# Need to know about TVS diodes



## Old3rail (Nov 28, 2019)

How do they get installed, and do they need replaced after a derail or other short?


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## Millstonemike (Aug 9, 2018)

They should not need to be replaced after a derail or short (maybe after a lightning strike)


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## Old3rail (Nov 28, 2019)

Millstonemike said:


> They should not need to be replaced after a derail or short (maybe after a lightning strike)
> 
> View attachment 557125


Thank you Mike, I forgot one, how do you determine what type you need? If there are different types


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## Millstonemike (Aug 9, 2018)

There are different types. *This Post* by GRJ lists the his specific recommendation and a source: Digikey electronic supply. Note that Digikey has favorable shipping for small parts via the US Mail shipping option ~ $5. Or you can peruse eBay if you can determine the seller's TVS diodes meet GRJ's recommendation.


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## Old3rail (Nov 28, 2019)

Millstonemike said:


> There are different types. *This Post* by GRJ lists the his specific recommendation and a source: Digikey electronic supply. Note that Digikey has favorable shipping for small parts via the US Mail shipping option ~ $5. Or you can peruse eBay if you can determine the seller's TVS diodes meet GRJ's recommendation.





Millstonemike said:


> There are different types. *This Post* by GRJ lists the his specific recommendation and a source: Digikey electronic supply. Note that Digikey has favorable shipping for small parts via the US Mail shipping option ~ $5. Or you can peruse eBay if you can determine the seller's TVS diodes meet GRJ's recommendation.





Millstonemike said:


> There are different types. *This Post* by GRJ lists the his specific recommendation and a source: Digikey electronic supply. Note that Digikey has favorable shipping for small parts via the US Mail shipping option ~ $5. Or you can peruse eBay if you can determine the seller's TVS diodes meet GRJ's recommendation.


Thank you Mike for the reply and help


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## Jetguy (Mar 22, 2019)

I have a slightly different opinion on some details regarding and let me back up the "why" behind placement. I fully believe in TVS diodes, however, I believe there needs to be more thought in power systems end to end. Before going into this answer, I've been meaning to place this into one of the transformer/TIU/TVS discussions for some time so it's maybe beyond the original question of just TVS placement, but I feel relevant.

Sure, you could put TVS diode right at the transformer. Sure it's easy. The what if?
TVS diodes clamp (short) voltages that are higher than the rating value. It's a simple device, open circuit like it's not even there until it sees a voltage = or greater than a threshold and then shorts. But here's the issue, enough spikes, they fail like most devices fail and when it fails, they tend to fail shorted. So now you have only the breaker inside the transformer keeping you from cooking that diode as it now conducts the full voltage and amperage of the transformer until it glows red hot. You could start a fire or meltdown and again, depending on a high amperage breaker inside an older transformer is just not good safety in my book. Th picture of the PW transformers with TVS diodes wrapped around the terminals is all over the web, but again, the what if and depending on that internal common high amperage breaker to trip is just not a great plan IMO.

Worse, let's say you have some electronic device- a MTH TIU, a Lionel Powermaster, between that transformer and the track. You do the opposite, you put the TVS diodes at the track or the output of say the TIU as many drawings I've seen place them. Well, again, when the TVS fails shorted now your massive shorted overload current is flowing through your electronic control device. Again, that transformer doesn't care one bit, it's going to provide more amperage than the rating for longer than you can believe and smoke anything between it and the short. Don't be fooled too by a TIU having fuses internally and thinking those 20A fuses are there to protect your now insanely valuable TIU. They are there to prevent a fire, and probably a poor job at that. In other words, sure, they are there, but between the high rating of 20A and the time it takes them to blow on a slowly crept up 20A load won't save anything you care about.

