What the MRC says.....
NCE has noted that the problem
described below and has duplicated it in their workshop. Jim dose not like the use of light globs as the primary means of
protection due to the high inductive rush created and would prefer
the use of the EB3. With the
10 amps system the EB3 is not suitable and a globe may be required.
Tests show
that this problem is limited to operation with sound decoder only
and the EB3 operates to specification with all other non sound
decoders.
The solution proposed by
Marcus Ammann with a globe that
bypasses the EB3 with a globe in parallel restricting the current to
a couple of hundred mill watts is currently the most practical
solution and should not cause any foreseeable problems.
Possible solutions:
Click on this link for a better way to use the EB3
[MRC
comment]
Click on this line for
Marcus Ammann bypass circuit
In conclusion a mix of the two
solutions would appear to offer the most reliable operation of the
EB3 in a layout incorporating all of a high quantity of sound
equipped locomotives
Technical
Discussion
EB3 and
limitations when operating with sound decoders
prepared by Marcus Ammann
Since installing a EB3 I have
noted slow response times with sound decoder and have done some
experiments. With CV 49 set at 10 (100 m secs), system works ok only
if one sound equipped loco is in power zone, but with two locos in
same zone, double headed or second in a siding but still powered,
the EB3 does not reset within a reasonable time. Changing CV 49 to
11 to 14 still no good. With CV 49 set to 15 (150 m secs) when there
is a short in the zone then all three status lights start flashing
and after a few seconds, command station trips. This is not a
desirable result.
If I increase current trip limit to 3 Amps then sometimes ok. Put 3
sound equipped locos in zone - no good, if I increase to trip limit
to 4 Amps still won't reset in reasonable time of what I would
expect say within 5 - 10 seconds. See results below.
These are my test results. My sound locos in all tests, all have
Soundtraxx DSD100LCs. I don't have any
BLI locos. I have wired up EB3
off the layout and on my bench with no other track connected and
away from any other track wiring and using only one channel at a
time.
Trip current 2 amps
1 sound loco = OK
2 sound locos = reset sometimes after at least 1 minute
Trip current 3 amps
2 sound locos = reset 5 to 40 secs and 50% greater than 1 minute
Trip current 4 amps
2 sound locos = resets after 5 - 10 seconds most of the time
3 sound locos = never resets in less than a minute
Above tests done on circuit breakers 1 & 2 and CV 49 and 50 set at
10 (100m
secs)
These results have been duplicated by the MRC and should be taken
into account when installing them in to a layout running a large
quantity of sound decoders.
Tests show that this problem
is limited to operation with sound decoder only and the EB3 operates
to specification with all other decoders.
Why:
Mark Gurries
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.
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.
Your light bulb solution works because it adds resistance in series
with the engines limiting the peak current. 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 and the engines will not run well.
Technical Discussion and 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 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.
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 hold-up 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.