Summary: Some Multifunction Decoders support an additional energy storage module to provide backup power when the electrical supply from the rails is interrupted.
Decoder accessory to store backup energy
- 1 Energy Storage
- 1.1 Overview
- 1.2 How Energy Storage Works
- 1.3 Storage Components
- 1.4 Wiring
- 1.5 CV Changes
- 1.6 Availability
- 1.7 Comparison of Energy Storage Systems
- 2 Video: Demonstration of a Keep-Alive by TCS
Some multifunction decoders support an additional external energy storage module to provide backup power when the electrical supply from the rails is interrupted by dirt, bad contact, insulated frogs, etc. This may prevent a locomotive from stalling when travelling over dirty track or turnouts with large plastic frogs, especially at low speed. Short wheelbase locomotives such as 0-6-0 steam locomotives are more susceptible due to their inherently limited number of electrical pickup wheels. Locomotives without flywheels can also benefit from energy storage. Sound Decoders benefit from energy storage systems by preventing dirt and bad rail pickup from causing audible crackling or static coming from the loudpeaker.
Typically this function cannot be integrated into a decoder because storing a useful amount of energy requires a sizeable capacitor. The capacitor (and additional components) must be installed in a separate location in the locomotive and connected to the decoder via a couple of wires.
The Lenz USP system is different. While it does have a small energy storage circuit on-board, the USP decoder has an advanced signal processing system that can read a weak or distorted DCC signal, thus keeping the decoder under control of the DCC system. With the Energy Storage concept, the train keeps moving if the DCC signal is lost, and control will only be regained when the decoder again sees a valid DCC signal.
Other commonly used names are "stay alive capacitor" and "electronic flywheel".
How Energy Storage Works
An energy storage module works by charging a large capacitor while the Digital Command Control signal from the rails is available. Capacitors store voltage, and attempt to resist changes in voltage by charging and discharging.
When the DCC signal on the track is interrupted, the capacitor begins to discharge which provides decoder power, which in turn supplies the motor. It can only do this for a short period since the capacitor will rapidly discharge. At some point the discharge will cause the voltage across the capacitor to drop below the minimum required for decoder operation.
The capacitor is charged from decoder's internal DC supply. Capacitors can be fitted if access to the decoder's power rails is available. During operation it will discharge into the decoder via those power rails.
Some additional components are required if a large capacitor is used.
Firstly, a large capacitor has a very low resistance when discharged. Placing a locomotive onto a layout with such a capacitor might trigger the command station or booster's short circuit detection. To prevent this a current limiting resistor is needed to control the inrush current. This however causes its own problem in that it limits how much current the decoder can draw from the capacitor. A diode can be added to overcome this limitation.
Secondly, a large capacitor can hold energy for quite a period of time (many minutes) if there is little current draw. A locomotive that was removed from the track and then returned later can remember what it was doing before and set off on its own. While it's away from the track, the voltage of the capacitor may drop to the point where there is insufficient power to turn the motor but enough to retain the decoder's memory that it was moving. To prevent this a discharge resistor can be added across the capacitor's terminals to slowly discharge it when not in use. This effectively resets the decoder so that its speed is zero.
The storage component is usually an electrolytic, tantalum, or a double-layer capacitor. The most common type is electrolytic because they are low cost and readily available. Electrolytic and Tantalum capacitors are polarity sensitive, installing them incorrectly will destroy them. Tantalum capacitors are also sensitive to heat stresses, so use care during soldering.
Double-layer capacitors (often referred to as gold capacitors) offer much denser energy storage than conventional capacitors but at a higher price and limited voltage rating. Usually at least four double-layer capacitor "coin cells" must be combined for this application. The benefit is the storage capacity can be 10 to 100 times more than with electrolytic capacitors of the same physical size. Despite the name, "gold capacitors" contain little, if any, gold.
Regardless of the type, all of these capacitors are polarized, meaning that they must be wired the correct way round.