The fix:
#1 ALWAYS use good fast circuit breaker or fuse protection on each channel on the output of the transformer. It should be the first thing out of the transformer. Again, the transformer is the SOURCE of the power and so limiting the current at the source protects any and all downstream devices from over current. Current is what smokes wiring, burns connections, overloads power flowing through devices. If you work on trains like I do, have you seen the small pickup wiring used in modern Lionel locos and cars? All it takes is a single derailment to smoke this wiring. Again, the idea here, is we limit the current at the source. That way, if any downstream load or device like a TIU, power master, or anything else like a train derails, a TVS shorts in failure, your wiring is pulled on and shorts out, we don't burn things up.
Also, as stated, these should be in each channel, however, I believe the common U should not have a breaker or fuse as an open circuit in common could lead to a floating neutral situation and all kinds of insane back flow currents between shared circuits. Again, just like a neutral in house wiring, it can be carrying returns from different sources and when that open circuits, now you back feed all kinds of circuits and wiring on your layout.

#2 But we also have to protect from voltage spikes. Think of voltage as water pressure. Electronics have a maximum voltage and beyond that, kind of like water pressure, they fail, not unlike a pressure spike in a pipe system literally bursts right through the pipe. Once cracked, now the current flows and a voltage spike causes the failure and then current flowing finishes the job of total destruction. This is where the TVS is the cheap solution and they do work for protecting from over voltage and voltage spikes.
So then comes the million dollar question where do these spikes come from and so where to best place the TVS so that it shunts the spike BEFORE it goes into your electronic devices (TIU, Powermaster, or other). The 2 places most spikes come from are the source (transformer) and the track and any accessories, trains, or whatever on the track.

One thing is that trains and the track itself, along with all wiring has some inductance and capacitance. A wheel rolling on a metal rail makes and breaks contact millions of times a second. Those make and breaks generate voltage spikes. Then you have inductance in motors and coils. Examples are track switches with coil actuators, the old gateman, old AC motors, just about anything that has a coil of wire. These devices create a massive spike every time they are turned off just like the spark coil in a car. This is why we need TVS diodes, to clamp these spikes shooting down the track and back into our control systems in both directions. In other words, they can blow up your TIU, your Powermaster, heck even your electronic control transformers (CW80, Z Controller, etc..) but also the trains on the track that have any electronics, even just simple sound systems.

Again, as this topic started, the old standby and position was put them at the transformer. Yes, older transformers, being a massive coil of wire flowing current, yes, can produce unintended voltage spikes as the load opens circuit. So yes, we are right to want to put the TVS there. The difference is, I advise putting the TVS AFTER a fast acting circuit breaker so that in case the TVS does fail and short, we don't create a massive meltdown because the fuse, breaker, or other means trips on the overcorrect short created when the TVS fails.
At the same time, we do need to protect the track since it can be a source of voltage spikes coming from the trains, the cars, all the accessories and anything else on the track system. Another not yet mentioned spike is the human element- static electricity in from us shocking the track. Yes, a lot of folks run trains in the winter and it's dry and you have a sweater on, carpet, and so sure, you yourself create spikes. So yes, minimally, putting a TVS at the track feed is a good idea.

Debates:
Sure, you can over do it. Yes, TVS diodes have capacitance, and so for signals like DCS that is in the power of the rails can be attenuated by too many TVS diodes. Also, if you don't follow my suggested rule of putting some kind of fuse or breaker between the power source and TVS diode, then when the diode fails shorted, then without a fuse or breaker to open the circuit, now your track is shorted and you have to find and test one by one each TVS to detect the one that failed.
Next good debate is what voltage rating should the TVS diode be? The idea is the TVS catches the spikes that would cause damage to the electronics on your layout. As an example, Lionel typically recommends no more than 18V AC for many trains and accessories. The problem is, that rating is RMS so actual peak value- not unlike a spike is higher. The point is, the TVS has to block voltages higher than X, but there is another part of the equation of how much and how bad is your layout at creating spikes above that voltage? In other words, let's take a value like 36V a common recommended value. OK, if you have a bunch of postwar trains, and you have say a gateman that is insulated rail or the weighted contact controlled as the trains go by, every time that actuates, you could be creating a massive repeated spike every time the train goes around. The point is, the voltage is a fine line. I this to be low enough to actually protect the electronics from their peak maximum voltage limit, but then also not shunt so much current and voltage the device self destructs from typical user and track generated spikes that are semi normal. In other words, the scary part is, if the TVS fails, it ws doing it's job and without it, very likely you were at risk for blowing up your electronics.

