TVS and Fast Acting Circuit Breaker help

After reading all the recommendations in this forum I added both.  I grabbed the fuse holder and glass fuse at autozone.  TVS from eBay.  I haven’t had a problem since but sure worth the piece of mind in my opinion.

once you did that did it help with sparks during derailments?

There's no real help for some sparks during most derailments, when you connect the rails together, you get sparks.  The issue really is, how well does the circuit protection stop the current flow when this short happens.  The real message here is to work on the trackwork to minimize any issues of derailments.  That's normally not that difficult with O-gauge 3-rail, it's pretty forgiving of less than perfect trackwork.

GRJ should I add more protection than just what the z4000 provides?

I don't think that will do much for sparks during derailments.  If you want the fastest acting circuit protection, you'd probably want to consider something like the PSX-AC board on each power feed.

Ok thank you, so I don’t need to add the extra breakers or TVS to my loops?

A TVS isn't a bad idea at each track feed.  If you have MTH DCS with the TIU, each channel has a TVS on the output.

any type of TVS you recommend, I don’t have a TIU, I just have legacy.

1500W 33.3V TVS at Digikey

does that go on the terminal block that comes from the transformer?

That's a good spot, the TVS goes right across the track feed from hot to common.

Thank you for all the help

Casper, I installed a TVS on every track feed and a PDX-AC on each of my two power feeds, just what John posted.  The PDX-AC is certainly fast acting.  I haven't fried anything since the installations. 

OK. I have a Z-4000 going to a TIU. I jumper my power wires from the Fixed inputs to the Variable inputs. I set the Variable inputs to Fixed. I power my TIU externally with a Z-1000. I use all 4 outputs on the TIU. Where exactly would the PSX-AC's go? Between the Z-4000 and TIU or after the TIU?

The PSX-ACs go between each Z4000 output and each TIU input.

Mark, thank you. Just curious  but why not between the TIU and track? Not being that electrically educated I'd like to know what happens electrically, both voltage and amperage, to the circuit and to the dc signal the instant I have a derailment and my wheels cause a short. Don

Having the PSX-AC before the TIU eliminates any possibility of it affecting the DCS signal.

GRJ, thanks for the info,

Don

Trestleman1, John is right.  Don't feel bad in the least.  I worked in electronics for 43 years, and you would think I would have thought of John's reason.  However, I didn't and first connected them after the TIU.  I was in a long discussion on the topic a few months ago and changed mine to match what John said.

Mark, I'd still like someone to explain exactly what happens electrically during a derailment/short circuit. Why do you get high voltage knocking out a TVS sometimes and high amperage other times or do you get both problems at once. Any explanation would be helpful. Don

Don, you're talking the impossible dream.  Every derailment, short, voltage spike, etc. is unique.  It's like lightning, each bolt of lightning is unique.  Even in the lab it's difficult to create a test environment where you can recreate the exact same overload or over voltage situation consistently.  Nobody can tell you exactly what happens during a derailment down to the last coulomb.

GRJ, That's funny, my wife says I ask for the impossible too. OK, I guess I'll defend against shorts and voltage spikes with PSX-AC's and TVS's and be happy running trains without problems.

Thanks again,

Don

@Trestleman1 posted:

GRJ, That's funny, my wife says I ask for the impossible too. OK, I guess I'll defend against shorts and voltage spikes with PSX-AC's and TVS's and be happy running trains without problems.

Thanks again,

Don

Don, I read your question and was hoping to see a reply from John that was better than mine.  Basically, I was thinking what John said.  Having worked most of my working days for power and telecom utilities, circuit protection (especially at the power utility) was a huge part of my job.  We used all kinds of protection devices since there was never a one size fits all fault (short circuit or voltage spike)   There are too many electrical variables to make a prediction.  What I highlighted in your comment is the perfect attitude to take. 

Mark, Yes, I was just trying to understand the problem and get as much information as I could before adding things like PSX-AC's and TVS's. If the information is not there, it's not there. I'll take your advice and happily run trains.

