place (that's what a TVS does, clamp the cable voltage) the bird still gets bounced. The only way to avoid this is to do the pinching near the bird, in this case within the 1 inch.

and that's why the TVS needs to go in the locomotive right by the board, or in the TIU right by the drivers.

If you want a much more academic understanding.... Take a look at the telegrapher's equation.

@SteveH posted:

Acronyms:

ESD - Eletro-static Discharge (static electricity)

MOV - Metal Oxide Varistor (electrical device with varying resistance that’s dependent upon the voltage applied to it)

And don't forget:

TVS - Transient Voltage Suppressor

To help describe what a "TVS" does I created this drawing.

@rplst8 posted:

To help describe what a "TVS" does I created this drawing.

Ryan, I like your drawing and how it helps illustrate what voltage spikes could theoretically look like superimposed on a sine wave.  However, since the voltage spikes we’re discussing are generated by collapsing magnetic fields in electric coils, the spikes are more likely to be out of phase with the primary power source.  Still quite potent nonetheless.  Thank you for taking the time to create this helpful drawing.

The real damaging spikes can be hundreds of volts of short duration, those do the real damage.

@SteveH posted:

Ryan, I like your drawing and how it helps illustrate what voltage spikes could theoretically look like superimposed on a sine wave.  However, since the voltage spikes we’re discussing are generated by collapsing magnetic fields in electric coils, the spikes are more likely to be out of phase with the primary power source.  Still quite potent nonetheless.  Thank you for taking the time to create this helpful drawing.

I was going to mention the spikes could happen anywhere during the cycle, but forgot. They are also probably not single spikes like I drew either, but rather impulses that ring a little bit with several “waves” of varying frequency. Capturing all that in a two dimensional sketch in the time domain is difficult.

The other thing I don’t address is how the transients affect the RMS value of the waveform. I actually wonder if it’s individual spikes that kill sensitive devices on the DCS and TMCC/Legacy boards. Or if it’s the overall sum of them that raise the RMS voltage. So many variables at play there during a derailment. It makes me appreciate dead rail DCC all the more.

@gunrunnerjohn posted:

The real damaging spikes can be hundreds of volts of short duration, those do the real damage.

Yes, I need to extend the y-axis a bit and make the scale logarithmic!

@rplst8 posted:

I actually wonder if it’s individual spikes that kill sensitive devices on the DCS and TMCC/Legacy boards. Or if it’s the overall sum of them that raise the RMS voltage.

The damaging spikes are usually only picoseconds wide, they don't have enough "pull" to affect the overall RMS voltage by any measurable degree.  It's the actual spike that does the damage.

I wish I was electronically smarter!!!!   

I did know that the ideal place for the TVS device is inside each engine as mentioned above.  I don't have the soldering talent or know-with-all to try to do that.  That's why I put one on each lockon - to be as close as possible to an engine.

thank you all for your interesting and educational replies.  I learned a lot in this thread - maybe the most important thing (FOR ME) was knowing that a shorted TVS device will short the track too.  also very helpful was knowing how to test a device to see if it's shorted or not.

as always, much thanks

walt

OK, now I'm more confused that I typically am

After I dismantled the Christmas layout and disconnected each lockon from the track, I took what I thought MIGHT be the offending TVS device and did the OHM test recommended above.

It showed '0', which according to that post meant it was shorted.  But yet the track that it was connected to was fine.

So i started checking 8 other ones and all read '0'.  That can't  be, right?  Was my meter bad, I thought.  I never before used that function but have no reason to think the meter is bad.

Did I misread or misinterpret the post above?  Or am I measuring it wrong somehow - I did NOT do one thing mentioned above and that was to remove one post of TVS from the lockon.  I kept the devices connected, but since they were not connected to track I thought that was OK.

As always, thank

walt

Walt, let me see if I have this right. With a lockon disconnected from the track, and no wires attached, it still shows a short across the terminals with a TVS installed? If that’s the case it means the TVS is shorted. What happens if you disconnect one lead of the TVS from the lockon?

And there may be a misinterpretation here, but a shorted TVS will not short the piece of track it is connected to. But it will LOOK LIKE the track is shorted when tested with the shorted TVS and lockon connected to it I hope that makes sense.

can you maybe post a picture of a typical lockon with TVS and wire connected?

Rod

Rod, you are correct.  The lockons were all disconnedted from everything but I did leave the 2 ends of the TVS in place (they are somewhat hard to install.

The lockons that I tested the TVS devices on are the original Lionedl type lockons.  You know, the are black, about 1 and a half inches long, and mabye an inch wide.  No lights.

As to a shorted TVS shorting the track, I was going by rplst8 's comment

Everything is packed away but I'll see if the box with the lockons is easy to get to, and if so, I will take a picture and post it here.  I will also disconnect one end of the TVS and test again.

The setting that I am using is the one with the "upside down" horseshoe like image.

walt

Well, I think I can say that it's 99.9% certain that you're were doing something wrong with the measurement.  I really doubt you have a whole handful of bad TVS diodes all at once.

I agree John.  That's why I mentioned which setting that I used

@walt rapp posted:

I agree John.  That's why I mentioned which setting that I used

I didn't see the mention of the meter setting, it should be on ohms.  With the leads open, the meter should read infinity.  If it's reading zero with the leads open, something is amiss.

sorry John, I had it in there but I see it's not so I don't now what happened to it.  I wanted to say it's the setting with the K and an "upside down" horse shaped image.  Hopefully the images are clear enough to see what setting I am using

Here are some of the requested images:

In the 1st one, it looks like both ends are still connected but the red one is not connected.

Here both ends are connected

That is reading an open circuit both times.  Hence, that indicates the TVS is fine and not showing a short.

Open Circuit on the left, probes shorted on the right.

It's a diode, you should be using diode check on that meter. That's why you might get readings you cannot make sense of.

Diode check increases the voltage and current the meter is checking with typically.

On diode check setting touch the 2 meter leads together and note what the meter does/shows. That is true shorted. Then test the TVS and see what it says.

Thanks John.  I was going by a comment that Bruce Brown made:

If the resistance reads "0", then the TVS is shorted.

Thanks Vernon, I'll retest with that setting.  Be right back!