FAQ - How Do I add a 'Stay Alive' to Hornby decoders?

[Chrissaf]

This FAQ has been written as a result of a ‘forum member’ request.

This tutorial will cover the two main types of ‘Stay Alive’ capacitors and where they connect to the Hornby R8249 based and Hornby TTS decoders. Although the tutorial will focus on Hornby decoders, the connectivity can ‘in principle’ be applied to any brand of locomotive decoder.

Before jumping directly into the ‘How To’ part of this tutorial, it would be useful to review some capacitor theory.

A ‘Stay Alive’ function needs to use a capacitor that has high capacitance values. Capacitance although measured in ‘Farads’ is expressed when used as a ‘Stay Alive’ more commonly in micro-Farads [µF]. The higher the µF value, then the more power that can be stored in the capacitor and the further the locomotive will travel under ‘Stay Alive’ power. This µF value size also impacts on the physical size of the ‘Stay Alive’ and the amount of space available within the locomotive body to accommodate it.

The µF values required for ‘Stay Alive’ capacitors will require the capacitor to be of the ‘Electrolytic’ type, which means that they will be sensitive to polarity. It is this sensitivity to polarity that means that it is not feasible to use a ‘Stay Alive’ on a DC Analogue powered locomotive. This is because the voltage polarities in a DC Analogue locomotive reverses; subject to the locomotive direction of travel and if you apply a voltage across an Electrolytic capacitor in the wrong polarity, the capacitor will swell up and potentially explode.

Therefore, a ‘Stay Alive’ should only be used on DCC powered layouts when a DCC decoder is fitted. The ‘Stay Alive’ capacitor is electrically connected across the output of the power rectification diode bridge that is included on all DCC decoders - see schematic below. This ensures that the voltage polarity across the ‘Stay Alive’ capacitor will always be the same correct polarity and never be reversed.

As can be seen in the block schematic diagram above; the ‘Stay Alive’ connects across the output of the ‘bridge rectifier’ which is producing the DC supply to power the decoder and ultimately under processor control the motor and functions. Therefore, when the track power disappears [poor connection or dirty track] the power stored in the capacitor can be used like a very small capacity battery backup to maintain decoder operation until the track power is restored. If the track power is missing for too long, then the pure DC power being fed into the decoder from the capacitor in the absence of a valid DCC signal on the decoder input [DCC Command Signal Line in the schematic] can cause the decoder to exhibit DC runaway. Therefore, it is recommended that when fitting a ‘Stay Alive, that CV29 on the decoder is edited to disable ‘DC Operation’.

It can also be seen in the block schematic above that the positive side of the ‘Stay Alive’ shares the same electrical connection point as the decoder Blue common positive return wire. This means that optionally, any DIY ‘Stay Alive’ that is added to the decoder can connect to the Blue decoder wire on its positive side.

It should be noted however, that the Hornby decoders do not separate out the negative side of the bridge rectifier as a separate wire. Some other brands of ‘Stay Alive Ready’ decoders do indeed bring out a negative wire from the rectifier bridge for this purpose. Just to be crystal clear, with Hornby decoders, you have to make the ‘Stay Alive’ negative connection directly to the rectifier bridge components on the decoder PCB [Printed Circuit Board]. This requires a very small soldering iron bit, good eyesight and very fine steady hand soldering skills.

The location for the positive and negative side of the rectifier bridge on the two main Hornby decoder types is reproduced below. Note that Hornby 4 & 6 pin decoders are based upon the R8249 PCB, thus the 'Stay Alive' positive and negative pad connection locations will be the same.

R8249

TTS

OR

OK so far I have discussed where the ‘Stay Alive’ is physically connected and touched on how they work. It is now necessary to consider the type of ‘Stay Alive’ to fit. You could of course, purchase a commercial ‘Stay Alive’ from a model retailer. But compared to the cost of the components, this is an extremely expensive way of obtaining a ‘Stay Alive’ as they use very common cheap components.

There are two types of capacitors that can be used for a ‘Stay Alive’, the common ‘Electrolytic’ capacitor and the ‘Super Capacitor’ which is also of an Electrolytic type.

Electrolytic capacitors increase in physical size as their µF values and rated voltage increase. This is not a big issue for large models with spare space capacity within them, but can be a significant issue in smaller models. This is a bit ironic, as it is typically the smaller 0-4-0 and 0-6-0 wheel configured models that are the ones to gain the most significant benefit from having a ‘Stay Alive’ fitted. Whatever the Electrolytic capacitor µF value that is chosen for use as a ‘Stay Alive’ its rated voltage MUST be at least 16 volts DC.

Super capacitors have extremely large µF values for their small physical size, but they have their own issues for use as a ‘Stay Alive’. Be aware that most 'Super Capacitors' have a low rated voltage; some are only 2.7 volts but most are 5.5 volts. You need a minimum 16 volts to use as a 'Stay Alive' with most decoders. If you apply a voltage higher than the capacitor rated voltage you will damage it.

