Decoders – Definitions Selection and Installations.

To fit a decoder into a loco, we have to determine a few factors namely, specific or universal decoder, size, current rating, features and how many functions. I will start with a few definitions of the available features.

Decoder - Specific or universal: Manufacturers have made specific decoders that fit in place of the light board in some popular locomotives. Some locos come with a DCC ready socket. The loco will be advertised as “DCC ready” Check with your dealer, this will save you from having to determine decoder criteria. If a specific decoder is not available, you will have to choose a general purpose decoder with the features you would like.

Size: The space available will determine the size of the decoder. Smaller decoders generally cost more.  In HO you can use N scale if the current rating is sufficient. Almost all decoders have a 1.0 rating, even “N” units.

Current Rating (capacity): This is the single most important requirement. Can decoder handle loco current? Larger scales locos require more current. See below for determination of loco current.

Silent, Quiet or Supersonic drive: All manufacturers provide this feature now. The decoder controls the motor speed by adjusting the current to the motor. It does this by altering the width of the square wave pulse to the motor, wider pulse, higher effective voltage and thus higher speed. This is called Pulse Width Modulation, and with the early types of decoders the frequency was lower, and it caused motors to hum, later decoders use a higher frequency above the audible range, hence we cannot hear the hum and hence this terminology. Without this feature, mechanisms are noisy. Different mechanisms react differently. Recommend this function.

Dither, load compensation etc is an extra feature that enables loco to operate at really low speed. This is done by altering frequency and amplitude even further to the current to motor. This effect overcomes mechanism resistance and inertia. Only is effective in first 1/3 of speed range.
BEMF is a form of control that enables the loco to remain at the same speed, once set, irrespective of a changing load, eg. When going up a grade, maintains same speed. A form of "cruise control". 
14/28/128 speed steps are the amount of different speeds (steps) from minimum to maximum speed. In 14 speed steps there are only 14 possible speeds at approx 1 volt increments and depending on mechanism this could cause jerky low speed operation. Use 28 or 128 for much better control.
Speed tables The way a decoder controls the speed of the motor can be altered in each decoder, either by user selectable tables or by adjusting CVs 2 – volts Start, 5 – volts max and 6 – volts med, to give desired loco performance, Speed tables are produced by altering speed steps, CVs 67 to 94. Adjusting CV 2 enables loco to start moving at speed step 1. Adjusting speed tables, enables loco performance to be matched with all locos irrespective of the type of mechanism. Particularly useful if locos are double/triple headed or banking.

Uploadable Speed Tables With some decoders you can create speed tables using a computer and a programming utility like Decoder Pro. Great method of storing CV values. Not available with all DCC systems.
Function Outputs:  This can be from the basic 2 to as many as 6 different outputs that can be independently controlled, normally lights. See decoder manual, Typically 40 to 100 mA.
Lighting Effects - Headlights front and rear will require 2 functions and depending on what prototype you model will determine what lighting effects you use, e.g. reversing and dimmable. For American prototype modellers there are Rule 17, mars, strobe, beacon, flicker, ditch etc that enhance the loco appearance.

Lights : Can be either incandescent 12 volt or 1.5 volt micro lamps or LEDs. You will have to determine what types are fitted to existing loco and if 12 volt no changes have to be made. If 1.5 volt or LEDs you will have to fit the required series dropping resistor. See decoder's instruction. Decoder will blow 1.5 volt lights if dropping resistor is not used. I prefer to remove original lights and fit “Golden” (no blue tinge) LEDs for best effect.

Two or four digit addressing. Four digit allows you to use the locos four digit loco number as its address, easy to remember. If using two digit, you have set some rule for 4 digit loco numbers eg first or last two digits. With five digits as with 42107 you will have to make arrangements here. Advanced decoders have 3 addresses, 2 and 4 digit and consist address. Some systems only support two digit addressing.

Programming: It is not as complicated as it sounds, for something as simple as adjusting CVs to modify decoder’s characteristics. "On the main" or operations mode is for easy adjustment of CVs while running loco (no read back of CVs) and service mode (paged, direct & register etc) on the program track. In this mode you can read decoder CVs actual values. Not all decoders and systems support all types of programming.

Smoke unit: If this is installed you can turn on and off by a relay controlled by a function output and switching track power, so this current does not go through decoder, so it does not have to included in decoder “current” requirements.

