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Hi everyone, 

I recently bought my first legacy engine and cannot wait to get it on the track! That being said, I want to make sure I am protecting it's inter-electronics.

I plan on purchasing the legacy control system and a tmcc powerhouse at a later point (one big purchase at a time); for the time being, I'm going to be operating the engine conventionally using a CW-80 from 2005. I plan on getting a TMCC direct lockon to protect from shorts before running the engine. I don't want to take any risk with a $500+ dollar engine!!

Will the lockon work when operating legacy engines conventionally with a CW-80? I figure the answer is yes but wanted to 'ask the experts' in case there's any hidden issues. My main concern is the small leakage current that is present when the transformer is throttled down. I was planning on buying the lockon anyway when I buy the full legacy system so I don't mind fronting the cost now. 

Thank you in advance! 

Last edited by Prr7688
Original Post

The TMCC Lock On is really only suitable for Command operation. Its breaker is controlled by a relay that doesn't put power to the tracks until the applied voltage is greater than about 12-13 volts AC. A better option is DCC Specialties PSX-AC. It allows variable voltage to pass through, its as fast as the Lock On or most modern transformers and bricks plus the trip point is variable by the user. You can set it to maximum current of your particular transformer.

BTW remember breakers and fuses protect the power source, not the load. Its the TVS that protects the load. 


The long and short of it all: A well executed power setup will include TVS diodes and discrete external circuit breakers for all utilized channels and all power sources, classic and modern, from your pre-war alphabet soup, through your classics like the ZW, to your shiny new ZW-L or Z4000.

As for more information on the why TVS diodes and external breakers are needed and such, here is some information I've compiled:

The TVS diode protects against voltage spikes caused by collapsing electromagnetic fields generated by the layout (relays, solenoids, motors, derailments etc).

A short video about those spikes from collapsing magnetic fields (scaled up from our trains, but lays out the scenario in an easy to consume way):


Since the source of these spikes are layout side, you want the protection layout side. Many people put these TVS diodes on the track, others put them on the output terminals of the transformer, and some do both!

With a ZW, it is easy to take 4 TVS diodes, shape them like this so they wrap around the binding posts, (A-U, B-U, C-U, D-U).

The 1.5KE36CA is what seems to be generally recommended around here and good for most any O gauge AC setup.

These diodes are sacrificial. They degrade as they protect against the voltage spikes. Just as your surge protector power strips recommend replacement every so often, same goes for the TVS diodes for similar reasons. TVS diodes can fail silently (no symptoms in operation), or fail spectacularly as a dead short, or fail annoyingly open preventing any power. There isn't a practical way for the layman to test the diodes for all the failure modes, so having a few on each buss and a replacement schedule (every few years) seems to be a reasonable balance.

Most people immediately think about their track and expensive locomotives, but don't forget to protect your accessory buss as well. They also have solenoids, relays, motors, and other sources of the EMF.

Some argue that power systems, like the MTH TIU, already have a TVS diode. That is true, and does offer protection, but it is thought to be better to have the protection closer to the sources, so the closer to the track, the better. Some even make it a practice to install these diodes directly in the locomotive so they don't have to worry about their club or friend's layout having the proper protection.

Most of all, the myth that fast acting circuit breakers (or fast blow fuses), whether included in a modern transformer, or externally are all you need is false - they do nothing to protect the sensitive electronics from these EMF voltage spikes. However, additional circuit breakers should still be considered essential. The main purpose of the transformers circuit breaker is to prevent the transformer from being damaged by an over current situation - this sometimes has the side-effect of also protecting downstream equipment (trains), but again, is not the principle purpose. If you have a huge honkin' tonkin' transformer capable of 15A output, but your train only needs 5 amps, then it should be on its own buss/block with a 5-7 amp breaker. This helps the small wires in your engine or powered rolling stock from carrying 15+ amps and melting in a direct short situation. For an accessory buss with small gauge wire, you definitely do not want 15+ amps going through that in a short situation, so again, an appropriate sized breaker.

Another need for external breakers on each buss/channel is that on classic transformers, not all potential power paths are protected. For example, a ZW with 4 channels, only has the U terminals behind the breaker. A short could happen between any two non-U posts with no internal overload protection.

For circuit breakers (and fuses), there is this misconception that circuit breakers trip the instant they are over their rating. i.e a 10 Amp breaker will trip at 10.01 amps or 10.1 amps, or definitely by 10.5 amps. That is generally not true. Just take a look at a generic breakers specification sheet:

cb trip curve

In this common example, you can see that constant 135% overload (13 Amps on this 10 Amp breaker) could take an hour to trip. ( As such, there are those who seek out magnetic or electronic breakers over traditional thermal ones. However, remember, in a dead short, something like a ZW can pump out 50 Amps minimizing the trip time of even thermal breakers (and also illustrating the need for discrete channel breakers - imagine 50 amps going through a small 24 gauge locomotive or accessory wire for an instant).

Most of all, this advice/recommendation for TVS diodes and discrete external circuit breakers pertains to all power sources, classic and modern, from your pre-war alphabet soup, through your classics like the ZW, to your shiny new ZW-L or Z4000.

If you are curious about the trip times of the classic Lionel transformers, the 5D testing documents have a nice chart to reference:


While thermal breakers have long trip times, electronic breakers actually trip in just over 1/2 a 60 HZ wavelength or about 9 milliseconds. This was verified on an Oscilloscope with both a TMCC Lock On and 180 watt brick. An amplifier monitors the current and opens the circuit at its set point virtually immediately. It doesn't depend on temperature increase to break a contact. 



I have been using a DCC specialties PSX1-AC for a number of years now , its lightning fast and can be configured with jumpers for different amperages and the option of manual reset or automatic when the short is removed. I am not familiar with the diode that is being discussed but am sure there are folks on here much that could explain that as well .  I used it with an old ZW before I got the bricks that run the layout now.  it was about$50 a few years ago. think it was purchased from Tonys train exchange a mail order shop that specializes in all things command control.  Rick

Electric circuit breakers are great, but the ones built into power bricks and transformers are designed to protect the transformer. It is still advisable to have discrete breakers on each circuit/channel appropriately sized for the expected load.

A loose corollary would be like your residential electric panel. Your power supply has a main breaker (100AMP or more service) that could be seen as the built in breaker of your train transformer. However, each circuit in your home has a discrete breaker sized for its intended load (15 AMP). You should do the same on your layout for discrete track blocks, accessories, lighting districts, and whatnot

And it is worth repeating that these breakers do NOTHING to protect against the transient voltage spikes discussed above, so TVS diodes are still a complimentary necessity.

Well, I have to disagree that you need another breaker for the PH180 bricks for powering your track, they have an excellent and very fast electronic circuit breaker.  I want 10 amps available on the power district.

Obviously, if you're running lower power accessories from the transformer, it's a good idea to provide individual circuit protection, but making it a blanket requirement is not valid, at least IMO.

Wow thank you all, this is a lot to consider! I did a project for a utility a few years back involving designing trip times for breakers/reclosers/fuses (such that the fault time lowered as it went downstream from substation) so what everyone is saying makes sense to me. I think I will hold off on buying a separate breaker for now as I hope its not too long until I have a full legacy setup, but I will buy a TVS Diode. 

Side question: do the TMCC powerhouses make any nose (hum or fan)? I cannot stand the whining fan noise that the CW-80 makes

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