No, it's not particularly useful for older trains, which don't have anything that needs protection from the inductive voltage spikes that seem to plague the newer stuff.
I've been doing some research on the way TVSs and MOVs are specified. I have found that the "10/1000 µs" power rating is the peak power calculated assuming constant clamping voltage and a particular current waveform. This is something called the "double-exponential" function. It's a particular pulse with an approximate 10 microsecond rise time and an exponential decay to half the peak in 1 millisecond. It is described in Semtech AN-9608, "TVS Diode Application Note", http://www.semtech.com/images/datasheet/transient_immunity_standards_bellcore_tr-nwt-001089_and_fcc_part_68.pdf . It is also Bellcore TR-NWT-001089, and U.S. Department of Agriculture, Rural Utilities Service, Rural Electrification Administration Bulletin 345-50, PE-60, "RUS Specification for Trunk Carrier Systems".
I worked out this function and integrated it to find that it has the same area as a 1426-microsecond rectangular pulse with the same peak value. (I found a reference to it that gave this value as 1500 microseconds.) So one of these pulses with a peak power of 1500 watts, as specified for the 1.5KE TVS, dumps about 2.14 joules into the device. The MOV that we discussed is however specified directly as being able to handle 3.2 joules, delivered in the same way.
I found this relevant article from EDN magazine: http://www.edn.com/archives/1997/041097/08df_05.htm Here are excerpts comparing MOVs and TVSs:
"MOVs gradually degrade with each transient event. Granular interfaces overheat and begin to short, resulting in a gradual decrease in breakdown voltage. Eventually, the MOV fails, and the device achieves a permanent low-impedance state, thus resulting in a loss of protection. MOVs turn on in a few nanoseconds and have high clamping voltages, ranging from approximately 30V to as much as 1.5 kV...."
"TVS diodes are solid-state pn-junction devices. A TVS-diode junction employs a large cross-sectional area so that the diode can conduct high transient current....TVS diodes are clamping devices. When the transient voltage exceeds the circuit’s normal operating voltage, the TVS diode becomes a low-impedance path for the transient current. The device returns to a high-impedance state after the transient threat passes. TVS diodes do not wear out and have no degradation of the electrical parameters, as long as you operate the device within vendor-specified limits."
(I am correcting some errors in a previous post.)
Bob Nelson
I have been reading this thread with interest. My question is this. Is this kind of protection needed or even valuable on the older post war Lionel trains or is is mostly for use on newer equipment with electronic circuit boards in the trains? Thanks.
Terry Thomann Fredericksburg, Virginia That is me on the left. My brother got the train TCA 09-64381
I would just wire them up and forget about them. Or you could do what Ben Franklin did and fly a kite in a thunderstorm. I don't think you need to test them. Bob Nelson has recommended a heavy duty TVS, which should handle almost anything.
Bruce Baker
Taranwanderer, you misunderstand the function of the TVSs. They are to protect agains voltage spikes, not short circuits. When you ran your test, the TVS did exactly what it should have done which is nothing at all. If you generated any voltage spikes, they would have been too small and too quick to discern without some test equipment like an oscilloscope.
I went to Vishay's web site to look at the data sheets for the TVSs. Bob Nelson has been quoting the capacity of the TVSs in watts, which is what the data sheets show. However, usually these devices are specified in joules which is the energy that they can dissipate, and is essentially constant regardless of the pulse width of the voltage spike or the peak voltage. I finally found the power dissipation spec on the last page: This Transient Voltage Suppressor diode has a pulse power rating of 1500 W for 1 ms. This is 1.5 joules. Figure 12 on the data sheet shows the transient thermal impedance which is 0.2 deg C/watt for a 1 msec pulse. 0.2 deg C/watt * 1500 watts is 300 deg C which will toast the TVS.
The data sheet is at http://www.vishay.com/docs/88301/15ke.pdf
The TVSs are going to be able to take a larger voltage spike if they are attached to a heat sink. The heat sink doesn't have to be fancy, just a couple of terminals will work, but leads should be short. Soldering wires onto the TVS leads is NOT a good idea as the wires are not a good heat sink unless they are something like #`14, and even then, they are probably not as good as the terminals on a transformer.
Another problem with all this is that we really don't know what the line voltage spike look like. Bob Nelson's advice about using a 1500 watt TVS is good as this will probably take care of almost anything except for a direct lighting hit. If the lighting hits your neighbor's house, your stuff may survive and it may not survive. If you want belt and suspenders, use a power strip with a surge suppressor in it along with several TVSs sprinkled around.
What you got is better than nothing, but not nearly as good as the ones that I recommend. Yours is not a TVS (transient voltage suppressor) but an MOV (metal-oxide-varistor). It is not as fast and doesn't clamp the voltage as tightly as a TVS.
Here are the data sheets for both devices: http://www.nteinc.com/Web_pgs/MOV.html http://www.vishay.com/docs/88301/15ke.pdf
And here are the Wikipedia articles for both types of devices, which may give you an idea of the differences: http://en.wikipedia.org/wiki/Varistor http://en.wikipedia.org/wiki/Transient_voltage_suppression_diode
"First--you just wire them ACROSS the A and U terminals on my ZW?" Yes, if A is the terminal you are powering the track with. If you're using any of the other terminals also, give each of them its own TVS.
