This page shows several methods of measuring DCC system voltages and currents. Also shown is an attempt to explain the voltage waveforms associated with model railroad DCC systems as they would be seen with an oscilloscope.
This page shows methods of indicating currents in H-Bridge circuits as used for model railroad DCC systems. Each method has advantages and disadvantages.
The following diagram shows a very simple schematic of an H-Bridge circuit with current indicating meters placed at various locations.
Text on the diagram indicates the type of meter and any special considerations involved.
Meter #1 - Is probably the easiest and least expensive to implement. The placement of a meter at this position will also indicate if there is a problem with the bridge itself when the load is disconnected from the booster.
Meter #2 - Is undesirable due to the drop across the diodes in the bridge that will reduce the track voltage by up to two volts.
Meter #3 - Is undesirable due to the considerably higher cost of meters that can accurately measure currents at the the operating frequencies of DCC systems.
Meter #4 - Is specific to the LMD18200 device. This meter can be taken out of the circuit without affecting the operation of the booster thus allowing a multimeter to be used when a problem is suspected. (Other motor control H-bridges may also have this feature.)
These are inexpensive "low tech" methods for measuring voltages and currents in DCC track systems. They are not as accurate as a high frequency AC volt and amp meters but if you only need an approximate indication these circuits will do the job.
The 1N5819 and 1N5820 diodes used here are Schottky types. This type of diode have a lower forward voltage drop (0.3V vs. 0.7V) than typical silicon diodes such as the 1N4000 series and can operate at efficiently at high frequencies.
NOTE: In the following section, for the purposes of illustration, all waveforms have a fixed duty cycle of 50 percent. No attempt has been made to discuss BIT widths or ZERO stretching as this is not within the scope of this page.
The DC power supply voltage used in the diagrams is 14 volts. The voltage drop that would normally occur across the transistors of a DCC booster's H-Bridge has been ignored.
The first diagram illustrates the basic function of full and half H-Bridge circuits as used for model railroad DCC booster systems.
The next schematic shows a Full Wave - DCC H-Bridge circuit in a very basic form. The DPDT reversing switch has been replaced by four transistors connected in pairs that are alternately switched ON and OFF.
Oscilloscopes have been added to the circuit in order to show what the waveforms would look like at various points in the system. Illustrations explaining the waveforms follow this diagram.
The next two diagrams show the voltages displayed on the oscilloscopes connected to the circuit.
The duty cycle has been fixed at 50 percent for illustration purposes as the normal DCC waveform is too variable to illustrate clearly.
The first diagram is for oscilloscopes A and B in the basic H-Bridge circuit shown above. The measurements are referenced to the common of the power supply for the bridge.
The voltages are equal in value but are 180 degrees out of phase.
The next diagram shows the voltage seen by the oscilloscope when connected across the output terminals of the bridge.
This voltage appears as a square wave that is twice the voltage of the power supply.
NOTE - At any given time, the instantaneous output voltage is never greater than the supply voltage to the bridge (14 Volts) even though the waveform appears as 28 Volts Peak to Peak.
The next schematic shows a Half DCC H-Bridge circuit in a very basic form. The reversing switch has now been replaced by two transistors that are alternately switched ON and OFF.
Oscilloscopes A and B are not relevant to this circuit as the output of the bridge is actually referenced to the common of the power supply.
The waveform for the "C" oscilloscope above does apply to this circuit and illustrates why a 28 volt signal appears at the output while the instantaneous voltage does not exceed one half of the maximum supply voltage.
The next diagram shows the effect of rectifying a DCC voltage with a full wave bridge. The voltage at the output of the bridge is equal to the input minus the drop across the diodes.
The small tick in the output of the bridge occurs when the polarity of the H-Bridge reverses. The size of the ticks is largely determined by the efficiencies of the H-Bridge and rectifier bridge and in most cases can be ignored. This is why filter capacitors are not needed for decoders.
Diodes used to rectify DCC should be Schottky or high speed silicon types. Diodes of the 1N4000 and 1N5400 series are not suitable for this application due to their slow turn-off times.
The next diagram shows a very simplistic explanation to Zero Stretching as it applies to DCC systems.
As in the above diagrams the rate at which the voltage changes polarities has not been considered. The important factor in Zero Stretching is the average time that the voltage to the track is either positive or negative.
The explanations for the circuits on these pages cannot hope to cover every situation on every layout. For this reason be prepared to do some experimenting to get the results you want. This is especially true of circuits such as the "Across Track Infrared Detection" circuits and any other circuit that relies on other than direct electronic inputs, such as switches.
If you use any of these circuit ideas, ask your parts supplier for a copy of the manufacturers data sheets for any components that you have not used before. These sheets contain a wealth of data and circuit design information that no electronic or print article could approach and will save time and perhaps damage to the components themselves. These data sheets can often be found on the web site of the device manufacturers.
Although the circuits are functional the pages are not meant to be full descriptions of each circuit but rather as guides for adapting them for use by others. If you have any questions or comments please send them to the email address on the Circuit Index page.
09 February, 2012