So as pointed out, they are a consumable protection device. You need to be able to test and replace them on occasion. You need to put them closest to the voltage sources that create the spikes, but also IMO best to have a fuse or other means of protection so when they do finally fail shorted, they don't cause an over current situation and burn out an expensive piece of electronics in the path (TIU or Powermaster).

I say all this from the perspective of replacing a PILE of TVS diodes in a stack of devices last week. Example was 3 Z controllers from MTH Z1000 sets, TIUs, and some other equipment. The interesting thing is the voltage values and the fact they STILL failed. Example, TIUs and Z controllers use the 1.5KE51CA in the variable voltage section, where the output of the TIU is a lower 1.5KE33CA, and then an often recommended TVS for transformers and track add ons is 1.5KE36CA
The point being, 33 is the lowest, and 51 is the highest, so I'm thinking if a 51 blows, that is one heck of a spike on the track. In the case of the 51s blown, the electronics were also damaged (FETs in the variable voltage circuit).
I'm just trying to give perspective here on why a TVS value is chosen. If the TVS value is too high, then spikes get through and can damage the electronics. If too low of the TVS voltage value is chosen, it does a great job of protecting electronics from voltage spikes, but may in turn fail more often due to catching more spikes from everyday events. Somewhere around the suggested value of the 1.5KE36 is a good value and proven by field testing and experience. Again, just trying to give perspective here and understanding. MTH TIUs have been using 33s internally, the variable channels have a 51 across the circuit as does the Z controller. By using externally 36s, I think we strike a value that should help protect the internal ones and provide a reasonable headroom for non damaging spikes on a given layout that don't kill TVS diodes too often.

As with any protection, people don't use it when it's too hard, too complicated, or too costly. Protection is a hard benefit to understand and define. You spend money and put effort into not destroying expensive electronics, VS doing nothing and rolling the dice. Using the wrong protection and having problems leads to the user giving up and removing it.


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## Jetguy (Mar 22, 2019)

And a topic for another day:
Specifically static protection (ESD) and even possibly lightning protection.
Given TMCC is a radio signal, and the need for your layout floating from ground, while also having ground present nearby to create the differential field compared to the signal.

So where and how to best place protection if any to deal with that and not kill TMCC signal, but also give a proper source to ground in the winter when dry conditions cause high likelihood of user created static shocks to the track, trains, accessories and so forth? Depending on the floating completely ground isolated state of most layouts, I think the TMCC base is the one thing known to be crossing true earth ground and your floating track state, and so one place I could see ESD killing the transmitter or worse in your expensive legacy base. And, yes, our club is using the TMCC buffer amplifier so in theory, that could kill it. Again, as far as I know, we intentionally use transformers and systems that all are not connecting the track side and accessory outputs to earth ground, the transformers are completely isolated. Essentially, our entire layouts are isolated and not connected intentionally to earth ground. The noted exception is that TMCC bases and their matching transformers do need a proper earth grounded plug and thus are a place where a user created ESD static shock to track or train or whatever then finds it's way through the TMCC base to earth ground and by doing so, presumably puts the TMCC base at risk for damage.

I'd love to hear thoughts and advice on this specific situation of TVS or other device to earth ground. Again, the reason for asking is a curiosity of the fact many people run trains more often in winter as staying inside also is a state where ESD risk is higher and a combined combination of train and electronics failures might happen. I've yet to see a discussion on this in my research- maybe it's something, maybe I'm wrong and it's nothing?