"As in, what you'd do to create a short-circuit?" Yes. "Am I understanding that right?" Yes. "Do they not conduct power across them except in a spike situation to absorb it?" Yes.
"Second--can I just walk in to my local electronic supply store (not Radio Shack, a real electronic supply store) and ask for a bidirectional 36 volt TVS?" You need to specify the power rating also. The one I recommended in all those posts is rated at 1500 watts (for a very short time). "Does that sound realistic?" Yes. It is very popular; so it's likely they have it if they have any.
Marty sent me this by e-mail. I am posting it for the benefit of anyone else who might be interested:
Bob, I understand your answer about using a lower voltage diode for HO since the voltage is usually less but since I have some 36 volt diodes will they still provide spike protection on his layout. Do the 36 volt diodes basically provide protection for circuits up to 36 volts? Thanks again.
Yes, the 36-volt ones will give almost as good protection, since the electronics probably won't be damaged by anything between 16 and 36 volts. The 36-volt TVSs have a minimum breakdown voltage of 34.2, which is the peak voltage of an RMS voltage of 24.2. So they should not be used on circuits with a higher RMS voltage than that, but are okay below it. The HO voltage is just an alternating square wave; so the peak is the same as the RMS. The 16-volt TVS I suggested has a minimum breakdown voltage of 15.2, which should provide a margin in case his voltage is a little higher than 12; but the 36 volt device would simply provide (much) more margin, with a very slight additional risk.
Here is a link to the Vishay data sheet: http://www.vishay.com/docs/88301/15ke.pdf
He should be able to use a lower voltage rating, since his track voltage is probably 12 volts. He will need a bidirectional TVS, because his voltage polarity will change with direction of operation for conventional operation or will alternate rapidly if he is using DCC. With 12-volt power, a 1.5KE16CA might be a good choice.
I had a friend ask me if this type TVS diode should be used on his HO layout? He is using locos with sound, etc. I told him I would think it was a good idea but I would check with the experts? If he is using DC power I would think a bidirectional diode would still be optimum? Thoughts?
Unless a TVS diode fails catastrophically -- apparently a very rare event -- you won't know if one has failed unless you have quite sophisticated test equipment; so my approach is:
1. Use more than one on your layout. At current prices you can buy 'em by the bag; It's unlikely that they wll all fail;
2 Plan to replace them every now and then, such as during once-a-year maintenance if you employ such a thing. For reasons of pure convenience I don't ordinarily solder mine but simply bend them around posts or screws and snug them down.
It is much easier to attach them to the posts on the outside of the transformer case or across track-lockons than than it is to do so inside. You can extend the leads if you have to . Personally I use quite a few barrier strips and often attach the the TVS diodes there. Just remember that they go across (in parallel with) the power/common leads and not in series with them.
Bob has also written that the closer the TVS diode is to the device (s) you are trying to protect the better -- another argument for using more than one per layout.
Don't forget that you need BOTH TVS protection against voltage "spikes" for your table-top electronics AND properly-sized fuses or circuit-breakers to protect the transformer and it's associated wiring from over-currents, i.e., "short-circuits."
You're welcome!
Yes, you can put them inside the case if you like. A TVS is not indestructable, but that "1.5K" in the part number means that it can take 1.5 kilowatts (for a short time). In this application, it is very unlikely they will have to handle enough energy that they will fail. You won't know when they take a bullet for your electronics. The only additional thing that I can think to suggest is what I mentioned above--you could also put them into the locomotives. That way they would still be protected if you run them on someone else's layout, while the TVSs on your layout would protect the other guy's locomotives running on your layout.
You might find the Wikipedia entry an interesting read: http://en.wikipedia.org/wiki/Transient_voltage_suppression_diode
If I understand correctly then, a TVS diode could be soldered to the A and U lugs inside the case of my ZW and they would be completely functional and out of the way. A TVS diode installed in this way would protect any locomotive operating on that circuit? Okay then, a couple of more questions please? Will the TVS diode last forever? Can they be damaged or destoyed and need replacing? Will I ever know if they absorb a spike? Is there anything else I can do to protect my locomotive circuit boards? By the way, for those considering it, I ordered the 1.5KE36CA-E3 diodes from the Mouser web site. They were on backorder for about a week. Great service in my opinion and very reasonable (cheap) prices. Thanks to Bob Nelson for his patience and helpful advice.
You are on the right track! And you can connect them at the transformers or at the track or at any convenient point in-between. They could even go inside your locomotives.
Bidirectional TVSs are necessary because you are using AC voltage, with the polarity reversing 120 times each second. But the same alternation means that there is no polarity to worry about when installing them, as you figured out.
I think I have read every post twice about using TVS diodes. I have ordered them and received them today. I am still confused about where they need to be wired into the track circuit. I use three vintage ZWs on my layout and run trains on six circuits using the AU and DU posts on each of the Zs. It appears to me that all I need to do to put the TVS's in play is to connect them somehow to the A and U or D and U post of each transformer? One end of the diode to the A post and one end to the U post? This would take 6 diodes. The TVS I purchased are bidirectional so I don't even need to worry about which end goes to the post? I guess they could also be connected to the track but I don't see any advantage to this. I don't use connectors as the leads are soldered to the rails. Am I on the right track? No pun intended. Thanks
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