For HO or OO gauge a capacitor as small as 470μF will provide some benefit but sizes of 2200μF and above are preferred. 2200μF will be able to supply power to a locomotive for a few tenths of second, enough to get it over a piece of dirty track at low speed. The endurance of larger capacitors of over 10,000μF can be measured in seconds.
Many locomotives will have insufficient or oddly-shaped space available to install a large capacitor but it is possible to use multiple smaller capacitors instead. They can be wired in parallel to increase the energy storage capacity or wired in series to increase the voltage rating. A combination of series and parallel wiring is also permitted.
The capacitor must be rated for at least 25V for the smaller gauges and 35V for the larger gauges (O and above). This rating is the maximum voltage that the capacitor can withstand without damage. If multiple smaller capacitors are used and they are wired in series then only the sum of the ratings needs to be at least 25V. For example two 16V capacitors wired in series would be suitable for the smaller gauges because combined they have a rating of 32V. If the capacitors are wired in parallel then all of the capacitors must meet the voltage rating requirement.
Series and Parallel as Applied to Capacitor Circuits
Capacitors behave the exact opposite to resistors in these circuits.
Capacitors in Parallel
Total Capacitance of capacitors in parallel is the sum of the individual capacitors. Three 100μF capacitors are the equivalent of one 300μF capacitor.
The voltage rating is equal to the lowest rated capacitor in the group.
Capacitors in Series
When capacitors are wired in series, the total capacitance is reduced. The voltage rating is equal to the sum of all the capacitor voltage ratings. For example, two 50V capacitors in series are the equivalent of a 100V capacitor. This is important for the low voltage ratings of Super Capactors often used in homemade energy storage solutions.
The total capacitance is calculated using the formula: 1/CTotal = 1/C1 + 1/C2 + 1/C3 ....
For example two 100μF capacitors in series equals 50μF total
Two 50μF capacitors in series:
- 1/C = 1/50 + 1/50 = 0.02 + 0.02
- = 25μF
A 50μF in series with a 200μF capacitor:
- 1/C = 1/50 + 1/200 = 0.02 +0.005
- = 40μF
There is no NMRA standard for wiring energy storage modules to multifunction decoders, and there are no known standard wiring harnesses or connectors that include the necessary connections. On many decoders the user must solder additional wires directly to circuit board, often requiring removal of some of the insulating plastic sleeve. More proficient soldering skills are needed to perform this task than are typical for installing a multifunction decoder.
DCCconcepts and Zimo use the convention of black wires for the energy storage ground and blue wires for the energy storage positive. TCS uses a similar convention but the ground wire is black with a white stripe.
The Lenz system uses 3 wires for U+ (blue), charge (pink) and ground (brown).
- Be sure to throughly read and understand the instructions provided by the manufacturer.
The energy storage module is fitted between the blue common lead and the 0 or ground lead. Many decoders will not have a pad marked Negative or Ground. To complete the circuit the module must be connected across the diode matrix that provides power to the decoder. This can be done by soldering to the cathode of one of the diodes in the matrix. This must be done on the decoder side, not the track side of the matrix. Be aware that one of the diodes in the matrix is the signal pick off for the decoder. That is not one to be used.
As always, consult the literature that came with the module for any additional CV changes required.
Analog Conversion Mode
Energy storage is not typically supported in Analog Conversion mode because there is no way for the decoder to distinguish between a command to stop (0V sent by an analog throttle) and a dirty section of track (0V received by the decoder). A decoder fitted with energy storage and with analog conversion enabled will still work normally, it will simply stop immediately when it receives no power.
Theoretically, it could be supported with the decoder assuming 0V means coast for a short period and then stop. The disadvantage being that there would be no way to cause an emergency stop.
Programming Issues with Energy Storage Modules
There can be unintended issues arising from the use of a "keep alive" module. In many cases the programming track output will only have power available during a programming read/write cycle.
The short, low energy pulses used during programming may be absorbed by the capacitor, leading to programming problems. The capacitors do not charge as they normally would, leading to issues like this.
