Meters for DCC.

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There are many questions asked by modellers interested in connecting DCC including: What is my DCC system voltage? How do I troubleshoot a voltage problem? How many locos can I operate before I need a second booster? How much current does a sound loco need? etc.

Explained below are some procedures that I have used to measure DCC voltage and current, with the equipment we already have and purchasing a couple of cheap components, measuring voltage and current can be possible accurately enough for our model railroads without purchasing the expensive specialist DCC meters shown below.

Measuring Voltage.

The DCC track voltage is 5,000 to 8,500 Hz (depending on how many “0s” and “1s” are in the packets) square wave A.C and is nominally 14.25 volts for HO. See the NMRA Standards for more details.

The electronics of a normal multimeter selected to A.C. assumes that the A. C. voltage being measured is the mains voltage for our home A.C. voltage supply that is 50 Hz in Australia and 60 Hz in the U.S. and the waveform is a sine wave. (DCC is a square wave). The meter reading is converted with the factors taken into consideration. Reading A.C. voltages of any other frequency and waveform, these “correction factors” will give an incorrect reading. Using an RMS Voltmeter will give a more accurate reading.

The need for measuring DCC track voltage.

To know the DCC track voltage is something nice, but not absolutely necessary. If the booster voltage cannot be adjusted, then what can be done. For each requirement that needs a voltage measurement, a “work around” can be used, so that these expensive specialist meters, are not necessary.

One of the great features of DCC are headlights and many other lights that the prototype had, can be installed in our models and operated just like they were on the prototype using the function outputs and the desired effect that decoder manufacturers provide, but using incandescent lights will need the operator to know what voltage is applied to the lamp, so that the lamp illuminates sufficiently. This is even more important when using 1.5 incandescent lamps that are in many of our locos now as standard.

Systems that provide high voltage to the track are easily recognised, by the extra bright headlights on the RTR headlights.

For the “best effect”, a sufficiently illuminated incandescent lamp is necessary. To achieve this, a voltage dropping resistor will be necessary with 1.5 volt incandescents, that are installed in later model locos. The decoder “function voltage” should measured to determine the correct voltage dropping resistor. See below.

that loco will run very fast at top speed and incandescent headlights are very bright, often blowing. See the section on “Checking Decoder Function Voltage” below to determine if you have this scenario.

Accurately measuring the DCC track voltage is really only required when a layout has more than one booster. These boosters should have similar voltages for trouble free operation of the layout.

To trouble shoot a “voltage drop” problem typically indicated by a loco slow running, loss of performance etc at a certain track section, you can use a meter selected to A.C. volts as described below.

Specialist DCC Voltmeters for our DCC layouts.

The two most popular units available are:

• Tony’s RRampmeter shown at left, a combination voltmeter ammeter with 3 versions available. The one shown is about US\$50.

• Pricom’s DCC Pocket Tester shown at the right. This tester is a full feature DCC system analyser that displays the DCC layout’s “health” that also has a DCC voltage selection. Two versions are available and can be bought from The Model Railroad Craftsman in Sydney starting from about \$130

Alternatives for measuring DCC voltage.

An accurate measurement can be made on some DCC systems by using a DC voltmeter. Connect one lead to either track and the other lead to the “earth screw” on the booster. This will show ½ the DCC volts. Multiply by two and this is your DCC track voltage. Not all systems have this “case” connection. Use this method or a specialist DCC meter to adjust, if possible, the DCC system track voltage.

Making a Voltmeter.

See Allan Gartner’s “Wiring for DCC” site for using a bridge rectifier and a DC voltmeter to measure DCC voltage.

DCC voltage adaptor. These can be made for about \$5 - 10 from a circuit at NCE Files at Yahoo groups

Photo left shows one assembled onto a piece of Vero board and mounted onto the meter with banana plugs and sockets for easy removal for other meter readings.

Troubleshooting a DCC layout.

This can be done using a digital multimeter selected to A.C. Volts. My four digital multimeters all show in the range of 13.5 to 14.2 volts A.C. while my DCC Pocket Tester shows 13.85 volts DCC. I have seen other digital multimeter displays showing from as low as 9.0 volts to as high as 22.0 volts AC. The Jaycar yellow QM-1500 at \$10 in the photo below, shows 13.8 volts A.C.

The normal (not a true RMS meter) digital multimeter is “set up” to read 50/60 Hz sine wave (domestic 240 volt supply) voltage and the way the meter samples the DCC square waveform, gives these variations. Check to see what your digital multimeter shows but don’t use this as an accurate measurement, of say, adjusting your DCC system voltage. Use it as a “guide” only and for comparison measurements.

To troubleshoot a problem that has caused a reduction in loco performance, take two measurements, one at the booster and the other at the problem area BUT you must have a load, otherwise the voltage drop will NOT be located. A 21 to 27 watt (1156 lamp) connected “across” the meter leads, will provide the necessary 1.5 Amp load. Compare these readings and if greater than about 2.0 to 3.0 volts (with 1.5 amps load), then there is an excessive voltage drop normally caused by a bad connection or poor wiring.