Don

@Trestleman1 posted:

Mark, I'd still like someone to explain exactly what happens electrically during a derailment/short circuit. Why do you get high voltage knocking out a TVS sometimes and high amperage other times or do you get both problems at once. Any explanation would be helpful. Don

Hi Don:

(Disclaimer -- I respond at risk of sounding rather condescending, which is absolutely not my intent.  You can also do a great deal of reading on the forum and elsewhere more information on these electrical subjects).  To 'generally' answer your question:

Shorts are an issue of current flow.  When a derailment occurs, you are directly connecting the 'hot' and 'return' of your AC power supply.  That in turn creates a condition where there is no load / resistance to impede or regulate current flow (amps) in the system.  This condition 'theoretically' demands infinite current at the ~18VAC from your power supply, which it of course cannot deliver.  At the moment of shorting, your PSX current breaker, wired in-line with your AC power supply, trips near-instantly, disconnecting the circuit and preventing your supply from melting down.  This entire sequence happens in less than a blink of an eye after the short condition occurs.

Voltage spikes (or 'voltage transients') are abnormal 'bursts' of volts beyond the designed supply voltage of a circuit.  They do not require a short condition to occur, but will happen often (if not always) when a short condition occurs.  Spikes obviously result from lightning strikes, but can occur whenever there is a electrical disturbance, (failed devices, components, shorts elsewhere, etc).  Voltage spikes are not (strictly-speaking) high-current events, and thus will not blow fuses or circuit breakers unless there is a coinciding high current condition, (which is often the case).  The TVS (transient-voltage-suppression) diode is connectedacross the 'hot' and 'return' of the AC power into the protected load, (as opposed to in-line, as with fuses and circuit-breakers).  When a voltage spike occurs, the TVS absorbs it, preventing the load from seeing that transient voltage and blowing it up.

Now is where GRJ and Mark's responses fit in perfectly. A derailment is first a short condition, but it also generates voltage transients.  The mix and magnitude of each during a derailment is as diverse as the stars in the heavens; you can never really lock down the exact details of each event.  That is why, ideally, we use circuit-breakers in-line right after the AC power supply to protect against shorts and high current conditions, and TVS diodes across the loads (track and / or locomotives) to protect them against voltage spikes.  GRJ (and others) have pushed this ad nauseam on the forum.

With all the track based circuit protection I have built into my layout, I couldn't help but reflect on how unnecessary it all becomes with my battery powered engines.

Patenary, That is an excellent general explanation of shorts and voltage spikes. I really appreciate the time and effort of your reply. I'm sure there are a few other newbie's who can learn from this also.

Don

@BOB WALKER posted:

With all the track based circuit protection I have built into my layout, I couldn't help but reflect on how unnecessary it all becomes with my battery powered engines.

Good point, Bob!! 

Just wanted to throw what I think is an often overlooked aspect of the situation involving transients (voltage and current) on the model railroad.  Thing is, it is mostly below 30v by regulation, so our skin keeps us out of most of this trouble--- most of our pain is the wallet.  Of course there is the exception--- the hot wire.  But we do have real experience above 30v, namely our 120v home--- we all know the hot wire is the one to watch.  Well, the other side is grounded. and the earth is just too big to change.  So, many of us see the small railroad in the same light:  hot side as source, common side as return.

The thing here is that the small railroad is not the same.  The reason for this is that in the small railroad's tracks, we have the electrical age hazard known as the extended conductive metallic surface.  So, it has been mandated that the toy train transformer shall be an isolation transformer.  That means that no wire of the output to the track, track signals, switches, or accessories is connected to the 120v neutral at the toy isolation transformer (the 120v neutral, "identified" as white or grey, or by having a "fin" on its insulation, is connected to the power system ground, which is most commonly the incoming service neutral.  (There are also delta systems, but these can be awkward as service drops in residential areas, and I'd rather not get into these weeds.)

