Aristo-Craft ART-7110 Electrical Nuances

Aristo-Craft's Signal Bridge, ART-7110, electrical nuances
Ted Doskaris
Revision GE-B1
(Original material of April 26, 2008 updated)
June 27, 2009

Revision GE-C  Added dimensional and signaling reference information
October 3, 2013

Revision GE-C1 Updated Philips LED glossary link, removed National Semi links
July 4, 2016



Sometime ago I purchased the Aristo Signal Bridge, ART-7110, with the intent of eventually using it in my yet to be built extended outdoor layout.
Also, around that time an Aristo FORUM member posted questions about the Signal Bridge that prompted me to take a close look at it - particularly from an electrical perspective.

To preface:


This product is a neat thing to have and adds a good deal of realism to the looks and operation of a train layout, and it is much appreciated that Aristo-Craft produces it.

The discussion that follows is in no way meant to denigrate it - rather, it is with the intent for  better understanding its electrical design and improving upon it - particularly with respect to its Light Emitting Diode (LED) circuits. (It is to be appreciated that an older version Aristo Signal Bridge includes incandescent lamps whereas the newer version like the one I have has LEDs for its lights.)

Though the Signal Bridge LEDs in my example of the product appear to work when applying power, the circuit configuration within it is deficient for best mode operation and can be potentially problematic - particularly with regard to the operation of the Yellow LEDs.
As such, the following discussion is somewhat technical and "dry" but addresses these issues and offers some suggested improvements.
Accordingly, I offer my apology to anyone who chooses to endure this discussion.

 

Description:


The Aristo Signal Bridge comes in a shallow, rectangular box with many pieces that must be assembled. There is an instruction manual booklet included in the box, too.
Shown below is the Aristo Signal Bridge, ART-7110, box



Shown below is the Aristo Signal Bridge box and its contents:


The Aristo Signal Bridge can be used to straddle 2 tracks.  When assembled, the narrowest distance is between the foot of the bases which measures about 11 & 3/4 inches.  The distance between the uprights is about 13 & 1/4 inches.  This leaves plenty of room for two parallel straight tracks placed at 5 1/2 to 6 inches centers.
 
The kit provides two signal lamp sub-assemblies, each associated with its own base housing having an electrical switch to control color illumination of 4 elements; thus, one lamp sub-assembly with its own independent lamp control can be dedicated to each of the two tracks when using two parallel tracks.  The lamp sub-assemblies are faced in opposite directions on the bridge.

This vignette does not describe the meaning of the signaling lights as to what colors are to be illuminated in a given railroad operation.  For this kind of information about railroad signaling lights, see the following references:

A one page chart defining signal light color arrangements is illustrated and described in the
"The Delaware, Lackawanna and Western Railroad Company
Color Light Signals Names, Aspects and Indicators"

A book about Railroad Signaling:
Title: Railroad Signaling



by Brian Solomon
Published by MBI, ISBN 0-7603-1360-1
and a Paperback version by Voyageur Press (ISBN-13) 978-0760338810

Now to describe the electrical aspects:

The instruction manual page 7 describes the electrical connections to a power source that a user must provide. Though there is no information in the manual as to what to use, the power source could include a so called "wall wort" providing it conforms to proper voltage and current requirements. It is obvious that voltage that can vary such as used for track power is not intended to power the Aristo Signal Bridge.

The Signal Bridge has two separate sets of light subassemblies, each captive via electrical cabling to a base unit that includes a small 3 position selection switch.


The selections are to be made manually by the user to illuminate a Red, Yellow, or Green light. In the course of making electrical measurements whilst operating the switch, I discovered it is what is known as a "make before break" type switch.
This means it does not electrically disconnect continuity from its prior position until it completes its connection in is new position. (In other words, if a green light were illuminated and you then changed the switch to its next position for the yellow lights, the green light would momentarily remain illuminated whilst the yellow lights would begin to illuminate.)
With such a switch, the lighting circuit would always be conducting current from its power source. In actual use, this would not be too noticeable when viewing the lights.

The light subassemblies are to be installed on the signal bridge overhead facing opposite to one-another. Each light subassembly includes 4 LEDs of the following colors: one Green, two Yellow, and one Red. The two yellow LED lights are wired together with the intent that they both are to illuminate as a pair when selected with the switch.

 

Operating voltage, AC vs. DC:


Shown below is an electrical schematic diagram of one of the Signal Bridge's base subassemblies. (I traced out the wiring to make the diagram, and both subassemblies are the same.) The selection switch is housed within a small circuit board in the base unit, and it also includes a 1000 ohm current limiting resistor intended for the proper operation of an LED when applying an appropriate voltage.


On page 7 of the Instruction Manual is a description of the voltage source connections to a subassembly. At fist glance this description appears to be rather vague and indefinite.

As can be seen Aristo cites 15 volts and shows (+) and (-) albeit with the wrong polarity assignments as to how the subassembly hardware is actually color coded for its wiring!
Although it may be normal convention to associate red with + and black with - for DC polarity, the hardware's actual wire colors are the reverse of this! As such, a user may be misled and confused when connecting power.
 
