A Guide TO Dcc Page 6 A Guide to DCC

6 5 EQ control of the sound. Each decoder is generally provided with documentation on what each CV does and the values to be programmed into them to do what is desired. While intimidating at first, programming CVs is not hard. Most DCC systems have detailed instructions on how to program decoders. However a typical user may have several decoders, and each decoder has a few dozen CVs, keeping track can be difficult. There are software programs available that allow users to save the CV settings for their locomotives. Other options include making a copy of the CV settings page for each locomotive decoder on the layout and mark up in pencil the settings, or develop a record on a computer spreadsheet. Regardless of the method, keeping track of what was programmed in will pay off in spades when troubleshooting or when trying to match what was done previously. BEMF and Motor Control Model railroad locomotives are attempting to simulate the movements of massive prototype machines. In reality, the friction between gears and even within the motor is a large percentage of the actual load on the motor. When trying to start a model locomotive that friction will keep the motor from turning until enough voltage is applied to force the motor and drive it loose. Once it is loose, it will move freely. This shows up as a jerk start on a model. So what is needed is a way to carefully apply power to the motor enough to get it moving, but then immediately back off as it breaks free. This is where high frequency and back-EMF motor control comes in. Instead of using straight DC, DCC motor drives use a very high frequency pulsed power drive to energize the motor. In this manner, it basically kicks small bursts of power to the motor to force it to turn slowly. But this still does not take care of how to keep it from over speeding when it starts to move. To control that, back-EMF control is needed. Any motor, regardless of quality, will generate electricity if simply spun. Or put another way, when energized and spinning, if power is shut off momentarily, there will be a voltage on the terminals proportional to the speed at which it is spinning. That is called a back-electro- motive force, or back-EMF. As described, the motor drive actually turns the power to the motor on and off in very short bursts. So a BEMF decoder is set up to read the voltage on the motor during the period of time the motor is off. These voltages represent forces which can be used by decoders to detect the speed and load on the motor and automatically compensate the power going to the motor. A control algorithm is used to adjust the power to keep the loco moving at a constant speed or to carefully control the motor at very low speeds where otherwise binding or even drag in the mechanism would stall out the loco. MOBILE DECODER INSTALLATION Decoder Installation - Fear and Trepidation or Wonderful Opportunity? Its just a matter of time, but at some point most model railroaders will come across an engine that they like but is not DCC equipped. It could be a new offering that does not come with a decoder, or it could be an old favorite from the DC days that would be nice to have upgraded. So how hard is it to install a decoder? Is it worth it? Decoder installs can be grouped into 2 basic categories DCC ready or not. Each requires a different approach and level of effort to complete. The first and most basic is to install a DCC decoder into a DCC Ready locomotive. Most commercially available locomotive models available today are DCC ready, including O, HO and N scale models. The principle feature of DCC ready is the motor is isolated from the frame and a wiring connector is provided that bring all the necessary decoder connections to one point on the model. These are equipped with either an 8-pin NMRA (more common) or a 9-pin JST (less common) connector. All major decoder manufacturers offer decoders pre-equipped with the appropriate mating connector. Older locomotives were designed to run off DC and generally had their motors directly attached to the frames. This includes both LIGHTING WITH DCC In addition to independent train operation and excellent motor control, DCC offers first rate lighting and lighting effects on locomotives and cars. Because the decoder is microprocessor controlled, and each output is independently controlled, exquisite lighting effects can be generated. These range from simple on/offcontrol of lights to special lighting options, including Gyralights, ditch lights, Mars lights, strobe lights, oscillating head lights, dynamo, Rule 17 dimming, dim-bright- offcycle, cabin lights and more. Decoders will generally have a listing of the lighting effects provided. Lighting effects are generally controlled by setting certain CVs to obtain the desired effects. CVs also control whether or not the light operates when going forward or reverse, or when it is stopped. Some effects are also interlocked with other functions. For example, ditch light effects are often interlocked with the horn function to create a back-and-forth lighting effect for a few seconds after blowing the horn. Most decoders can accept both LED and incandescent bulbs, and in general will provide roughly 14 VDC to the lights. Keep in mind LEDs are current limited and will require current limiting resistors to protect them. Many micro bulbs are rated at 1.5 volts and will also require resistors to operate. Some are provided with special voltage outputs custom tailored to certain lamps and manufacturers. A common feature is to provide 1.5 VDC in addition to the 14 VDC output. This allows direct connection of micro bulbs to the decoder without worrying about resistors. Lighting functions allow independent control of headlights, marker lamps and more for added realism. Photo by Bob Gallegos

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