TheRailwire
General Discussion => N and Z Scales => Topic started by: spookshow on October 06, 2014, 09:41:26 AM
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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
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photo of the drawbar?
it is a puzzle.
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A wizard did it.
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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?
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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.
(http://i745.photobucket.com/albums/xx93/spookshowphotobucket/drawbar1_zpsbf5c2cdd.jpg~original)
(http://i745.photobucket.com/albums/xx93/spookshowphotobucket/drawbar2_zps8a2f024d.jpg~original)
(http://i745.photobucket.com/albums/xx93/spookshowphotobucket/drawbar3_zps103458b9.jpg~original)
(http://i745.photobucket.com/albums/xx93/spookshowphotobucket/drawbar4_zpsa02e6d4b.jpg~original)
Thanks,
-Mark
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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?
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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
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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
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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
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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:
(http://www.maxcowonline.com/photohost/misc/morelightpickup.jpg)
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Ah, I get it! A wizard did it! :D
Cheers,
-Mark
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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
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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.
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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
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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
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Mark,
do you recall if the models which had a tender headlight and a single wire (plus the electrically active drawbar) had constant lighting or simply a 12V bulb (which would glow dimly when the model was running slow)?
It might have simply been a 12V bulb. If that was the case then the single wire just brought the other other electrical connection to complete the light bulb circuit (the other lead from the bulb would have been connected to the tender pickup. If the light was directional then there could have also been a singe diode in series with the tended bulb. Just like it has been done for decades on any plastic loco with directional lights.
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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
Actually Mark, what they did was very very old school. This is the way people did constant directional lighting before there were LEDs and before motors got really efficient.
Here's the key thing.
Look at the two diodes in series in my drawing. When enough current flows through those, no matter what else happens, there will be 2 x .7 = 1.4 volts across that pair of diodes. Anything you connect across them will get 1.4 volts.
Now, this isn't "without limit". If you just connect a pair of diodes across the tracks, they draw the maximum supply current from your throttle and they either burn up or trip the breaker. But notice, the diodes are connected so the current flows through them and then through the motor. You can think of the motor as just a big ol' resistor. It limits the current that can flow in the circuit. So the diodes don't burn out.
If you happen to have a copy of Peter Thorne's "Practical Electornic Projects for Model Railroaders", circa 1973, it shows this exact type of circuit and explains the theory behind it.
I know, electronically that is back when dinosaurs roamed the earth.
The triceratops was always my favorite.
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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?
What they did was wire everything in series instead of parallel. With the two wire locomotive, one of the wires is returning directly back to the chassis and rail while the other is connecting first to the light. Disconnect the one for the light and the motor will still work, and vice versa. With the single wire, all electrical current is flowing first through the engine and then through the tender (or the other way). Disconnect the one wire and nothing works.
The comments about magic (i.e. wizards) are not entirely silly, as silicon diodes, being semi-conductors, have an I-V curve rather than a set resistance. I find it a little hard to wrap my head around and I understand Ohm's law decently well.
The wiring method you've discovered may have fallen out of fashion because it was more prone to failure, since there are several points along the circuit where a failure causes the whole thing to stop working. It could also be that it didn't play as well with more finely tuned motors, or that it was just too complicated for most locomotive designers to understand.
Also, I could be wrong, but it seems to me there is also a resistor wired in the whole thing in series as well: the shrink wrap in the photos.
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With modern motors, that might only draw 60-80 mA at the most, and incandescent lamps
typically needing about 30 to light up well, what happens is that at lower voltages, the motor just doesn't draw enough
current through the diodes to really turn them on, so you don't get 1.4v and the lamps don't light.
The old-school motors easily drew 250 mA so that wasn't a problem.
The other issue is the heat from those diodes. We are talking about magic. Well, they don't keep that voltage drop
fixed at 0.7v by magic. They have to dissipate the remaining power as heat and they can get pretty hot at higher throttle settings.
Another reason this isn't used much anymore is that we've all gone to LEDs for headlights. With a simple series resistor,
most LEDs put out a pretty constant amount of light over a wide range of engine speeds.
Finally, of course, you have DCC, where the headlight is controlled by an independent fixed output from a decoder, completely eliminating the whole issue of needing constant voltage regulation.
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Clever. I remember when this was the popular way to get constant lighting, but like the others, have never heard of it being used in a tender this way. IN fact, it wasn't used much in N scale, partly because most N scale locos didn't have constant lighting, and partly because there wasn't enough room for the diodes in most of them...
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Mark,
do you recall if the models which had a tender headlight and a single wire (plus the electrically active drawbar) had constant lighting or simply a 12V bulb (which would glow dimly when the model was running slow)?
I'm pretty sure the locomotives I'm thinking of did not have constant intensity lighting. I guess that's the main selling point of the More/Lik design.
Anyways, thanks for all the information everybody. Very helpful!
Cheers,
-Mark