Tender back-up light confusion

[peteski]

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.

[mmagliaro]

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.

[jagged ben]

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.

 

[mmagliaro]

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.

[nkalanaga]

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...

[spookshow]

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