The other problem associated with the 120v neutral is the transient voltages which it may carry from time to time.  In my experience (limited) pulses just shy of 400v are most common, and associated with nearby electronic drives for electric motors running off either 460v or 277v.  You can rent a widget which will go through a whole roll of narrow paper tape printing out the voltage of each peak.  But unless you live above a car-wash, this is probably over cautious.  The postwar ZW had 1-mil shellac varnish insulation, and Lionel was proud of pointing out that this would withstand 1000v on the input (not to mention 105-C).  But not to worry--- the UL has raised this to 2000v for the power-limited Class 2 under 30v transformers, and already extended the reduction on these from 32v to 30v and to 660 VA to the toy transformer (UL697).  The really critical wire in many locomotives is that which may run as a connection between single roller pickups under motorized trucks at each end of the engine.  Lionel learned this lesson in 1954 with its Trainmaster H-66, quietly asking its service stations to replace the usual thermoplastic interconnection wire with a heavier wire having rubber insulation, from the triplex switch control harness.  The rubber insulation would char, rather than melt into one's skin.

The bottom line is that with the isolated wiring carrying alternating current, when the short recurs (per the string of sparks) each time, it is driven primarily by the collapse of the magnetic field in the 180w brick.  In full size practice, it is assumed that the motor load will contribute another 25%.  In this case the combined weight of 4 Pullmor motors could be compared to that of the 180w brick to estimate the added effect.  To protect say a TIU, from the overvoltage pulses, all its input  and output wires wires should have TVS protection, with say four or five times as much protection facing the the brick (transformer).  The phase that the voltage is in (positive or negative)  will determine the direction from which the voltage peak will come.  That direction will determine from what collapsing magnetic field it is coming; whether it begins at the beginning crossing of the current zero  (the so-called zero-sequence fault) will determine the relative effect at the source; the ohms impedance along the direction of wave travel will determine the reduction in voltage level along the path.  Worth noting that the ZW could produce a transient voltage of 39v, due to significant leakage reactance.  That reactance is significantly reduced in the 180w brick, I would say by a factor of 2.  The speed of electrical impulse over typical Lionel/MTH layout track and wiring is about 0.6 to 0.7 time the speed of light.  At 60 cycles, the source effect can be assumed to be simultaneous over the layout, provided the source effect is not steep-fronted.  Well, I think that covers it--- my head is hurting, a sure sign that it is mostly covered.  Well, a last reminder that to a certain extent, many of these TVS are sacrificial, and will eventually fail at the worst time. 

As for the current wave, that damages by its heating effect, and duration.  But that involves breakers, fuses, and other devices, and those are under discussion in another current thread here.

I think the Gunrunner has already mentioned the singular importance of putting a TVS on both wires of a pair whenever you decide one wire needs one. 

---Frank

Questions, (1) Does the TVS have a directional (Polarity) requirement or does it not ? (2) What test is used to check a TVS will function or has expired ? Resistance, Continuity ?

Thanks Dean

(1) The 1500W 33.3V TVS at Digikey has no polarity, it's a bi-polar TVS device.

(2) There is no definitive test for the TVS being open that can be easily done in circuit.  The testing involves increasing a current limited voltage pulse past the breakdown voltage and observing the "knee" in response of the TVS clipping the high voltage.  Obviously, the test for a shorted TVS (the most common failure mode) is easy, it's a short!

@gunrunnerjohn Good explanation. For anyone without an oscilliscope, can you think of any other way to effectively test if a TVS zener diode is conducting current when it's rated clamping voltage is exceeded?

There is a simple 'TVS Tester' in the 'Electronics Projects Developed on the OGR Forum by OGR Forum Members' post.

Link below in my signature line, see Project #4.

Follow up to my Question #2, scenario, a "Derailment" causes a fault (short circuit) , Circuit Breaker Trips, clear fault, clear short circuit (Derailment), reset circuit breaker, breaker trips repeatedly. Go to Track Block TVS where derailment has occurred, disconnect one TVS lead or both check continuity of TVS, with continuity TVS has collapsed "Shorted"and performed its function, replace TVS as it only has "one time" life function. Correct ?