The simplest way around this problem is to mechanically disconnect the capacitor from the decoder prior to programming. If the capacitor is soldered to the decoder, disconnecting is not an easy task. During installation a switch can also be installed which can break the connection to the capacitor. Mounting the switch in an easily accessible location may not be possible.
One proposed solution is the addition of a 100μH inductor (also called a choke) in series with the capacitor. It will slow the current flow into the capacitor, which will not be an issue during normal operation. During programming, it can reduce the current flow enough to make successful programming possible.
As inductors are not polarity sensitive, it can be easily fitted to either wire feeding the capacitor.
Some decoders have the ability to disconnect their energy storage modules when they go into their programming mode.
DCCconcepts refer to energy storage as "Stay Alive", they supply some decoders with the storage module included in the package or they can supply them separately. All of their current decoders support Stay Alive and all of them come pre-wired with two additonal wires to make it simple to attach their module without having to solder directly onto the decoder's circuit board.
Lenz Gold series decoders which support USP also support energy storage using their POWER1 or the larger POWER3 modules.
TCS makes energy storage modules and decoders with integrated on-board storage. All of their decoders manufactured after February 2012 are Keep-AliveTM ready or have it on-board. Some of their decoders require soldering wires together to connect the storage module (-KA series) or simply plugging them together with a quick connector (-KAC series). They also supply versions with a harness for their WOWSound multifunction sound decoder
Most Zimo decoders support energy storage using their SPEIKOMP kit and they supply instructions for building completely DIY modules allowing greater flexibily for capacity and physical size. Some of their decoders have the built in support circuitry and only require the capacitor itself to be added whereas others require additional external components.
Comparison of Energy Storage Systems
|Lenz||Yes||No||POWER-1||GOLD, GOLD Mini||Yes||Yes|
|TCS||Hardwire||No||No||KA1||All -KA series decoders||Yes||No|
|2-Pin Quick Connector||No||No||KA1-C||All -KAC series decoders||Yes||No|
|Auxilary Harness||No||No||WAUX-KA1||WOWSound Decoders||Yes||No|
|On-board||No||No||Integrated||All KAT and KAM series decoders||Yes||No|
|Zimo||No Special Port||No||Yes||10,000||SPEIKOMP, ELKOSODR||MX618, MX620, MX621, MX622, MX63, MX630, MX64, MX646, MX647, MX648||470, 6802||No||Electrolytic||Yes|
|25V Limited Port||No||Yes||10,000||ELKOSOMT||MX631, MX632, MX644||470, 6802||No1||Electrolytic||Yes|
|16V Limited Port||No||Yes||10,000||ELKOSOPL||MX633, MX643, MX645||680, 18002||No1||Electrolytic||Yes|
|16V Unlimited Port||No||Yes||Unlimited||ELKOSOGR||MX695, MX696||10,000, 20,0002||No1||Electrolytic||Yes|
|GOLMREG||MX645P22 + ADAPLU22||140,000||Yes||Double Layer||Unknown|
1 Modules are not supplied ready-made. Since only a single capacitor is required, there is nothing to make unless multiple capacitors are to be combined when more storage capacity is desired.
2 More capacitance is available by combining capacitors in parallel
3 All DCCconcepts decoders ending in SAP include the Stay Alive module in the package
4 DIY indicates that the manufacturer supplies instructions to build energy storage module from scratch
Double Layer capacitors are often referred to as "Gold Caps"
List of Supported Multifunction Decoders
- DCCconcepts: All multifunction decoders (specifically those listed as xxSAx)
- Lenz: GOLD series and GOLD mini series
- TCS: All -KA and -KAC (all decoders manufactured after February 2012), WOWSound.
- Zimo: All MX618, MX620, MX621, MX622, MX63, MX630, MX631, MX632, MX633, MX64, MX640, MX642, MX643, MX644, MX645, MX646, MX647 families