Keep Alive installation w/ Railpro

[Alan]

The reason for the bridge rectifier is because we do not have access to the output side of an LM internal bridge. DCC decoders offer access to the output side of their internal bridge and that is where KAs are designed to be connected. Ring is moving in that direction, although not there yet, with the pinout rearrangement on the new LM3.

Any KA, regardless of manufacturer, if connected ahead of a module will require a bridge rectifier.

DCC AC or unknown DC polarity voltage is derived from the rails and fed to the module (decoder). Inside the module the power moves thusly: --> Bridge rectifier --_*²--> Filter caps --> Regulator --> Stabilization caps --_*¹--> PWM controller --> Output to motor. The LM3 pin arrangement allows us to tap into where I marked as _*¹. DCC decoders and what we need in an LM is to be able to tap into where I marked _*².

It is literally nothing more than an additional 'wire' inside the module and a pin in the connector. Simple, right? Not so. It means redesigning the LM PCB, something that would be expensive for Ring.

We are not waiting on a Ring KA per se. We are waiting on Ring to make available connection to the output side of the internal LM bridge. With that, we could use any KA we wish. Should that occur then, Jacob, your KAs will still work fine. You will simply discard the bridge rectifier and attach you KA to the new LM wires.

[Alan]

A keep-alive for RailPro:

[Morebassman]

A video of this install would be great for the electricaly challenged! That's me

[TwinStar]

Here is my BLI E8. LM-2S, TCS KA-3, bridge rectifier, and 2 iPhone 6S speakers.

[nodcc4me]

Lots of room in there. 

[Alan]

I see you guys are all using big flat pack (GB case style) rectifiers. If you are cramped for space there are much smaller rectifiers available. For example, here is a SMD unit that would fit anywhere: http://www.mouser.com/ProductDetail/Bourns/CD-HD201/?qs=sGAEpiMZZMtQ8nqTKtFS%2fLRuoW8XJyK4EtCzdLtg3ZgpcRnkNzl%252bKw%3d%3d. Just solder wires to the SMD pads. Or you might consider the round RC case style units which are typically more compact than GB style.

Another consideration is the forward voltage drop. Typically, it is 1.4V for a common bridge rectifier. Units with lower drop are readily available. The rectifier in my link above only has a 750mV drop. For use with an LM, the lower the forward voltage the better. Lower forward voltages will reduce the, what Kevin calls additional oomph to get started, effect.

You can filter by both forward voltage and case style on most electronics suppliers web sites. Here is the Mouser filter:

[KPack]

...what Kevin calls additional oomph to get started...

That's a scientific term right there.  Perhaps I should trademark that?

Thanks for the link to the SMD rectifier.  I had wondered if there was something like that out there.  I'm all about SMD components....ever since I made the switch to SMD resistors and I realized how much space it free'd up.  If I was really organized I could etch a small board and solder on all the resistors I need and the bridge recitifier, then just solder the wires to the board.  Much cleaner.  But etching is nasty stuff and takes a good amount of work from what I understand.

-Kevin

[Alan]

But etching is nasty stuff and takes a good amount of work from what I understand.

Not true. Very easy to do. Check out this post: http://www.lkorailroad.com/control-panels-part-vii/ You see I use an old lamp, kitty litter pans, and a picture frame. Not exactly hi-tech but works great.

You may be thinking about the old negative resist process. Yeah, it was a PIA. Required an expensive UV bulb and was very finicky. In my younger days I made more junk boards than I did good boards using the negative resist process. Then positive resist came on the market. What a godsend. Made circuit board creation Fisher-Price simple. MG Chemicals is the dominant US supplier of positive resist materials: http://www.mgchemicals.com/products/prototyping-and-circuit-repair/presensitized-boards/positive-presensitized-600-series

The most challenging part of the process is not the etching of the boards. Etching is easy. The possibly challenging part is creating the circuit artwork. But for the relatively simple circuits needed for loco installation that shouldn't be a problem. The key is to draw with vector art, not raster art. I use Adobe Illustrator however it is an expensive program if you don't already own it. Inkscape is a free vector art program that would work great for circuit art. You could even use the Draw objects in MS Office PowerPoint if you wanted to. They are vector.

Print the art at 1:1 on an inkjet transparency and you are ready to go. 25 minutes later you will have a ready-for-solder circuit board.

Here are some examples of homemade circuit boards used on my railroad - power district circuit breakers and occupancy detectors.

