MTH’s new WTIU question from a (so far) non DCS user

The reason for passive mode was basically around 2 different themes:

#1 you had an old, early revision TIU that did not have fuses, and was highly risky for burning out the traces of the board since there are no fuses. Running passive mode would not run  any current through the fixed channels of the TIU- thus bypassing the risk.

#2 you had high current draw requirements so much so that running in passive mode bypassing current around the TIU made sense even with later versions that had fuses.

Just as a note- another weak point of the TIU was the banana jack terminals and the wire to ring terminals inside being loose creating a self destructive meltdown scenario on the terminals when higher current are passed through the TIU.

So- the WTIU would be subject to the same rules or scenarios: You either are running high (7-10Amps of current on a regular basis) and you don't want short circuit currents (12-20Amps) to pass through the TIU or don't want the possible risk of melting the terminals.

Passive mode typically requires inductive chokes put in series with your power supply (transformer) to prevent your transformer from degrading the DCS signal strength when using passive mode.

It's this simple of a concept. DCS signal is a high frequency digital signal riding on the power. In passive mode, your transformer, the TIU output channel, and the track would all be in parallel. The choke is in series at the transformer to choke off the DCS signal from basically being "shorted" by your transformer source. A choke is high impedance (resistance) to high frequency, but as you near DC or a slow 60Hz AC is lower resistance.

Note also, when running passive mode- we often ALSO supply the TIU with aux power in to keep the TIU logic powered regardless of channel status. So I would expect similar usage and external aux power when using the WTIU- because you want to keep the wireless portion booted and running constantly- rather than waiting time after you first turn on track power.

@Vernon Barry

Thanks for the speedy and detailed explanation!

@Vernon Barry posted:

So- the WTIU would be subject to the same rules or scenarios: You either are running high (7-10Amps of current on a regular basis) and you don't want short circuit currents (12-20Amps) to pass through the TIU or don't want the possible risk of melting the terminals.

Passive mode typically requires inductive chokes put in series with your power supply (transformer) to prevent your transformer from degrading the DCS signal strength when using passive mode.

The jury is still out as to if the WTIU will work in passive mode.  I don't know if anyone has even broached that question yet.

@Vernon Barry posted:

The reason for passive mode was basically around 2 different themes:

#1 you had an old, early revision TIU that did not have fuses, and was highly risky for burning out the traces of the board since there are no fuses. Running passive mode would not run  any current through the fixed channels of the TIU- thus bypassing the risk.

#2 you had high current draw requirements so much so that running in passive mode bypassing current around the TIU made sense even with later versions that had fuses.

Just as a note- another weak point of the TIU was the banana jack terminals and the wire to ring terminals inside being loose creating a self destructive meltdown scenario on the terminals when higher current are passed through the TIU.

So- the WTIU would be subject to the same rules or scenarios: You either are running high (7-10Amps of current on a regular basis) and you don't want short circuit currents (12-20Amps) to pass through the TIU or don't want the possible risk of melting the terminals.

Passive mode typically requires inductive chokes put in series with your power supply (transformer) to prevent your transformer from degrading the DCS signal strength when using passive mode.

It's this simple of a concept. DCS signal is a high frequency digital signal riding on the power. In passive mode, your transformer, the TIU output channel, and the track would all be in parallel. The choke is in series at the transformer to choke off the DCS signal from basically being "shorted" by your transformer source. A choke is high impedance (resistance) to high frequency, but as you near DC or a slow 60Hz AC is lower resistance.

Note also, when running passive mode- we often ALSO supply the TIU with aux power in to keep the TIU logic powered regardless of channel status. So I would expect similar usage and external aux power when using the WTIU- because you want to keep the wireless portion booted and running constantly- rather than waiting time after you first turn on track power.

Thanks for posting the clear schematic.

From the dept. of continuing education...

I re-read the section of Barry's book regarding passive mode, and have some questions. 

1) The book says that the disadvantages of passive mode usually outweigh the benefits, and that the main reason is to prevent over 12 amps from passing through the TIU (p. 119-120 in 3rd edition).  I looked at my Z4000 and the MTH brick for the Z1000; the largest breaker on any of them is 12 amps.  So, I'm wondering - how could one put more than 12 amps through the TIU if using a power supply from MTH (or any supply with a 12 amp or less fast breaker)?

2) There's no mention of chokes in the book, so I did some searching regarding function and available items.  The function makes sense to me, but I'm wondering about the size.  Seems like the choke should be capable of handling the expected maximum load, so it looks like a 3 to 8amp toroid coil would fit the bill.  One example from Amazon has a description of "uxcell a13071500ux0187 10 Piece Toroid Core Inductor Choke Wire Wind Wound 22uH 21mOhm 3 Amp, Coil" (the 3amp was the only one with the part number in the description).  Do you agree about needing to handle 3-8 amps?  (All of the chokes were under $2 each)

3) I never run more than 2 or 3 trains thru my TIU.  I had considered switching to passive mode to protect the TIU, but after re-reading the book I'm not sure that it's of much benefit.  Do you think passive mode is worthwhile when running no more than 3 trains?

Thanks in advance.

1. If I recall correctly the original main disadvantage of passive mode was a weaker signal the discovery of a 22uh choke has changed this

2. There are a lot of specifications to a choke. The inductance which we know that we want it around 22uh. The maximum current witch is what the choke is capable of handling, you would want this to be the same as your maximum output from the power supply so it doesn’t burn up during a short circuit event. The other spec. Is max saturation current which is the amperage when the choke is no longer as effective.

