Inrush Current (Sound Decoders)

Topics in this section:

BACKGROUND HISTORY OF PROBLEM
THE BASIC PROBLEM
PROBLEM: MULTIPLE SOUND DECODERS
TECHNICAL DISCUSSION ABOUT A SOUND DECODER SOLUTION


BACKGROUND HISTORY OF PROBLEM

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. But 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, 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.


THE BASIC PROBLEM

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.

A sound decoder have a relatively BIG capacitor 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. The capacitor 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.

The capacitors appearance of a momentary short circuit followed by a quick fade of the current is called HIGH INRUSH CURRENT.

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.


PROBLEM: MULTIPLE SOUND DECODERS

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.


TECHNICAL DISCUSSION ABOUT A SOUND DECODER SOLUTION

I have some thoughts about a simple low cost solution that DCC manufactures could include in their sound decoder that would solve the inrush current problem.  To learn more go here: Inrush Current Discussion