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Application notes for the N8101 DC Power Source

in an analog (non-DCC) environment

 

 

Analog track power

Simply put, a variable DC voltage is applied to two tracks with one being +DC and the other, -DC. Increase the voltage and the electric motor in the locomotive spins faster making the train go faster. If the train is required to reverse, track polarity is reversed. Also, what defines "forward and reverse" is dependent on which way the locomotive is facing when it's put on the track. The bottom line is that track polarity is not fixed.

Discussion here is limited to an input voltage maximum of 16-volts because that is the design limit of our N8101. For model railroading this includes track voltages for Z, N, and HO scales.

For O and S scale users, please check your maximum track voltage settings. In some cases, it may exceed 16-volts. We're sorry G-scale users, additional voltage regulation would be required for your applications.

Our N8101 converts analog track voltage (polarity not important) to a DC output voltage of known polarity via the on-board full-wave bridge rectifier. The N8101's big plus here is that this bridge rectifier uses a type of diodes known as Schottky diodes. These are very high speed diodes with much lower voltage losses than conventional silicon diodes used in most bridges. This is especially beneficial in analog operation because it is desirable to have lighting and special effects function at as low a voltage as possible. For example; it would be nice to have that roof beacon or Mars light working when to locomotive starts moving, rather than when it's up to road speed.

We believe our N8101 DC Power Source will provide model railroaders operating with analog DC the best approach to lighting using our LEDs, and dazzling special effects using our Simulators.

 

Analog power and special lighting effects

Coupled with the N8101, tests using analog track power show our Simulators function (with LED connected) as follows:

Simulators using white, yellow-white or blue LEDs (3.3-3.6 volt LEDs)

0-2.3 volts DC:  no function - LED dark

2.4-3.6 volts DC: Simulator full function - LED dark

3.7-4.0 volts DC: Simulator full function - LED very dim

4.5 volts DC: Full function - LED brightness is moderate but definitely useable

5.0 volts DC: Full function - LED brightness good

5.5-16.0 volts DC: Full function - LED brightness near full to maximum

 

Simulators using red or yellow LEDs (1.75-2.0 volt LEDs)

0-2.3 volts DC:  no function - LED dark

2.4-2.8 volts DC: Simulator full function - LED dark

2.9-3.1 volts DC: Simulator full function - LED very dim

3.2-3.7 volts DC: Full function - LED brightness is moderate but definitely useable

4.0 volts DC: Full function - LED brightness good

4.5-16.0 volts DC: Full function - LED brightness near full to maximum

Note: Our Simulator modules have an on-board 5-volt voltage regulator to provide constant power for the microcontroller and LEDs. This is a "pass-through" regulator which will allow lower input voltage through, but won't perform absolute regulation until the input voltage reaches at least 5.5-volts. It has an upper limit of 18-volts (hence, our spec. of 6-18 volts for input voltage). The on-board resistor for LED current protection is selected for operation at the (regulated) 5-volt level. That way, LED performance is stable throughout our specified operating range. The microcontroller however, begins full function at about 2.1-2.2 volts and is fully protected from excessive voltage when the regulator kicks in.

The benefit for analog operation here, is that as the track voltage is increased from zero upward, the special lighting effects produced by our Simulators are fully operational and stabilized long before LED intensity is noticeable. As voltage continues upward, the effect only becomes brighter, not different, until full regulation when brightness has stabilized at maximum.

Locomotive creep test

We tested 7 totally different N-scale locomotives from 3 different manufacturers on an analog test track for voltage levels at which the locos just started to move (creep). This was a relatively small sample, but yielded a wide enough variation that we expect a larger test would for the most part, just produce more data points inside the range. We found the lowest at 2.9 volts and the highest at 4.2. with the  average for the lot at 3.63-volts. Four locos were between 3.6 and 4.0 volts All locos were in very clean operating condition and the track was just cleaned. The test was a level surface with loco only, no rolling stock. Since the tests were performed under minimum load and motor drag conditions, we would expect normal operating conditions for a model railroad to yield slightly to somewhat higher locomotive startup voltages.

Based on these results, we feel the low-loss rectifiers in our N8101 provide an excellent method for power conversion to DC for our Simulators when operating in a typical analog environment.

Flicker control

The short answer is: Special effects lighting typically doesn't require as much flicker protection as "always-on lighting, and is some cases may not require any.

