DCC Inrush Current Problem

Background of the problem (July 28th, 2004)

Sound equipped locomotives have presented challenges to DCC that were not anticipated when the NMRA specification were written. The problem is the amount of capacitance that needs to be charged up to allow the sound electronics to function reliably with various types of DC power that is NOT pure DC. The capacitor charge current is a huge spike involving amp levels that far exceed the current capability of the both boosters and Circuit breakers. But it does not take long to charge. However, every time you add another sound equipped engine, the problem grows in size to a point it will become a major problem.

Prior to BLI (Broadway Limited Industries), sound equipped locomotives were few and quite a show item initially since it involved a lot a work to install a sound system. (But what head turners the sound units were!) Anyway, the problem existed but showed up at more of an annoyance level issue.

When BLI came on the scene, things changed quickly. Engines with sound became "Ready To Run" along with good quality construction allowed people to acquire more sound equipped engine faster than ever. Today most BLI customer come back for more and having many sound equipped engines on the layout has now become common place. Correspondingly, the problem is has now become a BIG issue.

Here is how the problem happens.

Electronic based Circuit breakers use Current Level and Time Duration to determined the difference between a normal momentary short circuit (normal stuff rolling down the track) versus a real short (caused by a derailment) where the short current can be sustained indefinitely.

Sound decoders have BIG capacitors in them that are used to store power to keep the sound going un-interrupted as the engine roles down the track make less than perfect electrical power pickup at all times. These capacitors must be charged up BEFORE the sound system will work. When they are first charged up, they look like a short to booster or circuit breaker. The short circuit current level fades quickly with time for it only momentary. The current goes to zero when the cap is fully charged up.

If the capacitor current fades fast enough below the short circuit trip level before the circuit break decides it is time to kill power, then everything works like you expect. No problem. If the current trip level is lowered or reduced, then the exact same capacitor current will not fade fast enough to clear and the circuit breaker will trip.

Adding more sound equipped locomotives is the same as adding more capacitors in parallel. Depending on your electronic circuit breakers setting and the peak current capability of your booster, people will get various degrees of success and failure with combinations of locomotives. The layout wiring also plays a part here too. So there are lot of variables involved on the layout side. What fails to function on layout A may work just fine on layout B.

Poor Mans DCC Circuit breaker: The Automotive light bulb.

Wiring a suitably rated light bulb with each track block solution works because it adds resistance in series with the engines which in turn limits or clamps the peak current value to a lower level. The down side with the light bulb is that if you have a lot of engines on the same section, the track voltage will drop a lot as the light bulb starts to glow. The engines will not run well.

Technical Discussion and discussion of possible solution at the Manufacture's end...

The amount of capacitance to put into a sound decoder will have a minimum and maximum requirement.

The minimum capacitance dictated for the circuit are typically concerns that are covered by the datasheet of the parts involved or an engineers experience with the circuits involved. But all of these specifications assume the power is clean and un-interrupted (Pure DC). In this case, we put the sound system on wheels with intermittent contact which changes the capacitance requirement to work beyond the minimum. The maximum, however, is determined by the decoder sound system designer through actual real world testing to handle the intermittent contact situation. Large capacitance capacitors will store more energy and keep the sound going through longer durations of power dropouts or dirtier track so to speak. Unfortunately there is no standard for "intermittent contact" in terms of time and strength. Another factor determining capacitor size is the requirement to have the sound unit work with old DC power. Many DC power packs have pulse power or less than pure DC power that was targeted specifically for motor control and not to run electronics. A BIG capacitor is again needed to filter out all those pulses to allow generation of enough pure DC to run the electronics. So the solution is all over the map depending on who did the design and what the design goals are. At the same time cost and space issue may become a factor in deciding what to do.

Soundtraxx discovered with the DSD sound decoders that there was not enough capacitance to make all the customers happy. Yet the size of the decoder was a big concern since not every would have the space to fit a large capacitor if it was factory installed. Although they never updated the DSD design (now that Tsunami DSD is to replace it), the did address it with the DSX by allowing one to optionally add extra capacitance. There is a technical note about how to do just this on the Soundtraxx website. Since decoder size is a Soundtraxx feature, having enough capacitance is a tough issue to address. I think the DSX approach is good idea that they should keep in the Tsunami product when it comes out.

QSI, which does the sound for BLI and Lionel and perhaps others, has the luxury of making decoders that are specifically design to fit in space provided by the locomotive from the day the locomotive design was started. Since the sound unit is guaranteed to fit, size is less of an issue and unlike Soundtraxx, can use less expensive and bulkier components. Their boards reflect just that design and thinking. They are huge compared to Soundtraxx boards.

Electrical Discussion

From an electrical standpoint, there are two parameters that determine the effective short circuit current level and durations.

1) The circuit impedance. Using Ohms law, V = I x R and re-writing it to I = V/R we can see that there is a direct relationship between the current, track voltage and circuit resistance. If the resistance goes down, the current goes up. The resistance is all the resistance in the complete loop of current flow from the booster to the track to the sound board through the cap back out all the way back to the booster. Typically this resistance is less than 2 ohms and typical track voltage is 14.5V. So the maximum current is really limited by what the booster will provide. So clearly every time the cap charges up, we WILL hit the booster current limit.

Part of that resistance is the resistance inside the cap which is called ESR or Equivalent Series Resistance. Its resistance value can be high relative to the layout wiring resistance. High performance caps will have low ESR and cheap caps will have high ESR. Low ESR will result in High peak Current Flow into the cap. High ESR will reduce or limit the peak current to a lower value. So the choice of cap can also effect the peak current value.

2) The capacitance value of the capacitor(s). Simply put, the more capacitance you have, the more energy you can store. Its a bigger rechargeable battery so to speak! That also means if the current available to charge up the cap is limited, the longer in time it will take to charge up to full.

So the worse thing to have is a low ESR cap with high capacitance. It will draw high current and sustain that high current for a long time. Just what the Circuit Breaker is looking for to shutdown. For a given size, cheaper caps will have higher ESR and Store less energy. So there are cost versus Size versus performance tradeoffs that must be made. The total capacitor solution will then vary with the application requirements.

The thing that bothers me is that there is a simple solution the high current cap charge inrush current. First install the minimum capacitance needed by the parts on the board. Then add the extra big capacitance for intermittent power holdup in parallel with the small cap but with a circuit change. Put a combination diode + resistor in series with the big cap with the diode in parallel with the resistor. The Resistor will limit the peak current or power to a safe level when charging up that is well below any trip limit. The diode allows the cap to bypass the current limiting resistor and provide full power to keep the sound unit running when power is momentarily lost. It cheap, small and simple to do. I hope the Soundtraxx Tsunami has that or fixes the problem some other way.

If the NMRA DCC body was to do something, it would be to define a inrush current profile that all booster and circuit breakers must pass and to recommend the circuit in question be designed to minimize the inrush current below this inrush current profile as best as possible.