Moreover, the manual shows "AC" as well - implying that either DC or AC of the 15 voltage value can be used, too.
Assuming this to be the case and visualizing an AC sinusoidal waveform, 15 volts AC is about 42 volts peak to peak or 21 volts peak with respect to its 0 crossing. Since current will only flow in one direction for a selected LED since it is a diode, then the current that flows will correspond to the applied half wave voltage (or 0 through 21 volts peak).
However, using 15 volts AC will result in an averaging effect having a lower perceived level of light emitted by the LED.
Given the component values shown in the diagram, approximately 14 ma will flow for a given LED switch selection setting if applying 15 volts DC, but in the case of the application of 15 volts AC, an approx. 19 ma peak current will periodically result . If, however, 21 volts DC (flat line) were applied, a steady 19 ma current will result. More on this later.

Shown below are the Aristo Signal Bridge base & lamp subassembly underside views with the circuit board and with the lights' covers removed.


Shown below are the base & lamp subassembly underside views with the circuit board removed.


Shown below are the base & lamp subassembly underside views but now with the circuit board removed and turned over so its component side can be seen.


Note the round 1000 ohm 1/4 watt resistor and rectangular metal slide switch for selecting the LEDs.

Having taken apart the signal bridge subassembly for examination of the wiring reveals some concerns:

The below picture of the Signal Bridge is a schematic diagram that depicts its factory wiring.
The signal bridge includes two separate, identical base unit / light subassemblies that have identical circuitry and wiring.
Shown here is the electrical diagram of a base unit subassembly on page 1 of 3.


It is to be appreciated that LEDs used in the example Aristo product are not identified, but appear to be 8 mm to 9 mm in size and would be expected to be similar to other like kind LEDs that have a typical specification of 20 ma forward current whilst developing 2 volts across it.

 

The two electrically parallel yellow LEDs present a concern as explained:


Since the resistor that is in a circuit common to all LEDs is optimized with a 1000 ohm value for  a current of approx. 20 ma going through a single LED - like that for the Red or Green LED - then the situation for each of the Yellow LEDs is to only admit a proportion of the current by one half  (approx. 10 ma), thereby risking falling short of  electrical operational specifications. This will result in diminishing the light power (luminous flux) for both yellow LEDs. This degradation may be subjectively noticeable as a brightness deficiency in the light of day environment for outdoor train layouts.
But there is more to it as one of the two electrically parallel yellow LEDs can dominate the other in current draw:
Though it is intended that the two yellow LEDs being placed in an electrically parallel fashion is for both to illuminate at the same time, one of them can conduct  more current than the other, thereby risking differing luminous flux for the two - unless these LEDs are matched to virtual perfection as to their characteristics. Such a process is called "binning" and would have an added cost. It is not known if Aristo-Craft specifies this of its LED vendor.

 

Operating voltage:


As previously mentioned, the Aristo Signal Bridge can be expected to operate on 15 volts AC according to the manual that accompanies this product. Moreover, the manual includes expressions of (+) and (-) that is not normally associated with alternating current.
Examination of the Aristo signal bridge circuitry shows that it's design is a direct current (DC) device.
It should be recognized that the Aristo instruction manual may be a holdover written for older version Signal Bridges' incandescent lamps where the use of AC and DC type voltage is not of concern.
That said, applying AC in newer version Signal Bridges would allow for it to function with user selected LEDs only being illuminated for each half cycle of applied voltage since a bridge rectifier is  not included in this product.
It is common industry practice to express AC voltage values in terms of  root mean square (rms) which in this case of Aristo's 15 volts AC means a peak amplitude will be about 21 volts. At this time it is unknown exactly what Aristo's LED electrical specifications are; however, as previously stated for a similar device in this application it is typically specified to operate with 20 ma whilst developing about  2 volts across it. Since 21 volts less 2 volts divided by 1000 ohms yields 19 ma, then using a DC power supply of 21 volts would be sufficiently appropriate. It is to be appreciated that 15 volt AC applied voltage would only operate (illuminate) the LED/s at every half cycle, but the 21 volt DC flat line applied voltage would be within anticipated specifications along with the expectation of best luminous flux.
That said, the little circuit board 1000 ohm current limiting resistor appears to be a 1/4 watt part (250 mw). Thus, this resistor would be dissipating  approx. 360 mw of heat if 19 ma were allowed to pass through it. By doing so it would eventually likely fail from over heating.
In order to prevent this resistor from failing it can be changed to a physically bigger 1/2 watt size, or the applied voltage should not exceed about 18 volts DC which would result in a reduced (approx.16 ma) current through an LED - though with diminished luminous flux.
For comparison, if using 15 volt AC with its 21 volt peak value, the resistor would not be stressed since the developed heat would be averaged to a non critical amount due to the effect of the half wave current flow.