To achieve the 16 volts with ‘Super Capacitors’, you connect more than one 'Super Capacitor' in series. Doing that makes the rated voltages of the individual capacitors accumulative. So three 5.5 volt capacitors in series gives the equivalence of one capacitor rated at 16.5 volts. Six 2.7 volt versions in series give 16.2 volts and so on.

However, connecting capacitors in series reduces their overall equivalent capacitance. This is not a major issue with 'Super Capacitors' because of their extremely high individual values that you are starting out with. For example, putting three 0.1F Super Capacitors in series gives an overall capacitance of 33,333uF.

An acceptable 'Stay Alive' can operate on a capacitance as low as 3,300uF, although 6,800uF would be preferred. As you can see, three 0.1F [5.5v] Super Capacitors in series giving 33,333uF is more than 10 times the ‘Stay Alive’ capacitance that could be used as a minimum.

The final design requirement for a home brewed 'Super Capacitor' Stay Alive, is in-rush current protection. You MUST include a resistor and diode in series with the capacitor. Note however, that the resistor and diode themselves are wired in parallel before being wired in series with the Capacitor [see schematic below].

The ‘in-rush current protection’ works as follows.

At the instant in time that the locomotive is powered up, the ‘Super Capacitor’ will look electrically like a ‘short circuit’. The resistor then limits this short circuit current draw to a level which is below the controller short circuit trip current setting. As the ‘Super Capacitor’ charges up, the capacitor impedance [resistance] increases thus reducing the in-rush charging current. Once the capacitor is fully charged, then the capacitor now looks like an ‘open’ rather than ‘short’ circuit and current flow stops.

Comparatively low value standard Electrolytic capacitors do not normally need additional in-rush current protection as the time that they are in their ‘short circuit’ state is very much reduced compared to ‘Super Capacitors’.

Further considerations that need to be taken into account when fitting a ‘Stay Alive’ to a Hornby decoder.

Commercial decoders that are factory designed with a ‘Stay Alive’ have additional circuitry to deal with this potential issue.

As previously discussed; ‘Stay Alive’ capacitors generate an in-rush current when the locomotive is initially powered up, more so if the ‘Stay Alive’ is based on ‘Super Capacitor’ components.

The 'Stay Alive' capacitance is not an issue for normal running as full track voltage / current is always available to charge up the capacitor.

However, once you remove the loco from the track to transfer it to the 'PROG' track, the capacitor discharges through the decoder (i.e. it is doing its 'Stay Alive' function). Thus when placed on the 'Programming Track' the capacitor is very nearly discharged.

The 'Programming Track' normally has no power on it, so the capacitor cannot recharge. When you then initiate a 'Programming' command, the power sent to the 'Programming Track' is very brief and severely current limited. Thus the capacitor can suck all the energy from the programming packet in a vain attempt to recharge itself. This current drain means that there is no power left for the decoder to be energised enough to read or react to the low power programming packet.

Therefore where a 'Stay Alive' is fitted to decoders that were not designed for it, the ‘Stay Alive’ can sometimes [but not always] affect CV programming. The recommendation is therefore that the 'Stay Alive' capacitor should be connected via a micro plug 'n' socket so that it can be easily and quickly disconnected if a change of CV is required. In the majority of cases and for many or most users; once they have configured the decoder CVs they rarely need to change them again, so this issue doesn't arise if the ‘Stay Alive’ is connected after the decoder has been fully configured.

One way round this (not possible with the eLink) is to put the DCC Controller in POM mode (Programming On the Main) which Hornby call 'Operate Mode' (a switchable mode option on the Elite and for which a version of POM is standard as default on the Select). Thus in this mode a decoder CV can be written to whilst the ‘Stay Alive’ remains fully charged on the main track power.

Be aware that in POM and/or Operate mode you can only write to a CV......you cannot usually read one. Although some brands of decoders support reading a CV via POM if RailCom is supported and enabled; but in the main, the normal 'Direct Mode' (Programming Track) should be used for reading CVs, which of course could then potentially be impacted by the presence of the 'Stay Alive'.

One final consideration; if the 'Stay Alive capacitance value is very high, then the pure DC power being fed into the decoder from the capacitor in the absence of a valid DCC signal on the decoder input can cause the decoder to exhibit DC runaway. This can be more noticeable when shutting down the DCC system at the end of a play session. It has been reported on the forum by some members that a large value 'Stay Alive' (when fitted) has made the loco shoot off on its own (DC Runaway) when the DCC track power is switched off. This can be eliminated by editing CV29 in the decoder as previously discussed in this tutorial to disable ‘DC Operation’. Internet references to 'Stay Alive' CV configurations usually also mention CV11 being set to zero as well as disabling 'DC Operation' in CV29. Just be aware that current Hornby decoders do not support CV11, so the Internet references to CV11 can be ignored.