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Convert DCC track voltage to DC for lighting your rolling stock



For this discussion it is assumed that the rolling stock you're going to add lighting to, already has trucks that are capable of electrical pickup from the track. The Kato Cupola Caboose, Kato Phase III or IV Amtrak Superliner Passenger Cars, and Rivarossi Heavyweights with wheel pickup kits would be some examples. The bridge inputs (~) described here will be connected to both trucks, each oriented for separate rail pickup.

Adding electrical pickup type trucks to non-configured rolling stock will be covered later as a separate issue. FYI, Kato #800461 caboose truck sets are $5.00 a pair and can be adapted to Atlas or other "cabeese". These come with metal wheelsets and the phosphor bronze pickups. They can be ordered directly from Kato. For other types of rolling stock there are other options.

An overview:

  1. To a device like an LED which requires a straight DC voltage source to operate, the DCC signal on your track looks like AC. This is because like AC, with respect to "ground" (zero voltage reference point), this signal also goes negative (minus volts). This is one of the reasons DCC will allow you to run two different  locomotives in opposite directions on the same section of track. Truly marvelous, but pesky to an LED or other DC device.

  2. To convert  this "AC" source, we use a bridge rectifier (like our tiny N301S, or our incredibly super-tiny N302S)  to filter out that part of the signal that is negative voltage and allow only the positive (plus voltage) part to pass through. Voila... DC!

  3. Next, we add two more components. First, a capacitor to help smooth out the filtered DC and act as a small "storage battery" for when your rolling stock travels over gaps in the track or slight dirty spots. This capacitor "charges up", then discharges whenever voltage isn't present to help reduce flicker in your LED. Second, a current limiting resistor to protect the LED from burning out.

  4. The capacitor is a polarized type called an electrolytic capacitor. That is, it has plus and minus connections. To function properly, it must be connected with the minus lead hooked to the minus (-) connection on the bridge rectifier. Hooking it up backwards will destroy the device and it may even "pop" like a tiny firecracker. These electrolytic types come in several varieties. The most common are the wound film type that look like small cylinders. A much more compact version is the tantalum capacitor. They're more expensive, but much smaller for the same capacitance value. Our N3100 is an example of this type. Capacitance values are defined in microfarads (μf), the bigger the number, the greater the capacity of electrical charge it will hold. For our purposes, 500 to 1000μf will do the job. Capacitors are also rated by maximum allowed input voltage. Always make sure the capacitor your going to use is rated higher than your voltage source. Otherwise... "pop" like a tiny firecracker. For this application, a 16 volt unit is fine.

  5. The resistor value is chosen using a simple formula based on the voltage source (Vs), the voltage of the device (Vd), in this case an LED, and the device current (I). With LEDs, the current is normally expressed in milliamps (ma) or thousandths of an amp. For example, 20ma can be stated as .020 amps. The formula is:

  1. The first step is to determine the voltage source. A simple method is to temporarily connect a bridge rectifier across the track on a DCC layout and measure the output of the bridge. The bridge has four connections. Two are marked with a tilde symbol ( ~) denoting the AC inputs, the others are marked plus (+) and minus (-). Connect the AC inputs to your track (carefully), and using a multimeter (voltmeter), measure the output on the plus and minus leads. Be extra careful not to short the outputs or the bridge could be damaged. On a N-Scale DCC layout, you should expect to see approximately 11.35 volts. This value is Vs.

  2. Let's use one of our 2x3 LEDs as an example for the other two known values. The 2x3 has a device voltage of 3.6 volts and current rating of 20ma. To calculate what value resistor we'll need, we subtract the device voltage from the source (Vs-Vd) which gives us 7.75 volts. This value is then divided by our device current I (20ma or .020) which gives us 387.5. This is the calculated value of the resistor we'll need to provide 20ma to our LED from an 11.35 volt source. Resistance is expressed in Ohms (Ω, the Omega symbol). Since you aren't likely to find a 387.5 ohm resistor, we'll pick the next higher standard value, or 390 ohm. We suggest you use a 430 ohm resistor to give the LED a little extra safety margin. It won't effect the brightness much and will ensure a long LED life. But, let your conscience be your guide.


The basic circuit:



  1. If you plan to connect more than one LED, connect them to the bridge outputs (where the capacitor is connected). For details regarding multiple LEDs circuits click on the "LED circuits" link next to each lighting product, or you can get there directly by clicking here.

  2. If you have very limited space and need to use one of our N302S Micro Bridge Rectifiers, we recommend using our N5032 multi-stranded Flexible Wire for hookup to the AC inputs of the bridge. This flexible wire should be pretinned at the end before connecting. The reason for using this wire is that the tiny pins on this incredibly small bridge rectifier are very small and can be damaged or broken if flexed too much. Using our flexible wire will allow the wire to flex during mounting the bridge and protect the pins. For output connections, our magnet wire of the N5032 wire can be used.

One more thing regarding bridge rectifiers... they are constructed using 4 silicon diodes. These diodes have a voltage "drop" of about 0.6 volts DC. What this means is that you can expect to have about 0.6 less voltage on the output of the bridge than you have on the input (12 volts AC or DC on the input, about 11.4 on the output). If you're doing the calculations noted above to determine proper resistance for an LED and you have a know source voltage, taking this voltage drop into consideration will give you a more accurate result.

That's it in a nutshell. Please email us if you have questions or run into difficulties. You can do that by clicking this link: nsupport@ngineering.com




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