Sound decoders:  These are available complete with motor control or they can be separate units providing just the sound. Easier if sound decoder has motor control, requires less space, example the Soundtraxx DSD100LC.

Determining Decoder Current

How much current does the loco draw? This can be determined by using a DC throttle and running loco at full throttle with a maximum load and monitoring the current. Use a multi meter set to DC 10A in series with one track lead. This reading is "operating" current. Lock the wheels or hold flywheel to prevent them from turning, this is the "stall" current. Manufacturers refer to this as "peak". Lenz does not use peak current. Lenz has stated at http://www.lenz.com/products/decoders/index.htm  "While the stall current is an important number if the locomotive motor can actually stall, we have found that most modern locomotives do not stall.  Instead what is important is how much load the motor will place on the decoder during its maximum load. To determine the maximum continuous draw of a locomotive, run the locomotive at 16volts DC and hold back the locomotive as you would if you were cleaning the wheels.  The current draw measured is the maximum continuous current draw.  In general HO locomotives draw less than 1 amp continuous and N scale locomotives draw less the  0.5 amps continuous."

Decoders now in 2005, have really improved with motor control, more features, smaller size and lower cost. With the use of can motors in models in the last 10 - 15 years, these currents are less than 1.0 amp. If you have an open frame or older motors these may draw up to 3 amps. Check with stall test as above. Decoder manufacturers know what sort of motors are in our locos and have designed the decoders accordingly and the ratio between operating and peak (stalled) current has reduced significantly, so the decoder will be able to handle all conditions. Most decoders made these days have short circuit and overload protection.

Below table shows the locos that I have measured the currents for max load running and stall currents. In most cases I have used a general purpose HO decoder with a 1 - 1.3 A operating current with a 2 A peak. I could have opted

Model	Max Load	Stall
DJH 53	.3	.8
Lima 38	.4	.7
Austrains 36	.4	1.25
Lloyds 30	.2	.9
K & M 48	.35	.8
AR Kits 45	.35	1.4
Mains West 44	.65	1.2
K & M 442	.5	1.4

for smaller decoder in most cases. In one 48, I used an N scale 1 amp decoder. Ultra small decoders cost more.

Once decoder current is determined, then decide how many functions and what lighting effects. Most decoders produced in the last year support most of the above features. The best way to decide, use one of the below units in an easy installation and program CVs and try to understand all features, you will be surprised what you learn.

General Purpose Decoders for HO and N scales We can safely fit 1 to 1.5 Amp decoders in HO and 1.0 Amp in N scale. If in doubt check loco current as above. HO examples are TCS T1,  NCE D13SR, Lenz LE1014 or the Digitrax DH123D. All are silent/quiet running. N examples TCS M1, NCE N12SR. You can fit both of these in HO if pushed for room but more expensive. Remember any brand decoder will run on any DCC system.

Installation:

Remove loco body and check for available space to fit decoder.

Remove minimum amount of weight if necessary to fit decoder.

Remove any noise suppression items fitted to motor terminals

Totally isolate motor from chassis of loco and use meter to check isolation.

Remove or modify existing light board and fit decoder, secure with Blu Tak.

Wire up decoder as per decoder instructions. Trim wiring for neat job.

Wire up lights if required, remembering to check for resistors if 1.5 volt lamps.

Remove decoder light wiring or stow if not connecting to lights.

Double check wiring and ensure nothing will foul the body when fitted.

Place loco (without body) on program track and attempt to program its road number as its 2 or 4 digit address. See DCC system manual.

If wiring is correct, command station will read CVs and display.

If unable to read CVs - check wiring. If all is ok, fit body and run loco.

Loco will run without programming on default address of 3.

DCC Ready Locomotives: Just plug the “Plug and Play” decoder into the socket in the loco. Note some early Austrains locos, the DCC ready socket/plug was incorrectly wired. See Austrains NSW 442, SA 700 and the early NRs. Decoder installation. Check with the dealer or DCC books or manuals.

Steam Loco Installation: The decoder is normally fitted to the tender in a tender model. This will require wiring between loco and tender and with extra tender pickups for better running, there will be 6 wires including headlight. Wiring should be very flexible and small to reduce the possibility of tender derailment, an example, 21/.09 and approx 1.1 mm in diameter or similar. With wiring from tender to loco you cannot separate units, which makes for more difficult handling. You may consider fitting a miniature plug/socket.