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Capacitors
Frequently I hear: “Well, if 220 µF comes with my decoder,
why not add another 10,000 µF? I’ve got the room.”
This approach is effective and inexpensive. I recommend the
circuit in figure 4. It is slightly more complicated than just
adding a big capacitor in parallel with the existing ones. It
adds a resistor and diode to the mix. Why? The resistor limits
the inrush current, reducing the load on the DCC system. The
diode bypasses the resistor when the track voltage drops so
the capacitor can supply power to the decoder.
Here’s how to select the components for figure 4.
4: Circuit to add more capacitance to any decoder
See text for component values and connections.
4
DCC Impulses Column - 4
NMRA DCC standards allow track voltage as high as 27 volts for
scales larger than N and 24 volts for N scale decoders. Thus, the
capacitor should have a working voltage of 35 volts or higher to
fully comply with the standards, especially for O and G scales. I
find that most folks run their smaller scale track voltage in the
teens and can get by with 25-volt capacitors. Size and cost both
go up with working voltage. There is a compromise.
The “capacity” of the capacitor is measured in microfarads
(µF). The more microfarads, the more energy stored. I usually
find 2200 µF to 4700 µF the most useful range.
If you need, say 1000 µF, and a capacitor that size won’t fit into
your space, look at several smaller capacitors. In this case, two
470 µF capacitors wired in parallel will give you about 1000 µF
and may have a more manageable form factor. Be sure that you
keep the polarity consistent (wire all the + leads together and
all the – leads together). Insulate your connections.
The resistor should be ½ watt with the value determined by
the size of capacitor.
Capacitor Value
Associated Resistor Value
Up to 2200 µF
100 ohms
Up to 4700 µF
220 ohms
Up to 10,000 µF
470 ohms
The diode should have a working current rating equal to or
larger than the decoder’s current rating. Its voltage rating
should be 30 volts or higher. For common 1-amp decoders,
MRH-Mar 2013