I wired my clubs new layout and we have had issues in the past over heating TIUs and this new layout the decided they wanted to use 360W PowerMasters that would be capable of 20A to the track so the TIU is in passive mode with a 28A choke. I made a board specifically to hold this choke

3. That’s something you’ll have to answer for your situation, I think Vernon Berry’s two cases at the top of this thread still ring true.

@Mallard4468 posted:

Thanks for posting the clear schematic.

From the dept. of continuing education...

I re-read the section of Barry's book regarding passive mode, and have some questions.

1) The book says that the disadvantages of passive mode usually outweigh the benefits, and that the main reason is to prevent over 12 amps from passing through the TIU (p. 119-120 in 3rd edition).  I looked at my Z4000 and the MTH brick for the Z1000; the largest breaker on any of them is 12 amps.  So, I'm wondering - how could one put more than 12 amps through the TIU if using a power supply from MTH (or any supply with a 12 amp or less fast breaker)?

Because, the transformer can deliver higher than 12A, hence why they limited to a 12A breaker, Further, it's a thermal breaker, so for how long and how much higher will it pass higher than 12A BEFORE it trips? Besides that, even when circuits are rated for higher amperage- as you cross a 10A threshold there is so much power that it will find a weak link- a less than ideal solder joint, a less than ideal contact or mechanical connection. A KNOWN weakness and still present in the WTIU is the ring terminal wiring to the binding posts is STILL a physical connection. When not if, you pass high current through those and one heats up and softens the plastic- that reduces the tension, resistance increases, now gets hotter, it both oxidizes and gets looser- even more resistance- total failure.

2) There's no mention of chokes in the book, so I did some searching regarding function and available items.  The function makes sense to me, but I'm wondering about the size.  Seems like the choke should be capable of handling the expected maximum load, so it looks like a 3 to 8amp toroid coil would fit the bill.  One example from Amazon has a description of "uxcell a13071500ux0187 10 Piece Toroid Core Inductor Choke Wire Wind Wound 22uH 21mOhm 3 Amp, Coil" (the 3amp was the only one with the part number in the description).  Do you agree about needing to handle 3-8 amps?  (All of the chokes were under $2 each)

The choke needs the current rating of your power source + some margin. So generally, I would find a 3A rated choke would not be the right size.

3) I never run more than 2 or 3 trains thru my TIU.  I had considered switching to passive mode to protect the TIU, but after re-reading the book I'm not sure that it's of much benefit.  Do you think passive mode is worthwhile when running no more than 3 trains?

This is somewhat of a personal decision and highly suggest you do a risk matrix analysis:

#1 What is the cost of even one failure- to replace or pay for repair of the WTIU? These are roughly $400 street price.

#2 what is the likelyhood of failure? Remember there are factors to this so it's not just one answer. How much current are we running? What about peak current situations? Can we be sure we never get the loose binding post failure- and that failure is further worsened by the current? Take into account the number of terminal per track- so 4 total in normal pass through mode- a total of 4x4 or 16 total failure points and any one failure is bad.

#3 What does passive mode cost? Some wiring and some right sized inductors. I have been using and recommending these https://www.digikey.com/en/pro...P1770TA-220M/5429725 however, that one is out of stock. This search is for 12A rated versions https://www.digikey.com/en/pro...pWqaRoQIAmstFzLy2yoA So back to the question- about maybe $5-6 per channel is what passive mode "costs".

#4 What is the disadvantage frequently brought up when using passive mode? That the TIU no longer kills output power with an emergency stop button request. My counter argument is- if you actually use emergency stop function- it required powering down the remote, powering down the logic of the TIU to reset. Now that the wireless is integrated, I don't know yet (just saw my order from TW for the new WTIU attempt on an old card) works the same way, but bottom line, it felt like emergency stop was a little too "emergency" and a lot less just stop and let me try again.

Thanks in advance.

Again, the catch22 right now, I cannot say if passive mode works on the new WTIU, if there are any other gotchas or knowns, I don't know how they implemented E-stop in the new version and firmware.

Edit- let me add another perspective. My other hobby was/is 3D printing. One of the known failures and something that did and still does plague that industry is failing/melting/burning electrical contacts often related to the heated bed. Heated beds in the past when powered off the same DC supply were typically well over 100Watts and often near 200 Watts. Common voltages were 12V to 24V DC- so honestly- within the similar ballpark of voltages, current, and power of some O scale trains and power systems. As an example the typically 40-80W heater of the extruder melting the plastic did not burn terminals and typically is under 5A of current- even less on 24V. However, typical bed currents were much higher than 5A, often 8A to much more. Even when using larger terminals and connectors- often well within rated usage - still failed!!! They would get hot, soften and oxidize the contacts, then also melt the plastic, get hotter because of higher resistance and then in some cases even result in a fire.

What I find is also true in this hobby. Below the 5A threshold, it's not that you cannot have a failure, it's that the likely hood greatly decreases. As you cross over 5A and the closer you get to 10A, the likelyhood of finding a weak link, something that goes high resistance, and then self destructs is just that much higher risk.

And the big thing is- do you want that happening inside a $400 device, or do you want it external in some $4-$5 components externally?