 

Suggested changes:


A simple change that includes resistors:

A simple change to correct for the yellow LEDs operational deficiency is shown below for a base unit subassembly on page 2 of 3 in the schematic diagram.
Basically, the change includes reconfiguring the yellow LEDs parallel connections to a series connection and isolating them with a dedicated current limiting resistor.
Since an LED device specified luminous flux is at a specified current flow through it, this is the preferred condition for the two yellow ones for best consistent performance as a series connection guarantees the same value of current will flow through both of the yellow LEDs.


Note that the resistors shown in the above diagram should be 1/2 watt types or better. This includes changing the present 1000 ohm 1/4 resistor to a bigger 1/2 watt power rated version.

 

A simple LED driver in the form of a constant current generator:


Shown below is a schematic diagram of factory wiring with changes employing an example of a constant current generator to drive all LEDs as well as to correct for the yellow LEDs operational deficiency. This changed circuitry also allows for AC and DC operational voltages. By reconfiguring the present circuitry associated with the current limiting resistor with a constant current generator, all LEDs can best be operated as they are current dependant devices. These changes will also allow for a wider range of input voltages to the signal bridge, including accommodating both AC and DC voltages without regard of applied polarity.
Shown here is the schematic diagram of a base unit subassembly with these changes on page 3 of 3.


The bridge rectifier shown in the above schematic diagram may be used in common for both base unit subassemblies.

Note:
The resistor values in the schematic diagrams are nominal and may be slightly different once I actually implement a change like those described above to the Aristo Signal Bridge. Accordingly, at a later time I hope to add information to this discussion.

It is to be appreciated that other approaches using purpose designed ICs for LED drivers are also available from such companies as National Semiconductor.

Moreover, Aristo-Craft (or a user) may wish to consider adapting the Pulse Width Control (PWC) with voltage regulator circuit design that is commonly installed in many of the locos for application to the Signal Bridge - thus, allowing it to also operate on track power.

 

A possible Signal Bridge application:


Connecting an Aristo Signal Bridge to Aristo's Remote Turnout (switch) machine:

Sometime ago an Aristo FORUM member had asked how to implement this using a 22 volt DC power source.
At that time I did not appreciate the level of detail as previously discussed, particularly with respect to the two yellow LEDs.

Shown below is an example of the Signal Bridge connected to Aristo's Remote Switch Machine accessory terminals along with the use of a 420 ohm 1/4 watt resistor as per the schematic diagram. Though this resistor value will allow for the LEDs to illuminate and will serve to prevent the base unit 1000 ohm resistor from overheating when applying the 22 volts DC, a switched selected LED won't illuminate with the best possible luminous flux.
As to connecting the two base unit / light assemblies to the remote switch machine, I show in the diagram a simple approach to having the accessory terminal SPDT switch select one OR the other base unit lights. Of course the light assemblies when normally installed are facing in opposite directions, so if you desire to get a bit fancier as to having control of the lights / colors in both directions at the same time - the little circuit board wiring will have to be removed and reconfigured accordingly along with other supporting circuitry.


 

Reference material:


Philips LUMILEDS
LED Technology Information

The TI LED Lighting Overview includes the following statement (Their more recent web site version is worded somewhat less technical.):

"Regardless of type, color, size, or power, all LEDs work best when driven with a constant current. LED manufacturers specify the characteristics (such as lumens, beam pattern, color) of their devices at a specified forward current (I_F ) not at a specific forward voltage (V_F ). Most power supply ICs are designed to provide constant voltage outputs over a range of currents, hence it can be difficult to ascertain which parts will work for a given application from the device data sheet alone. With an array of LEDs, the main LED in the array is driven with the same current. Placing all the LEDs in a series string ensures that exactly the same current flows through each device."

National semiconductor had a white paper, "Matching Driver to LED" (Note: Though this paper described high power LED lighting, including arrays of lights, the same principles apply to the 20 ma LED types that are used in the Aristo Signal Bridge.)

The following are selected excerpts:

"Two features set LEDs apart from other lighting sources: first, LEDs are driven by current, and second, the forward voltage across an LED is low and is DC. The typical forward voltage, VF, ranges from 2V to 3V for InAlGaP LEDs, and from 3V to 4V for InGaN, but the luminous flux of an LED is proportional to the forward current, IF.
...
it is important to note that current is the independent quantity, reflecting the fact that current control is the key part of an LED driver.
...
Placing the LEDs in series guarantees that the same current flows through each device,  each individual device
...
A second disadvantage of the series-parallel parallel array is that the LEDs will not share the drive current equally unless their VF is matched. This requires them to be binned by the manufacturer and increases their cost.
...
Dimming of the light output of LEDs is done with pulse width modulation in order to maintain a consistent color or color temperature of the light. Above a certain frequency (generally 200 Hz) the human eye cannot distinguish the individual pulses, and by adjusting the pulse width while keeping the ‘ON’ state LED current at a certain level the average level of light perceived varies accordingly. ..."

End