What I have learned is you really want to know is what the current could be when connected to a new DCC power source. In other words, you really need to know the motor Resistance. Measure it this way. Place an ammeter in series with the test track, and hook up a volt meter in parallel to the test track. Turn all lights, aux equip(sound,smoke generators)OFF. Run the loco, at 6-8 volts or so. ( Smooth DC but not critical). #1. Hold the loco so that the drive wheels slip.(Do Not Press Down). Record the amperes. That is the 'Irun'. This is the maximum average current demand while running. A DCC decoder must be size selected to provide these amperes continuously. #2. Press down on the loco to temporarily lock the drive wheels. (Do this quickly to avoid overheating and damage)Record both the voltage and amperage. (Do it several times and average the data). Now use this formula R(esistance)=V(olts)divided by A(mps). Next, calculate Peak current on a DCC supply as Ipeak=12V divided by R in ohms. You will now have 2 numbers; the Irun and Ipeak. I used 12V for HO/N scale you could use 14 to 16V for S/O/G scale. A DCC decoder needs to be able to supply these peak amperes without damage. Most DCC decoder manufactures will provide an Ipeak current rating.[don't rely on Irun alone, what happens if your loco stalls on a hill with a heavy train]. certain 'high performance' motors will have a very low resistance, and subsequent high peak current demand. [early proto 2000 PA's for example] If the calculated peak current is too high, consider using a decoder with a built in short circuit protection [usually more $$$] or adding a discrete resistor(R2) of several ohms in series with the motor leads to increase its effective resistance.[Be careful the resistor will get hot. It needs to have a watt rating of at least Irun*Irun*R]. I cannot tell you what the R2 value should be, that you will have to determine by how much you want to lower the current draw. Now since there are many different ways to skin a cat, I would welcome further input/suggestions. ------------------ When in doubt, empty your magazine. Member #33
Chris, Although the procedure you describe is a good starting point, I would like to make some comments : 1) I don't think it's a good idea to turn off all lights / accessories, as the current rating of a decoder includes the current draw of all functions, i.e. a decoder rated at 1 Amp is able to provide a total of 1 Amp (motor + lights + any function accessory). 2) I think the Ipeak current measured by your method should be used as Irun for selecting a decoder, as this is the current draw that the decoder will see when starting an engine. The power to the motor is chopped but always of maximum voltage (12V, for instance, for N or HO), so that when an engine is starting the decoder will see the full Ipeak current. Hope this helps. Jean-Louis Simonet visit traiN'ternet: http://www.trainternet.org/
Having never run DCC (yet), but having a good electronics background, here are some suggestions from years of burning stuff up..... Is 12 volts REALLY the maximum voltage you'll see across the motor? I've seen up to 18 volts on my DC system. Look at the maximum voltage your system can send to the motor and use that to calculate the maximum current draw. This will not be the maximum short-term current, but the running current. Also include all lights being turned on at the same time. What's the maximum peak current on your system that the decoder will see? If you quickly switch from one direction to another while the motor is still turning you'll draw MORE than the stalled rotor current. The reason is that the motor is not generating back EMF to hold the current back, but is instead generating forward EMF to draw even more current through the motor. Picking a decoder that can handle more current than the locked rotor current is a necessity. You'll burn it up otherwise. The decoder will fail from over-current conditions OR overheating. It you wrap the decoder all up in heat shrink or tape you have to de-rate it. Remember to install it so that it can keep cool. I know it won't be easy, but it's another one of those things you have to plan for. My solution? Pick a good & beefy decoder, don't seal it all up in tape, and have a good time! If you burn one up, get an even beefier one and do it again! Mark
Mark, I picked 12 volts as that is a nominal amount that most systems run on. DCC is different than DC, my system max voltage is 14V, most DCC systems are able to regulate the voltage to the tracks. I can go from 11V to 18V, and with the use of diodes I can even change that. a question: why wrap the decoder in tape, I just use a little insulating glue or if there's room just let it hang loose in the tender. ------------------ When in doubt, empty your magazine. Member #33
The current rating of the decoder, or any other electronic equipment is partially based upon its ability to cool itself off in free air. Any material placed around the decoder that will thermally insulate it will cause it to operate hotter, which will reduce the rating of the unit and/or contribute to its early demise. Tape, RTV adhesive, or Goop will all add insulation to the unit and make it operate hotter. It's a lousy thing to hear, isn't it? Hence, get one with a higher rating than "required" just to make sure that you'll have enough "real" capacity in the decoder. Mark
Mark_Athay, All the decoders that I know of are designed to work under these conditions (i.E. wrapped under their own wrapping - except some Lenz decoders). As they are specified to work this way, I don't see what the problem is. Jean-Louis Simonet visite traiN'ternet: http://www.trainternet.org/
I'm going to have to agree with JLS on this one Mark. I've got about 7 engines with decoders using the RTV method and I've had no problems nor does the decoder or tender seem hot. ------------------ When in doubt, empty your magazine. Member #33
Because the run-of-the-mill digital multimeter cannot accurately measure the amperage of the DCC square wave, you must get a Rampmeter or build something like it. http://dccspecialties.com/products/pdf/RRampMeter-Application-Notes.pdf
If you know how to solder and follow instructions, you can build the below DCC amp meter. I have built two. The electronics on the PC board convert the DCC to DC for the meter which is set to the 20ma scale. http://home.cogeco.ca/~rpaisley4/DCCammeter10.html This guy uses the below meter. He just does not show the meter. He use to. I have three of these meters and they read about 13.7 VAC on out club NCE Power Pro system when we need a voltage reading. I would buy at least two and maybe a third for the car which is in the 12 volt range. http://www.harborfreight.com/7-function-multimeter-98025.html Get some clip leads also. http://www.harborfreight.com/18-inch-low-voltage-multi-colored-test-leads-66717.html The only ones who ddon;t like the amp meter are those who have never used it. Rich
Motor current It is so much easier to put a multimeter in series with one lead to the motor and measure DC current at 12 VDC. That is what everyone does. I lightly touch the drivers for a little more more load. Many do that. Some lock to motor which can strip gears if done wrong. I keep a charged 12 volt get cell rechargeable battery for this purpose. This style battery is used in many cheap battery back ups for PC's. I picked up a couple surplus for various projects. Rich