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## gunrunnerjohn (Nov 10, 2010)

In general, the location of a TVS protection diode should be as close to the circuitry being protected as possible. For locomotives, this obviously means in the locomotive. Of course, inside the locomotive is also the most labor intensive place to install the TVS diode, not to mention the possibility of damaging things taking the locomotive apart. That being the case, most people limit themselves to placing them somewhere on the track.

I don't see an issue with the TVS and TMCC signals, simply because the TVS is actually not connected across the TMCC signal. The TVS is connected across the transformer power feed, that's regardless of where in the power path it's located. The TMCC _earth ground_ connection has no connection to either side of the transformer power output, hence the capacitance of the TVS doesn't enter into the picture as far as TMCC signal strength is concerned.

OTOH, for MTH DCS signals, the TVS across the power is a factor in signal strength, and much has been written about factors that degrade the DCS track signal strength. All of my electronic products that are intended for installation in locomotives or rolling stock have protection for the DCS track signal in the form of a 22uh inductor to limit attenuation of the track signal. This is also why too many TVS devices on a single TIU channel can end up making the "cure" worse than the "disease" as far as DCS signal strength is concerned. The Littlefuse 1.5KE39CA can have over 1,000pf of capacitance, at the 3.27mhz nominal frequency of the DCS signal, that equates to a capacitive reactance of less than 50 ohms across the DCS signal. If you have a number of TVS devices across the track, you can very easily seriously degrade the DCS signal.


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## Patrick1544 (Apr 27, 2013)

Wow. That’s a big lesson.


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## Old3rail (Nov 28, 2019)

Patrick1544 said:


> Wow. That’s a big lesson.


That's what I thought, a very good one. John


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## Lehigh74 (Sep 25, 2015)

Thanks Jetguy. Good explanation. I think I heard once that TVS diodes fail closed, but I had forgotten that and sometimes I wondered if mine are still good, but now I’ll know if any fail if I have a short that I can’t figure out. I’ve been putting them at each barrier strip where I run power to the track.


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## T-Man (May 16, 2008)

For anything in electronics seek out a data sheet. For this TVS diode I found this attachment.


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## gunrunnerjohn (Nov 10, 2010)

TVS diodes typically fail shorted, but not always. They can also degrade where they are letting higher amplitude spikes through, and finally they can fail open as well, though not nearly as often.


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## Madman (Aug 22, 2020)

gunrunnerjohn said:


> TVS diodes typically fail shorted, but not always. They can also degrade where they are letting higher amplitude spikes through, and finally they can fail open as well, though not nearly as often.


*How would we know when a TVS fails ? Mine are located across the terminals of my ZW transformers, as Mike has shown in his post. *


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## Jetguy (Mar 22, 2019)

Madman said:


> *How would we know when a TVS fails ? Mine are located across the terminals of my ZW transformers, as Mike has shown in his post. *


When they short the transformer, smoke and glow red hot until the 12-15 Amp breaker inside your ZW finally trips or you notice your track shorted and it's not from something else.
The other small fraction of the time, they fail open circuit like a blown fuse.

So here is the problem: testing a TVS diode.
It's a device that looks open circuit like a blown fuse under the rated voltage.
When a voltage equal to or higher than the rated threshold voltage, they short circuit.
If the TVS is in circuit, meaning attached to the transformer and your track, to test it, you would have to send a voltage equal to or higher than the rating to trip the diode into conduction- meanwhile since it's connected to your track and transformer also potentially sending them the high voltage.

So no, to best test a TVS diode, you kind of want it out of the circuit to see if it properly conducts when we expect it to. Conversely, you can check in circuit for a short, but if you have more than one TVS say at track feeds around the layout- then if one is shorted- all appear shorted.

