Installing a TCS KA3 or KA4 on an LM-3S without a bridge rectifer

[William Brillinger]

Today I have confirmed that you can install a TCS KA3 or KA4 Keep Alive on the LM-3S without a bridge rectifier.

WARNINGS:

• This only applies to RailPro LM3 modules. Older Modules need a Bridge Rectifier to install a Keep Alive

Wire the Keep Alive as follows:

Module 9 Pin Blue (Common Wire) to KA3/4 Blue +12v Wire
Module 6 Pin Yellow (Negative Wire) to KA3/4 Black/White Ground Wire





Cheers!

[Alan]

Bill, we determined this would work back on the original KA thread several months ago. Did you get additional information from Ring or by confirmed are you saying you wired one up and tested it?

[William Brillinger]

Ring has indicated that this is the "correct" way to do it. (if you must, etc. etc.)

And I have now Wired and Tested it.

[Alan]

Ah okay. Essentially the same thing he told me. Recall our discussion about preventing damage due to shorts on the outputs? Wired this way there is no protection for those outputs. The motor outputs are sturdy enough to tolerate a fault but the lighting outputs are not. Hence the if you must, etc. Make certain your lighting wires are done right and no wires rub on anything. Would hate to see you lose a module.

[CPRail]

Does installing with a bridge rectifier provide the protection for lights, etc.? I've done all mine so far with bridge rectifiers, but this looks like a much easier install. One less piece to fit it.

Aside from having one less piece, and the potential to void the warranty, what are the pros and cons of with or without a bridge rectifier.

Please keep the explanation simple, I'm not an electrical engineer. As it is, I'm always amazed that I can get things to work on the layout. KAs and Bridge Rectifiers are at the top end of my limits...most of these components are filed under Magic Boxes. As long as you connect it EXACTLY as instructed AND the Magic Smoke stays in them, they work.

[Alan]

Ha! Proof that all electronics are indeed smoke powered. Why else would they quit working when they lose their smoke?

Installing with a bridge rectifier does preserve the protection of the light outputs. Not because of the bridge itself but rather the fact the bridge and KA are attached upstream of the built-in module protection. Installing a KA using the blue wire and ground pin places the KA downstream of the built-in protection thus rendering the protection useless. It is only a problem if a short circuit occurs on one of the light outputs. Otherwise, it is fine.

You hit the two main differences - piece count and warranty. Although, as has been mentioned before, there is no way Ring could determine that a KA killed the module so the warranty issue is more one of your own personal morals. I once bought a new Walthers turnout only to open the package and discover a used piece inside. Clearly not every model railroader takes the high road.

Kevin has conformed that a bridge rectifier installation may require a touch more throttle to get moving than a non-bridge installation. Given that every layout has a different level of electrical integrity you may or may not notice this effect. There should be a slight reduction in top speed as well however if you do experience a difference it will be very small.

There are some very small bridge rectifiers available if space is a problem.

[jimw]

Alan, I am puzzled as to why the addition of a bridge rectifier to the light (blue) common would result in a voltage drop to the motor circuit?  Was Kevin's installation different from Bill's diagram above?

Please excuse my ignorance if this is a stupid question. My brain circuits for electronics are obsolescent, since they were wired 40 years ago!

[Alan]

The rectifier is attached to the red and black wires (power from the rails).

[jimw]

Thank you, Alan. That clarifies my misperception.  Am I correct in assuming that the capacitor (KA) as diagrammed will only provide relief from light flicker, but not sound or motor?

Jim

[Alan]

Definitely will stop light flicker. For motor and sound we can't say for sure without knowing the internal circuitry of a LM3. It may, it may not. Bill and Kevin's experience can shed some light.

The rectifier/cap on red black scheme will provide temporary power to motor, sound, and lights.

[William Brillinger]

Installing the KA on the Blue (Com) and Yellow (Neg) lines provides constant internal power to the LM and eliminates sound and light drop outs. In my experience, a locomotive will remain in motion for up to 15 seconds with this arrangement. Small power disruptions are a thing of the past with KA's properly installed using either method.

Here is the wiring diagram for installing a KA with a Bridge Rectifier:

[jimw]

Bill: That makes sense to me, with the KA capacitor protected (from track polarity reversals) by the bridge rectifier. Your new diagram shows a LM-2, but I presume a LM-3 (or any track-powered device) could be wired in this manner.    Since the bridge rectifier in this schematic handles the full locomotive current, I expect it will require a higher current rating than those used in light-only circuits.  I would appreciate any current information you could provide on these specifications, as I am considering this approach for my installations (and that "Magic Smoke" is SO expensive!)

Alan: Does this configuration mitigate the concern about shorts you referenced in your May 12 comment?

Jim

[Alan]

Alan: Does this configuration mitigate the concern about shorts you referenced in your May 12 comment?

Jim

Yes.

[Bill]

I believe the bridge rectifier method might prevent using it with a dcc controller.  Anyone checked this out?

[Alan]

I believe the bridge rectifier method might prevent using it with a dcc controller.  Anyone checked this out?

Shouldn't. RP doesn't get its control signal from the rails. If you open up a LM you will see the very first thing the red and black wires go to is a bridge rectifier (4 SMD diodes). Adding an external bridge for KA creates two bridges in series. DCC signals can't pass through a rectifier so a LM can't receive signals via track.

As I understand it, the CI-1 is radio broadcasting the DCC signal to the LM.