[Alan]

Swapped emails with Tim Ring today. It is confirmed you should NOT connect a keep-alive to the ground pin and the blue wire of a LM3. Our assumptions were correct. The blue wire is south of the current limiting circuitry of the LM meaning an accidental short of an output could lead to the keep-alive blowing a huge wad of current through the LM. Not good.

The keep-alives you guys are building is the way to go with RP. Or, clean your track. 

[melarson]

It is confirmed you should NOT connect a keep-alive to the ground pin and the blue wire of a LM3.

I have to say, this makes me wonder why the new "negative" pin on the LM-3 is there?  If not for keep-alives (and I assume also batteries), then what?  It is astonishing to me that Tim would make such a connection available that is apparently so dangerous if miss-used.  Did he say what the purpose of the negative lead is supposed to be?

Michael

[G8B4Life]

I have to say, this makes me wonder why the new "negative" pin on the LM-3 is there?

I have to say I wonder as well.  Given Rings statement on the LM-3 

A 'Negative' terminal is better for battery powered connection and will be more flexible to add a keep alive product.

why would you only go half way with the redesign of the LM board needed to bring these connections out to the user? Maybe Ring has some wizard circuitry up his sleeve that'll allow his KA's to be attached to the blue and negative wires.

Speaking of circuitry, Alan I'm constantly amazed with the homemade circuitry work you post on your blog and now here. Inspirational, helpful and I must say better "fire under the backside" tutorials than others I've seen out there.

- Tim

[Alan]

Tim did not say what is the purpose of the negative pin although to be fair I did not specifically ask. His answers were the typical, vague, non-technical responses I have come to expect from him. No warranty, risk of fire, blah, blah, blah. The one nugget of information he did supply is that there is no LM over-current protection in a blue wire / ground pin capacitance application. Since that was the key piece of information I was seeking I stopped there. I make it a habit not to post email conversations publicly online but you aren't missing much. That really was the only juicy answer he provided. He did say he is aware some people are connecting KAs in this manner but quickly followed with another bout of no warranty, at your own risk, etc.

When you think about it the answer could not have been any different. The ground pin is immaterial. The root problem is huge capacitance on the V_cc rail at the load. Absolutely the worst place to put it in any electrical circuit.

One good use I can envision for the ground pin is to add a small non-lethal amount of capacitance. Just enough to keep the LEDs lit and the sound system from resetting when momentary loss of rail contact occurs but not so much as to fry the output transistors from a short circuit. Much like what other DCC sound decoders do with their external capacitor. This wouldn't do anything for the motor function but, as I understand it, momentary sound resetting is a common annoyance for some. It may fix that problem.

Tim isn't going to reveal granular design details. Nor should he. He has a business to protect. We can only speculate and deduce from basic electronics facts. We suspected, and Tim confirmed, the blue wire / ground pin is not a safe place to connect a motor KA. All this exercise did was confirm you guys are doing it the right way with your homemade KA circuits. At least on LM1, LM2, and LM3 units. Who knows what is in store for LM4s.

[Alan]

Maybe Ring has some wizard circuitry up his sleeve that'll allow his KA's to be attached to the blue and negative wires.

KA with built in current regulation - done.

Mark Gurries lists regulated/not-regulated on the web page Bill linked to earlier however, regulated in the DCC KA world means voltage regulation as noted in Mark's answer to question #4. We need current regulation. If I were a betting man my chips are on a RP KA product that has built in current regulation. Then blue wire/ground pin makes total sense. The addition of the ground pin may be laying the groundwork for such a device.

It is worthy to note our entire discussion revolves around protecting your LM. If you are confident you can install in such a manner that a short circuit will never occur inside the locomotive then putting a store-bought KA on the blue wire/ground pin will work. Essentially, a $70 gamble on your part. Tim Ring on the other hand knows people will make mistakes and the last thing he wants is more warranty costs or bad public image by not honoring warranty. When he receives a returned dead module he has no way of knowing that a KA killed the module. Hence, he is doing the logical thing by saying don't attach a KA.

[William Brillinger]

@Alan

I just want to take a moment and publicly thank you Alan, for your invaluable contributions to this forum.

I hope I speak for everyone when I say I really appreciate your time and input here.

There are so many great people here and I am thankful for every one of them, but your efforts stand out as above and beyond.

Thank you.

[Alan]

I hope I speak for everyone when I say I really appreciate your time and input here.

Thanks Bill. It is fun playing RP detective trying to piece together the clues! They are just clues however. Being thrown into the conversation for little more than conversation's sake. If the thread helps someone build a better RP KA for themselves then so much the better. Glad to be a part of it.

Odd isn't it, for someone with no need whatsoever for KAs to get so engrossed in the subject? Clean your track dagnabbit!