Voltage drops only become evident with current flowing in the circuit. The higher the current, the higher the produced voltage drops. All wiring creates voltage drops and is dependant on wiring length and size. Wiring with thicker wire reduces this drop.

As shown in the photo, I made an adaptor to mount the lamp and painted it black to reduce the glare (for the photo). I have a part of the layout where trains with triple headed locos slow down. I connected the meter and this is why, 9.9 volts AC compared to 13 volts AC at the booster. All DCC systems will have different values but there will be similar voltage variations, indicating a fault. Note: some entry level systems provide more that 22 volts DCC to the track. See AMRM Aug 04 issue.

Decoder Function Voltage measurement. See also. Measure the function output (blue wire) of the decoder and the white or yellow wire (negative) and select the function on. This DC voltage will be about 0.5 to 1.0 volt less than the DCC track voltage. See AMRM Dec 05 issue. This method is suitable for systems that don’t have the “earth screw” case connection as detailed above.

Measuring Current:

On DCC layouts, all the locos, coach lighting and power bus connected accessories on the whole layout are powered by the booster, not like in DC where each loco is powered by a separate cab with its associated transformer. How many locos can I operate till I need a second booster is a common question. This has been explained in many books using 1.0 amp per loco for the calculation. Thus a 5.0 amp system will only be able to operate about 5 locos. This is an unreasonable calculation if using locos with the later type of can motors.

Connecting a DCC ammeter permanently will indicate how much current is being used by the layout at all times. Only when the current reaches the booster’s capacity, will it be required to add a second booster. On my club layout with 8 operators running trains with 10 – 12 locos, some sound units, the ammeter shows about 1.5 to 2.0 amps all the time. Plenty of headroom from our 5.0 amp booster. Remember, during an operating session, at one particular instance in time, not all locos are running. My “current” measurements for per loco are: idle 30 – 50 mAs, running non sound 150 – 300 mAs and sound 200 – 400 mAs. Illuminated passenger coaches and guards vans, depends on how many lamps and types. A coach with five 40 mA lamps would draw 200 mAs, similar to a loco. A train comprising 1 loco and 6 lighted passenger coaches could draw approximately 1.5 amps. With all the operating variations an ammeter is an ideal way to demystify the “how many locos can I run” question. Ammeters show “real time” readings.

For all but the largest layouts, a single 3.0 to 5.0 amp booster will be sufficient to supply all the power initially. Expanding the layout and adding accessories will obviously require more power but having the ammeter, will tell you when. I consider the ammeter to be essential, for trouble free operation, plus you learn about your layout with it. The photo left shows the bridge rectifier connected to the booster. The current draw shown is for three Soundtraxx DSD-100LC sound equipped locos, triple headed (consisted), hauling a train of 25 bogie wagons. The 12V21W lamp I use for power division is starting to glow at this current demand and the track voltage is 10.6 volts DCC. The lamp and the bridge rectifier reduce the track voltage. With some DCC systems, like my NCE, you could adjust the voltage to compensate for this voltage drop. Some modellers have recommended operating on lower voltage than the 14.25 volts DCC. They have reported better control, have had less trouble and this lower voltage will keep the decoders cooler, especially sound units. There is a lower limit and by introducing bridge rectifiers for the ammeter and 12V21W lamps for power division (a later DCC Workshop topic) will reduce this even further. Each modeller’s own setup will determine if he needs to or can adjust the DCC voltage for these additional items and whether he needs to.

Making an Ammeter:

Referring to the below diagram and photos, all that is required, is a 6 amp bridge rectifier, Jaycar part number ZR-1314  at \$1.80 or four fast acting 6 amp diodes and a suitable meter. If fitting a permanent ammeter a 10 Amp digital multimeter, Jaycar part number QM-1500 at \$10. Total cost of the permanently installed ammeter - \$11.80, real cheap. The bridge rectifier or diodes will reduce the DCC voltage to the track by about 1.5 volts. I have added a SPST switch to bypass the ammeter when not utilising the meter, so as to give maximum voltage to the layout.

On the right are all the parts for making an ammeter, the square 6 amp bridge rectifier or 4 very fast 6 amp diodes and a switch. The 5 Amp panel meter has the bridge rectifier mounted on the rear with the bypass switch mounted on the top.

Meters can be mounted behind the layout facias in some suitable location.

Fitting an ammeter to your layout.

The schematic below shows how to construct the Simple Ammeter. The bridge rectifier can be inserted in EITHER track feeder. The bridge rectifier reduces the track voltage by approximately 1.0 to 1.5 volts, so a Bypass Switch is inserted for when the Ammeter is not required, if you want the maximum voltage to the track. Some DCC systems allow the track voltage to be adjusted, so this voltage drop could be compensated for.

To DCC for Novices