October 2019 55and rough spots, they also
need to supply a certain volt-
age to meet the requirements
of both the mobile decoder
and the sound circuits.
The mobile part of the cir-
cuit requires about 3 to 5 volts
(V), but the sound circuit
requires about 6 to 8V. Given
that the super capacitors used
in most stay-alives are rated
at 2.5 to 3V, if you do the
math you quickly realize at
least three are required, and
four give a margin of security
that makes electrical engi-
neers comfortable.
That’s why most stay-alives
have four or more capacitors
- manufacturers have to
design for the lowest common
denominator and build in a
safety margin. It’s the capaci-
tors that determine the final
size of the stay-alive.
Yes, I did say most stay-
alives have four or more
capacitors. The ESU stay-
alives have only one or two
capacitors ❸. Although I’ve
never been able to get ESU to
confirm or deny this, the gen-
eral consensus is they use a
booster circuit that kicks the
voltage up to the minimum
required. The trade-off is a
smaller device at a higher
cost, about twice some others,
but a shorter run-time.
One recent development as
a result of the continuing
trend of miniaturizing com-
ponents are the new KA1 and
KA2 Keep Alives from TCS.
These new Keep Alives use
much smaller capacitors and
are almost half the size of
the company’s previously
smallest ones ❹.
I’ve used these in tight
installations and found they
do an excellent job of keeping
the locomotive running and
the sound on. I expect given
this trend in miniaturization,
we’ll see decoders with built-
in stay-alives in a few years.
There are some downsides
to using stay-alives. First,
since they supply a DC power
source, dual-mode decoders
can interpret this as the loco-
motive running in a DC envi-
ronment, so the dual-mode
option should be turned off in
CV29. SoundTraxx specifi-
cally points this out in its
instructions. Stay-alives also
can interfere with program-
ming, especially on the ser-
vice mode track. Again,
SoundTraxx has specific
instructions on how to deal
with this in its instructions.
Some readers have told me
they’ve installed a latching,
magnet-activated reed switch
to turn the stay-alive off dur-
ing programming. I’ve also
heard from one reader who
has had locomotives take off
uncontrollably after program-
ming on the service mode
track and asked me to warn
readers of the potential for
this. I’ve never had any of
these issues, but I do about
90 percent of my program-
ming on the main.
For some folks, using a
stay-alive in all their locomo-
tives may be an expensive
option, so which locomotives
benefit the most from them
and under what conditions?
First, short-wheelbase
locomotives like small diesel
switchers and especially small
steam locomotives are most
likely to stall on dirty or
uneven track. Unfortunately,
these also are the ones that
are most difficult to add a
stay-alive to. However, the
new TCS Keep Alives may
make many of those installa-
tions possible. If you have lots
of turnouts with unpowered
frogs, then you might benefit
from stay-alives to prevent
stalling. Finally, even long
wheelbase steam locomotives
can be prone to stalling, espe-
cially if they only pick up
power from the drivers.
In all cases, if a locomotive
has a tendency to stall often
or is exhibiting symptoms of
rough running and flickering
lights, then I would give a
stay-alive a try.By Larry PuckettDigitrax Power XtenderSoundTraxx
CurrentKeeperTCS Keep Alive❷ Wires or plugs. Some stay-alives and decoders come with
bare wires, but others use polarized connectors that aren’t
directly interchangeable. If you decide to mix and match,
you’ll also have to do some cutting and soldering.❸ Three wires. ESU PowerPack stay-alives have three wires
and only one or two capacitors, yet can power a LokSound
decoder for several seconds. These are the only stay-alives
recommended for use with LokSound decoders.❹ Small packages. The redesigned KA1 and KA2 Keep
Alives from TCS are about half the size of their bigger KA3
and KA4 brothers, so they will fit in many tight installations.