Tender back-up light confusion

[spookshow]

I recently purchased a More/Lik brass SP 4-8-4 and while putting it through its paces I've run into a bit of a head-scratcher...

Unlike most brass steamers, this one actually has a working/directional back-up light in the tender. Although how it actually works has me completely stumped. The electrical pick-up scheme is seemingly very traditional/old-school - IE, the right-side drivers pick up one rail and the left-side tender wheels pick up the other rail (with current from the tender transferred to the locomotive by way of a single stiff wire on the drawbar).

So, for that back-up light to work I figured that all the tender wheels must be live and that the light would work regardless of whether or not the tender was actually hooked up to the engine. But no, that is not the case. It only works when the two are hooked up.

Basically, it seems like the tender is getting its right-rail current from the engine (through the drawbar). But if that's the case, where is the engine getting left-rail current?? I mean, WTH? I must be missing something obvious, but I'll be damned if I know what it is... 

Thanks,
-Mark

[victor miranda]

photo of the drawbar?

it is a puzzle.

[eric220]

A wizard did it.

[nkalanaga]

Maybe the tender is actually grounded to the same side as the loco, to prevent shorts if the cab and tender touch, and then picks up power through the insulated wheels?  Are there any wipers in the tender trucks?  Or are the tender trucks mounted on insulated bolsters, allowing the shell to ground through the drawbar to the loco, with wires connected to the truck mounting screws inside?

[spookshow]

The wheelsets only have one live wheel each (left side). There are no wipers, so presumably current flows from the wheels, to the axles, to the trucks, and then into the tender chassis/shell.

Two wires go into the tender PC board - one is soldered to the inside of the tender shell and the other is soldered to the top of the drawbar peg. It's kind of hard to tell, but the drawbar peg appears to be insulated from the rest of the tender chassis.









Thanks,
-Mark

[Sokramiketes]

Is that an insulating bushing on the pin on the tender?  And a wiring coming off the pin, meaning the tender frame is one polarity and the locomotive, drawbar, pin, wire the opposite polarity?

[spookshow]

Is that an insulating bushing on the pin on the tender?  And a wiring coming off the pin, meaning the tender frame is one polarity and the locomotive, drawbar, pin, wire the opposite polarity?

That's what it looks like to me. But if the locomotive is sharing one polarity via the drawbar, where is it getting the other polarity from? Nothing works (motor or tender light) unless the two units are connected.

Thanks,
-Mark

[victor miranda]

tradition is tender left pick-up and engine right.
and no one follows the convention?

I think the secret is the tender to engine wire.
the engine picks-up one side and the tender the other.
the wire brings the engine pick-up to the lamp.

there may be a similar three diode arrangement in the engine.
that should supply the voltage for the lamp.

victor

[spookshow]

OK, but if they drawbar connection simply provides right-rail current to the tender light (from the engine), where is the engine getting left-rail current? Like I say, it doesn't run unless the tender is hooked up.

Sorry if I'm being dim - I'm not much of an electronics expert.

Thanks,
-Mark

[mmagliaro]

There is only one physical and electrical connection between the engine and the tender.  It is through
the drawbar.  The engine will not run unless the tender is connected, and the tender back-up light will not work unless
the tender is connected.

Therefore...
The tender light must be connected in series with the motor.  My guess is that the lamp is connected
across those 2 diodes which are in series with the motor.  The 2 diodes drop 1.4v no matter what, so the lamp gets
1.4v all the time and burns with constant brightness. 

I can't see the details of how that little circuit board is wired in your photo, but my guess is that the 3rd diode in there is
wired in reverse, across the other two, so that when the engine is in reverse, current still flows from engine to tender and the
engine runs, but now the lamps only get 0.7 volts, so they don't light up.  That's actually pretty crafty.

Like so:

[spookshow]

Ah, I get it! A wizard did it! 

Cheers,
-Mark

[victor miranda]

one day I will learn to write as clearly as Max.

I think his circuit is a good guess of what is there.

touch a wire to one rail and the drawbar (imitate a loco) and the tender lamp has a 50-50 chance of lighting.

a silicon diode has a .7 volt drop in forward, so two will give you about 1.4 volts and that is high enough
to light a 1.5 volt grain-o-wheet lamp.
the third diode it to let the current run the other direction. one assumes forward.

victor

[peteski]

Max's diagram makes sense (except that there is a similar constant lighting circuit in the locomotive, also in series with the motor).  The 3-diode board is a very good clue.  Cleverly designed circuit.

[spookshow]

It's probably hopeless trying to explain this to me, but I'm going to go ahead and ask anyway...

Up to now, all of the steamers I've encountered that have working back-up lights (and at the same time, have one-rail tender pickup / the-other-rail driver pickup) have required a second wire running from the locomotive/motor to the tender light. What did these guys do that eliminated the need for that second wire, and how come nobody else figured this out before? Was More (or Lik) just that much smarter than everyone else?

Thanks,
-Mark

[victor miranda]

it kinda depends on how you look at it.

the constant voltage/lighting diode bridge is not new.
it can be placed anywhere you can isolate the connections.

no one felt a need to light the tender?

most times you see it, it is on the engine attached to one of the brush caps.

No reason it can't be set in the tender.
You have to insulate the drawbar from the tender as well as the loco frame. 
think of it as a strangely shaped wire?

victor