Measuring Track Voltage
Summary: Measuring track voltage is a different process than one used for analog control systems
- 1 Measuring DCC Voltages
- 2 Adjusting the Track Voltage
- 3 Notes
Measuring DCC Voltages
There are several ways of measuring the voltage on the track when Digital Command Control is used. They range fron simple and inexpensive to expensive tools.
The oscilloscope is the best method. Not only can you observe the amplitude of the DCC waveform on the track, you can also visually assess the quality of the signal. A distorted signal will be readily apparent. With the addition of current clamps, the amperage being drawn can also be measured.
While professional bench top 'scopes are not cheap, in recent years low cost scopes with LCD displays and a number of sophisticated functions have appeared below $1000. Some can be found for as little as $500. Tektronix has also entered the budget scope market, with some excellent products. While Tek is known to be one of the best, there are other brands that come close to equaling Tek's reputation. Where Tek scopes excel is their patented input circuitry, which gives far better results than low cost budget scopes.
Of course, there are a number of low cost options available for smart devices. Several "pocket scopes" are on the market which use an iPhone or other device as their display. For DCC measurements, they are adequate, and can often be found for less than $200.
Making Measurements with an Oscilloscope
Using a scope to measure track voltage is rather straightforward. Assuming you know how to use the scope, a 10:1 probe is connected to one rail, and the ground clip onto the other. After a few adjustments, the display will have a trace of the waveform. Most modern devices have built in math functions, so the display will probably include voltages and frequencies present. The 10:1 probe should be trimmed first to ensure you get an accurate trace. The scope should provide a reference signal to trim the probe for the correct wave shape.
Remember: the scope's reference point moves as the DCC signal changes. The trace is misleading as it isn't really 28Vpp, but 14Vp on each rail.
A simple analog or digital multimeter can be used to measure track voltage. This is a little more involved due to the nature of the DCC signal.
Method 1: Rail to Chassis Ground
This is the method recommended by Digitrax.
The measurements will be made between one rail and a common reference point. On a Digitrax booster, there is a "GND" or ground terminal which is used as the reference. It is also possible to use the metal case/chassis as the reference point.
Measurements are made using the DC Volts (DCV) mode, on the 20VDC range, if the meter does not autorange.
- Measure from Rail A to Ground. Record this value
- Ignore the sign, as DCC has no polarity.
- Measure from Rail B to Ground. Record this value
- The track voltage is the sum of the values found on Rails A and B
For Power Cab owners, NCE recommends measuring the output of the Power Cab using an AC Voltmeter. The measurement is made at the back of the Power Cab Panel (PCP) and should read approximately 13.8VAC. NCE does caution that getting accurate readings with any meter will be difficult.
|Scale||Recommended Track Voltage, Volts|
NMRA Recommended Voltages by Scale
The table shows the NMRA recommendations for voltage. There will be some variations in the measured voltage. Other sources may indicate slightly different voltages. Most decoders can handle much more than the nominal voltage before damage occurs.
According to the table, an HO scale layout should have 14V on the track. Using the above method, the readings should be 7V between each rail and the reference point, for a total of 14V. Digitrax recommends 15V, with a tolerance of 0.6V. The manufacturer of your DCC system may have a different recommendation.
Why Does the Meter Read Half of the Voltage
The reason lies in the periodic nature of the DCC Waveform.
The voltage on the rail switches on and off, creating pulses. The On time is equal to the period the voltage is Off. For an analog DC Voltmeter, this results in only 50% of the torque needed to move the needle. For a digital meter, it will have an equal number of samples for the on and off periods, which when averaged, result in a reading of half the actual voltage.
Why will a meter on DCV Measure Zero Volts between the Rails?
Again, the answer lies in the periodic nature of the DCC signal.
When measuring voltage between the rails using the DC Volts scale, the average voltage is Zero. A DC Voltmeter expects the current to always flow in the same direction. Since the current flows from A to B, then B to A, the rapidly reversing currents seen by the meter do not create enough torque to move the needle, or in the case of a DMM, it is displaying the sum of the two, which is zero. Unlike the AC Volts function, where the current is expected to reverse direction on a periodic basis.
Adjusting the Track Voltage
Should adjustments be necessary, such as matching the amplitudes between boosters, consult the manufacturer of the DCC system.
- DCC has phase, not polarity. If positive and negative voltages were present on the rail, using the Digitrax method results in a reading of Zero Volts on the DC range.
- Phase is relative: With DCC, one rail is always the inverse of the other: If Rail A is ON, Rail B is OFF.
- When Rail A is ON, current flows from A → B, and when the rails are inverted, A ← B
- Ground does not refer to earth ground, it is just a common reference point for all measurements. Your DCC system should float.