Woodland Scenics Plug N Play Voltage

[rickb773]

I just bought a second hand W.S. Plug n Play building and I really do not want to spend another $45 for the "official" W.S. wall wort and controller.

Can anyone tell me what the input voltage is to the building light (and do I need to use a resistor)?

I cannot find anything on these via google searches.

Many thanks,
Rick

[Chris333]

Their site says:

JP5770 - Power Supply -
INPUT: 120VAC, 60Hz

OUTPUT: 24VDC, 1000mA

[rickb773]

Their site says:
JP5770 - Power Supply -
INPUT: 120VAC, 60Hz
OUTPUT: 24VDC, 1000mA

I am asking about the input to the light itself (output from the controller to the light bulb/LED) not the wall wart.

[mmagliaro]

I took a read through their FAQs.  You need a "Light Hub" that you power with the wall wart, and then you plug the lighted buildings or other accessories into the Hub.  There is an FAQ about "what voltage" the hub outputs, and their answer is that the outputs are "current sources not voltage sources", and that each port on the hub can output a maximum of 30 mA.  That STRONGLY suggests that the current limiting is done in the light hub, so therefore yes,  if you want to power your lighted WS building directly from a power supply, you need a resistor in between.
30 mA is too high for an LED, so they are probably expecting there to be multiple LEDs in parallel inside the buildings.
All this is to say, if it were me... I'd begin by conservatively experimenting with resistors.

Let's a guess at 3v for the drop through a white LED.
If you use a 12v wall wart (I doubt you really need a 24 volt one),  (12-3)/.020 = 450 ohm.  So I'd probably start with a 680 ohm resistor and see what happens. If it is really dim, then go down to, say, 470, then 390.  I would not go below 390 at 12v, since 9/390 is 23 mA, and by then, you should surely see some decent light levels.

All this assumes you can't crack that building open to see how many LEDs are in there and how they are wired.  That would really help in picking the resistor.

[nkalanaga]

And assumes that WS hasn't anticipated us reverse-engineering their designs, and painted all of the resistors, so we can't read the color codes!

[mmagliaro]

And assumes that WS hasn't anticipated us reverse-engineering their designs, and painted all of the resistors, so we can't read the color codes!

Ha!  But... I'm not expecting resistors inside the buildings.  Since each hub port's maximum output is limited to 30 mA, I am assuming there are resistors inside the hub to limit the current.  But if they are in the building and are unmarked, just use the ol' ohmmeter to see what the resistors really are.

[spookshow]

I connected one of their street lights to an adjustable low-voltage power supply (with an initial setting of 1V) to see just how far I could push it. What I learned is that they don't light up at all until around 2V, and then get nice and bright at around 2.7V. So, 3V does sound like it's the max (although I'm content with the brightness I get at 2.7V). Now, that doesn't mean that their building lights are the same, but I'd bet good money that they are.

-Mark

[peteski]

I suspect that WS does not use resistors as current limiters in their hub, but some other means of limiting the current (like maybe constant current diodes).  Many LED power supplies for higher power applications use constant current power supplies where someone can connect one to several LEDs in series, and in either case they would glow with the same brightness since the current is limited.  That would explain the 24V power supply. With that much voltage available (at limited current) the WS hub can handle a range of multiple series-connected LEDs.  This is all speculation mind you as I have never cracked open or even handled one of the WS lighting hubs.

Typical white (and blue) LED requires around 3V to illuminate.  Yes, they will start emitting light at voltages around 2.5V, but full brightness is around 2.9-3.2V (depending on the LED).

It would be interesting to find out what is the open circuit voltage at one of the hub's outputs. 

[Maletrain]

When trying to get the desired light intensity with various LEDs, why keep trying different individual resistors?  Why not use a variable resistor to get the exact effect you want?  Then maybe measure that resistance and replace the variable with a fixed value resistor.

See https://www.ebay.com/itm/334541121236

[Lemosteam]

I had some leftover connectors from their street light systems that I snipped off for the connector to use on some RC batteries. When I removed the shrink tubing near the connector for the lights and de-soldered them, there was an 0603 resistor under the shrink wrap.

The resistor reads 681 (680 Ohm) used to light three streetlights per connection. See image below.

Not sure if it is the same for the structure lighting or not, but rather than buy their bulky system, I just used 1w resistors to my brightness liking and connected them to my own 3v power source.

[mmagliaro]

Using a pot to find the "best" resistance is a fine idea.  I just suggested trying some discrete resistors because sometimes people don't have a pot handy, or they are trying to just make the LED work with what ever they have lying in their electronics junk box.   The find a good one, solder it in, and move on.

Peteski, it would make sense for them to use a better current-limiting circuit than just a simple resistor, although the fact that they say a "maximum of 30 mA" doesn't make sense in that context.   If one hooked up a single LED, how would the hub "know" to limit the current to only 10 or 20 mA to avoid buring out the LED?

[peteski]

Using a pot to find the "best" resistance is a fine idea.  I just suggested trying some discrete resistors because sometimes people don't have a pot handy, or they are trying to just make the LED work with what ever they have lying in their electronics junk box.   The find a good one, solder it in, and move on.

Using a pot is a good idea but if it is accidentally turned to zero ohms, the excessive current can damage the LED.  Adding a series resistor with the pot (using lowest safe value for the LED) will prevent that from happeing.  When the pot is turned to zero ohms, the series resistor will lstill limit the current to a safe value.

Peteski, it would make sense for them to use a better current-limiting circuit than just a simple resistor, although the fact that they say a "maximum of 30 mA" doesn't make sense in that context.   If one hooked up a single LED, how would the hub "know" to limit the current to only 10 or 20 mA to avoid buring out the LED?

Assuming that the WS hub uses constant current circuit for its individual outputs, that will limit the current to whatever level WS designed.  The voltage across the LED will develop based on that current.  With 24V as maximum voltage, the hub's output could power from one to about 7 series connected white LEDs, and the current always will stay limited to 30mA

I suspect that specifications WS supplied for the circuit are not exact. They designed the hub to work specifically with their lights, not with modeler supplied LEDs, so the "max" specs they supplied are just approximate.  Of course this is all just a speculation on my part.

[mmagliaro]

Well, it appears somebody else has done the heavy lifting for us.
Look up "Sumida Crossing"  WS Lights, and you will find a nice investigative article.  This fellow took measurements,
and took apart one of the light hubs to see what the circuit looks like.  It's pretty darn simple.  A rectifier and filter,
then a one-transistor driver with a pot in the base lead to control the LED brightness.  Other than that, it just has a 685 ohm dropping resistor in series with the load.   So on a nominal 24v supply, with a 3v white LED, we get 21/685 = 30.6mA, which nicely agrees with the WS FAQ comment.  I still think that is way high, unless every accessory they sell for their lighting system has at least 2 LEDs in parallel.   Those in-line resistors that Lemosteam showed for the streetlight system explain a lot.  Those streetlights must only have one LED in each one, and would burn out at 30 mA, so they stuck another resistor in the actual connector wire to cut it to 15.
--
So after all that, it would probably be prudent to just hook up a 12v supply with 680 ohm resistor in-line to the structure, and see how it looks.   
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You also have to worry about the polarity.  Are the terminals or wire to the building marked in any way?  With white LEDs, you really don't want to connect them in reverse.  They can be damaged by much lower voltages than other types.