A more in-depth explanation follows:

 

When including devices that produce special lighting effects, such as our Simulators, the term "flicker" really needs to be defined a bit more accurately. In a typical model railroading environment, "flicker" is the result of momentary power loss to rolling stock lighting such as passenger car interior lights, caboose marker lights, or locomotive lighting. These are normally lights that are on all of the time. When dirty track, wheel sets, or track gaps cause temporary power loss, the flicker that occurs is quite noticeable. This is a classical case of flicker. Controlling this pesky problem is performed by adding a capacitor or capacitors to the circuit which "charge up" under normal track power, and drain into the circuit to keep the lights on during momentary power losses. Depending on the type and number of lights (and track voltage level) the amount of capacitance required to eliminate the flicker problem (either totally, or mostly) can be up to as high as 1000μf for circuits with multiple lights. Capacitor requirements for good flicker control for most single-light circuits, typically ranges from 300μf to 600μf. Again, this value totally depends on how much current the light or lights draw and how long the period of time is that the capacitor(s) must provide power.

With special effects lighting, when power is interrupted, a Simulator's microcontroller is reset causing the effect to restart. Depending on the particular effect, this reset may or may not appear very obvious, and as a result may or may not be objectionable to the observer. When a Simulator is powered through one of our N8101 DC Power Source modules, the tiny 10μf capacitor on-board the N8101 is not large enough to keep a microcontroller from resetting. This capacitor is primarily for filtering of DCC and AC rectified input voltage. The addition of external capacitor(s) would (might) be required.

Now, unlike the very obvious flicker with always-on lighting, two things are quite different with effects lighting. First, with very few exceptions (ditchlights and steam-era class lights come to mind), the special effect either varies lighting intensity (i.e. marslights and gyralights), turns lights on and off (FREDs, flashers and strobes), or some combination of both (beacons, alternating flashers, etc.). Second, very little power is required to keep a Simulator's microcontroller alive (about 1/2ma at 2.1-2.2 volts). The simulator's on-board regulator draws a little power, but it is minimal. When power is momentarily lost, if the associated lighting for the effect is on, that light source will be responsible for nearly all of the power drain in the circuit. One 2-volt 20ma LED draws 40 times as much power as the microcontroller. A 3.3-3-6 volt LED will draw 60 times as much. If the effect is in the off mode, only the microcontroller is drawing power. So... how much external capacitance is needed with special effects lighting? It...depends.., but, for the most part, not nearly as much as always-on lighting.

The old saying: "beauty is in the eyes of the beholder", really comes into play here. The "it...depends" really depends on the type of effect and how it "appears" when power is momentarily interrupted. Figure 1, below shows where additional capacitance would be added to a typical lighting special effects circuit.

                                                  Figure 1

The capacitor or capacitors are connected across the DC output (Simulator input) as shown. If multiple capacitors are used (i.e. our small 100μf N3100 capacitors), they must be wired in parallel with each other (all of the + connections jumpered together, and all of the connections jumpered together).

Rating the "need" for capacitance

Below is a what we would call a starting point for the determination of adding capacitance to a lighting effects circuit. We've rated the "need" from least to most based on what is likely to have visual impact on the realism of the effect. While this is our opinion, it should only be treated as a guideline, because beauty really is "in the eyes of the beholder". What is truly important, is how the effect appears to you.

Least

FREDs and Strobes (none to possibly 200μf)

Beacons and alternating effects (100μf to 300μf)

Marslights, Gyralights, etc. (200μf to 500μf)

Ditchlights, steam classlights, etc. (300μf to 800μf)

Most

 

Analog and "always on" lighting

The N8101 is well suited to provide constant-polarity DC power for general lighting of model railroad rolling stock. This would include passenger car and caboose interior lighting, marker lights in general and locomotive cab, class, and number board lights. Its very-low loss schottky rectifiers will ensure that the majority of track power (regardless of polarity) is passed through allowing polarity sensitive devices like our LEDs to function as soon as their threshold voltage is reached. Having current carrying capacity of 200ma, the N8101 is powerful enough to support any typical rolling stock lighting requirement with room to spare. As noted above, the on-board 10μf filter capacitor is not intended to provide any meaningful flicker control. External capacitor(s) are required for that purpose. The diagram in Figure 1 covers the connection of capacitors to the N8101's output solder points.

When wiring passenger cars for interior lighting (in most cases, multiple LEDs), we always recommend wiring the LEDs in parallel with each having a protection resistor selected based on maximum expected track voltage. If you need to review parallel LED wiring, more information is available here.

Other power needs

While the vast majority of track powered applications for rolling stock (other than motors in locomotives)  is lighting, there are certainly other cases where polarity sensitive devices might need power. Since the N8101 will support up to 200ma of current, this could include tiny motors, solenoids or other DC actuators, or miniature circuitry to perform various functions. The N8101's tiny size will allow it to be place just about anywhere.

 

2008 Ngineering