So theoretically a "New" TVS would read open no continuity ? With out a scope is there a way to check that a "New" TVS is going to function, as we all know "QC" maybe a real variable.

@SteveH posted:

@gunrunnerjohn Good explanation. For anyone without an oscilliscope, can you think of any other way to effectively test if a TVS zener diode is conducting current when it's rated clamping voltage is exceeded?

The way they're tested in an industry setting, say for certification of the component for use in a critical system, is basically what I said.  The part is placed in a test stand, and a current limited pulse supply is connected across it.  The resultant voltage waveform is analyzed to see if the TVS device meets it's specifications.  I believe I've seen some mention for bipolar devices of using AC so you can test both polarities at a time.

A simple bench test can be done using a high voltage current limited bench supply.  Say you have a diode with a 36V breakdown rating, and you have a 50VDC power supply with constant current capability handy (doesn't everyone?).

Connect the TVS directly across the power supply feeds, monitor the current through the TVS device as you slowly increase the voltage.  At some point, usually somewhat above the breakdown rating, the current will start to go up fairly quickly with voltage.  You can check that "knee" in response against the data sheet for the TVS device.

Don't forget to do the test again with the TVS device flipped around, it's a bi-polar device and you need to test both polarities.

@rtr12 Thank you for your work compiling the lists of many of great resources available on this forum and for mentioning the TVS tester design John created.  Your lists are a great resource.

@gunrunnerjohn thank you for the spelling out the additional testing method.  Would something like an 120V AC Variac connected to the input side of a FW bridge rectifier with it's pulsed DC output feeding through a current limiting resistor wired in series with the TVS, substituted in place of the smoothed waveform variable DC power supply you suggested, be an acceptable method for measuring the current knee?

I haven't tried the variac, but that should work.  The reason that the real testing labs use a specific pulse generator is they're also testing the response time of the TVS.  To just determine if the TVS is working (somewhat), that would be sufficient.

Although the predominant failure mode is shorted, they can also fail open.  A TVS can also slowly degrade over time and not meet it's specifications.   That's a lot harder to accurately test for without more sophisticated test methods.

@gunrunnerjohn posted:

I haven't tried the variac, but that should work.  The reason that the real testing labs use a specific pulse generator is they're also testing the response time of the TVS.  To just determine if the TVS is working (somewhat), that would be sufficient.

Although the predominant failure mode is shorted, they can also fail open.  A TVS can also slowly degrade over time and not meet it's specifications.   That's a lot harder to accurately test for without more sophisticated test methods.

Before this gets too far afield, I don't know how you would use the pulsed DC or any AC method without an oscilloscope. If you had a meter was truly peak-reading, as distinguished from peak-responding,  you could do it, but those aren't common devices in the hobbyist world.

@PLCProf posted:

Before this gets too far afield, I don't know how you would use the pulsed DC or any AC method without an oscilloscope. If you had a meter was truly peak-reading, as distinguished from peak-responding,  you could do it, but those aren't common devices in the hobbyist world.

Well, that's a reasonable point, I never actually considered using AC for these tests.  Of course, since I find it tons easier to do this kind of stuff with a 'scope, I don't put much thought into doing it without one!

The simple DC test is the one I figured was easy to do, and I'm sure that will do a basic test.  Several years back this issue came up and I tested one using a DC bench supply.  I just repeated that test.  With a 50V HP supply, I set the current limit at 30ma.  Using the 1500W 33.3V TVS at Digikey part, the knee is right at 36 volts, as soon as I ask for more voltage, the current shoots up to 30ma almost instantly.  Flipping the part yields the same result.

BTW, if you leave the 30ma flowing for a spell, you can feel the part slowly warming up.  I doubt I'm any where near it's power dissipation limits, but it is certainly inhibiting any further voltage increase.