Dave's DCC Tips #1
How to Keep the Smoke Inside your Decoders
As we all know, DCC decoders (along with other electronic devices) work on compressed magic smoke. Once you let the magic smoke escape, the device no longer works. Sometimes the smoke is very sneaky and you do not see it escaping, but the result is the same - the device no longer works. Here are some tips to help you keep the smoke inside your expensive DCC decoders.
- Before wiring a decoder into a loco, use a multimeter on Low Ohms or Diode Test range to check that there is no connection between either motor wire and the two track pickups/loco frame, or any of the lighting wires. If you do not own/know how to use a multimeter, buy one and learn to use it before fitting decoders.
- Likewise check that there is no connection between the lighting wires and the two track pickups/loco frame. There is one special case with some locos where the lighting common terminal is connected to the loco frame/one track pickup, and in this case you must not connect the blue decoder wire to the lighting common terminal - the blue wire must stay unconnected.
- When fitted, the decoder will apply up to 12 volts across the motor terminals, or across the lighting wires between blue and white or blue and yellow. It is best to try low voltage such as from a 1.5 volt battery across these wires before fitting the decoder. Some locos use low voltage 1.5 volt globes or LEDs. In this case there is likely to be a bright glow at 1.5 volts and you either need to add series resistors (1K for LEDs, probably less for 1.5 volt globes) or use a special decoder with 1.5 volt (for globes) output, or with built-in LED resistors. If you fail to take the appropriate steps before fitting the decoder, you will briefly have very bright lights before the smoke escapes from both the lights and the decoder.
- When making the wire connections, never rely on just twisting the wires together. The connections must be soldered and properly insulated. The best method is to use heat-shrink tubing threaded over the wires before soldering. An alternative is liquid insulating tape, a paint-on product available from suppliers such as Jaycar. In this case you must apply multiple coats and ensure the coating is thick enough with no missed spots.
- Make sure you insulate any unused wire ends. I do not recommend cutting unused wires off as you may wish to add extra lights or features later.
- Never use ordinary PVC electrician's insulating tape. It will deteriorate under the heat conditions from the motor and decoder, creating a sticky unreliable mess. The only insulating tape you can safely use is high-temperature Kapton Tape, available from our own Gwydir Valley Models. Likewise you should not use ordinary double-sided foam tape as it breaks down and crumbles with heat. Use Scotch 4011 Outdoor Mounting Tape. I bought mine from a newsagent.
- Remember that the decoder will generate heat and may soften plastic parts such as the loco shell.
- If the decoder has a metal heat-sink, follow the manufacturer's advise and make sure it is able to cool correctly.
- If the decoder has heat-shrink tubing over it, do not remove this unless advised to by the manufacturer.
- Make sure no metallic parts of the decoder (tracks, solder joints, etc.) touch any metal parts of the loco. Use Kapton tape or 4011 mounting tape.
- If the loco already has an 8 pin socket, no harm will come if you accidentally reverse the plug, but the lights may not work correctly and the loco will run backwards. Rotate the plug to the correct orientation rather than trying to fudge it by setting the Reverse bit in CV 29. Take care as some locos actually have a 10 pin socket with the two extra pins used for a speaker connection. Make sure you do not accidentally plug the decoder across these extra pins.
- If your loco has a 21 pin plug, fit the decoder so that these pins go up through the printed circuit board of the decoder into the decoder socket - not the way you would expect. Note that one pin is missing and there will be a corresponding undrilled hole in the PCB for this position - this ensures you fit the decoder the right way around.
- After fitting your decoder, do not place your loco directly on the main track. Put it on the low-powered programming track first and try to read the decoder. If that fails, you quite likely have a potentially smoke-releasing wiring problem. Sort this out before putting the loco on the main track.
- Never test the motor or lights by applying voltage across them while a decoder is fitted to the loco. If you do need to test the motor or lights, you will need to disconnect at least one motor wire or two of the three lighting wires from the decoder before testing.
The precautions listed above will help you prevent the expensive magic smoke escaping from your decoders and locos.
Dave Heap © 2013