Mixing Ma Digital and analog

Back when I operated my analog Marklin gear, I managed to mostly stay away from sectioning the track by using the catenary to feed one loco
while operating the other off of the track. It was also quite cool because if I was only running the one electric loco, I could set the track voltage to a suitable level to always keep the lights in the passenger cars on regardless of the loco power (since the cars got their lights power from the rails). Of course, it looked a bit weird when the lights in the passenger cars dimmed because I was modulating the power to a second loco that was running on the rails, but that's a different matter. I think I can safely say I got my start as an EE, setting up the circuitry for that train layout.
Now, fast-forward to the present. The old gear is getting dusted off and missing pieces are being replaced and digital equipment is pervasive. It's everywhere, even in the "grab boxes" of special deals. But if I want to probe into the world of digital train operation, I still have my old analog stock to consider. I know that there are conversion kits to make them all digital but that's not a path I'm ready to follow. At least not yet. So I'm wondering if I can get analog and digital to safely co-exist on the same track. Obviously, I can resort to rail sectioning but that's sub-optimal. I was thinking along my old trains of thought and figured maybe I could set up a digital feed on the rails and run analog on the catenary. That would have the side benefit of the old passenger cars' lights getting fed all the time.
I ran this idea by a guy in one store but he wasn't too happy about it. He'd much rather convert my old loco to digital (which, of course, is where the money was in it for him). He said he believed the analog train would interfere with the digital gear, which is possible, I suppose, but not something I'd really accept his word for just like that.
The way I see it, there would be two electric circuits sharing a common earth (the rails). One circuit would be the digital mix of constant-voltage AC with overlaid signalling in the center rail and the other would be the analog varying voltage in the catenary. Now, it's possible that the resistive losses in the rails may cause some (moderate) voltage variations due to the extra current from the analog loco, but that can be reduced by proper wiring. I don't think I have to worry about ground loops (this is hardly HiFi audio). The only major worry would be transients from the analog loco (like if there's a short break in catenary contact so there's a spark; happens now and then) doing nasty stuff to the digital equipment, but I kind of think they have to be prepared for that anyways, so isn't that a lot of what those special rails for connecting the cabling are supposed to do?
A lot of rambling, I know. I just wanted to run though my arguments and hear if there's anything I've overlooked. Thoughts or experiences, anybody?
/Tirsten
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
<<snip>>

Short answer: No.
You're likely to let the smoke out of the analog power supply. It'll be far safer to go all digital.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Joe Ellis wrote:

Why?
If the only thing they share is the earth (rails) and the analog feeds catenary only, how will I smoke it? I'd be more concerned about transients from the analog upsetting something in the digital controller!
Marklin's power supplies (transformers) are pretty robust. If they overheat, a thermal switch kicks in and shuts them down (temporarily).
Now, in all fairness, I think your statement has great value in any scenario where a 2-rail feed is involved, as you then have to keep careful track of polarity so that you don't accidentally put a loco on the "wrong way" and somehow create a bridge between catenary and live rail. Especially if you back-feed high voltage from the digital side into an analog DC supply. That's probably instant death to many DC supplies! So I'll assume you're thinking in terms of DC on a 2-rail system. But I'm asking about Marklin's AC 3-rail system, which is a different ballgame alltogether.
/Torsten
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Then they're not "co-existing" on the same track, are they? You actually have separate circuits. I'd still be worried about what might happen as a result of a derailment, though. You could have some very expensive results for trying to save a little money.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Joe Ellis wrote:

I rambled too much so the message was hidden in all the words :-)
Sorry about that.
/Torsten
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Joe Ellis wrote:

In 46 years of modelling I've never seen the smoke come out of a Maerklin analogue power supply - the current limiting cutouts eventually fail (safe) if they're treated harshly enough over exceptionally long periods and then the voltage tap wiper gets bent out of shape when amateurs take the top off.
Regards, Greg.P.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
The two systems should coexist with no problems if you only tie the power at one point, the common rails as you have done before. I will note that the DCC systems provide power that doesn't change for the lights so you may want to completely convert to that system and then you will only need ot run with the center studs for everything and leave the catenary for analog locos.
-- Why do penguins walk so far to get to their nesting grounds?
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Torsten Lif wrote:

Once you start adding operational signal blocks you're back to rail sectioning - a single block Digital railway only works at the 'trainset' level.

That is quite workable - so long as your old locos are all overhead types.

There should be no inerference between overhead analogue and center rail Digital. The only real linkage might be during a derailment, and that can be catered for by placing a car tail light bulb with a wattage rating slightly higher than the transformer output in series with the analogue controller output.

Throw away the special rails - or at least snip out the suppression components! Digital won't work with them as they distort the Digital signal!!!

Go for it - with the provisos of removing the radio/TV suppression components from those feeder sections and adding the resistive load (car tail lamp bulb) to the analogue circuit.

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Greg Procter wrote:

Let me see, if I understand you correctly, you're saying I take a 35W 12V car lightbulb and hook it in series with my analog feed to the catenary. Hmmm. As long as the loco operates normally, current will be far less than what will even begin to warm up that filament so it will have fairly low resistance. Should work. This is to prevent the analog supply from feeding too much current "backwards" into the digital?

I was referring to the "new" special rails that come with the digital kits. Something made me think they contained some kind of electronic componentry, but that may well be an error on my side.

Cool! Thanks.

(Who can't even spell his own name!)
/Torsten
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Torsten Lif wrote:

It can potentially work in any of three ways: - Digital into the analogue supply. - analogue ito the Digital supply. - analogue and Digital forming a loop.
As it is relatively slow to react, it probably won't restrict instant current flow through the Digital electronics, but the Digital amplifier should already include short term protection.

Ok, but as you were talking of long stored Ma I guessed there would be some "interference suppression feeder tracks" in the box.

I don't know - I make my own feeder tracks using the basic track sections.

A change is as good as a holiday!
Regards, Greg.P.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Greg Procter wrote:

I never actually had any of those. Living way out in the country, we didn't care much about radio interference.
No, I was thinking in terms of the new starter kits and (possibly) getting a set with the new C-rails for my kid to play with.

Good answers as always. Thanks, Mr. P!
/Torsten
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here. All logos and trade names are the property of their respective owners.