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A Brief History in DCC: Inrush Current

Posted by on Jun 25, 2015 in Tips DCC Components |

Courtesy of Mark Gurries NCE DCC Site

Sound equipped locomotives have presented challenges to DCC that were not anticipated when the NMRA specifications 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 DCC boosters.  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 circa 2000), 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). The problem existed but showed up as more of an annoyance level issue.

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

Here is how the problem happens.

Electronic based Circuit breakers use Current Level and Time Duration to determine 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, which 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 a booster. 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 breaker 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 breaker’s 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.

The minimum capacitance dictated for the circuit are typically concerns that are covered by the datasheet of the parts involved or an engineer’s 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 their early DSD sound decoders that there was not enough capacitance to make all customers happy. Yet the size of the decoder was a big concern since not everyone would have the space to fit a large capacitor if it was factory installed. Although they never updated the DSD design (the Tsunami replaced the DSD), SoundTraxx did address it with the DSX by allowing one to optionally add extra capacitance.

QSI, which did the Quantum Q1A sound for BLI and Atlas, amongst others, had the luxury of making decoders that were specifically designed to fit in a space provided by the locomotive from the day the locomotive design was started.  Since the sound unit is guaranteed to fit, size was less of an issue, and unlike Soundtraxx, used less expensive and bulkier components.  The QSI Q1A boards reflected just that design and thinking. They are huge compared to Soundtraxx boards.

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. The maximum current is really limited by what the booster will provide. 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.  It’s 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. 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.  It’s 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.

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 shut down. For a given size, cheaper caps will have higher ESR and Store less energy.  There are cost versus Size versus performance tradeoffs that must be made. The total capacitor solution will then vary with the application requirements.