Hence, why I was saying, if you put a fuse in series with the TVS (ideally a slow blow IMO) then the TVS diode if it does fail and short, blows the fuse. If you put an LED and resistor in parallel to the fuse, then when the fuse opened circuit and the TVS is shorted, the LED lights up telling you TVS failure. The fuse should be of a value that won't blow when the TVS is shunting normal spikes. By doing this, you now have a fuse you can unplug periodically, and then use a TVS tester to test the diode as it would then be out of the circuit.

To my knowledge, I have not seen people suggesting this or any diagrams or pictures of people using this.


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## Jetguy (Mar 22, 2019)

Again, understanding what a TVS does and how it fails.
A TVS diode has a voltage rating, and it's open circuit until it reaches that threshold and begins conducting.
When a TVS fails, the idea is that it began conducting from a voltage at the threshold, then because that voltage and power was so great, it breaks down the structure of the TVS so that the TVS now stays shorted, even below the threshold where it should not be conducting. Since this happens across the power of the transformer, instead of just shorting the spike, now it's conducting and shorting the transformer and that energy is turned to heat further self destructing the TVS, not to mention shorting your transformer and track so you cannot get power to the trains.

It's possible that either a spike voltage or some power surge is so great, all in one swoop it just blows the TVS sky high into open circuit. In the words, the spike or surge was such an event, the TVS just blows open circuit internally like a fuse.

Also, as gunrunnerjohn pointed out, spikes and surges can cumulatively damage a TVS over time. In other words, it could be on big spike that causes failure, or thousands or millions of smaller ones that eventually breakdown the device into failure.


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## Jetguy (Mar 22, 2019)

Let me try to address your situation from what I know from previous posts:
#1 you previously were running post war locomotives without any electronics.
#2 you recently got at least semi modern MTH and Williams loco with electronics.
#3 you are powering with a ZW transformer and wish to add some protection specifically TVS voltage protection.

My concern:
We have not addressed what you are using for current limit (amperage) protection.

Are you using external circuit breakers or fuses between your ZW transformer and the track or your accessories?

The reason for asking is trying to help you get proper protection. If you just protect against voltage, that's not doing anything for amperage. If you do have fuses or breakers, lets examine this as a system, and combine both voltage and current protection for maximum protection of your trains and equipment. I say this, because once most folks get into modern stuff with the sounds and experience, they often buy more.

The reasons why we do this:
Modern trains have electronics and they have both current and voltage limitations.
Accidents happen, derailments, shorts, voltage spikes, etc..
The cost to repair even one modern locomotive can be quite a shock.
Protection in the form of breakers, fuses, and TVS diodes is trivial to the cost of even one repair.


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## Firewood (Nov 5, 2015)

I run old-school cab control with a mix of recent Williams, MTH and Lionel. Modern electronics are more a confusing necessity than hobby enjoyment for me, so I don't have any plans for a command control system. I guess my MTH locomotives are the most electronically loaded. My two MTH bricks seem to have a fast trip-out but I'm probably being naive. Would I put a TVS on each control block to protect locomotives?


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## Jetguy (Mar 22, 2019)

Yes, ideally.


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## gunrunnerjohn (Nov 10, 2010)

One dark side of too many TVS diodes is their impact on the MTH DCS track signal. A typical 1500W TVS of the type you'd use for O-scale has hundreds of picofarads of capacitance. Here's a commonly used one, and it's also on the output stage of the MTH TIU.

1500W 33.3V TVS at Digikey

If you have multiple TVS diodes in one track segment across the track power, you can significantly attenuate the DCS track signal. The typical TVS diode we use has about 400pf of capacitance at it's rated breakdown voltage, and that capacitance goes up the lower the voltage is across the component. 400pf at 3.27 is a capacitive reactance of around 120 ohms. If you parallel three TVS diodes at various places on one power district, you have a combined capacitive reactance of around 40 ohms. That will seriously impact the amplitude of the DCS signal on the tracks.


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## Firewood (Nov 5, 2015)

Thanks guys - much appreciated.
FW


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