Have we seen the Light ? - DCC and Light Bulbs



Because DCC systems are capable of outputting several amps of current, they have a very sensitive automatic cut-out system to protect in the case of short circuits. Some systems are capable of in excess of 4-5 amps which is sufficient to cause significant damage if current is allowed to flow for any period of time. Consequently, the automatic cut-outs on DCC systems are virtually instantaneous - far faster than any DC controller.

Instantaneous cut-outs can have their advantages, but if not properly catered for, can cause inconveniences, the most common of which is the situation of an entire DCC layout shutting down in the event of a short.
On a small layout with only one or two locos in operation at any one time, this probably won't be a problem, but on a larger layout where there are several operators active at any one time, the whole layout shutting down can be an inconvenience.

There are several solutions to this problem and also several considerations.

Factors to be considered include:

  • Model railway layouts should not have shorts if properly wired
  • If shorts are occuring, they are actually telling you that there is a problem which needs fixing!
  • The practicalities of a whole or part layout cutting out
  • The prototypical scenario

On a layout with correctly wired turnouts (see Wiring Live Frog Turnouts), the only reason a short should occur is if a loco derails on a turnout and positions itself such that it creates a direct short between the two running rails.
In a situation where there is only a single operator, it really does not matter if a whole layout shuts down, although it will cause all DCC sounds to 'restart'. Where there are multiple operators, consider whether it makes practical sense to keep other operators operating while there is a derailment.
If one operator is re-railing a train, does it make sense that another operator can run a train past in the other direction, potentially colliding with the derailed vehicles or colliding with the hands of the re-railing operator ? Probably not.
In the prototypical situation, once a derailment occurs, everything local to the derailment stops and all signals are placed at danger. Interlocking systems have the effect of relaying this back to previous signals such that a whole area may be stopped. Why should our models be any different whether at an exhibition or otherwise ?
This really comes down to an operational rather than technical issue but is often resolved with a technical solution.


So what are the technical solutions ?

Some modellers wire a 12V car light bulb in series with the track supply for each of their separate power districts. On a small layout with only one district, this has no benefit. On larger layouts, it has the benefit that when one district is in a short condition, it doesn't bring all the other districts down as well.

So let us analyse the use of light bulbs:



The power of the lightbulb used is specifically chosen such that under normal operating current conditions, the light remains unlit.
When a short occurs as in Track District 2 in the diagram above, it causes an increase in current in the district where the short occurs and this causes the lighbulb to illuminate.
The lightbulb in itself has the effect of limiting current to around 2 amps.
Although the district where the short has occurred stops, all other districts remain operational.

There are two very important facts to note about this solution:

  • Although the lightbulb illuminates and limits current to around 2 amps, current is still flowing through the actual object(s) causing the short.
    Depending on the properties of the object(s) causing the short, even 2 amps is sufficient to cause heating and melting damage to both models and electrical equipment, including decoders
  • The automatic cutout on the command station will never operate because it has been overridden by the lightbulbs


What do the Manufacturers say about the light bulb solution ?

Lenz advise that while the light bulb solution won't cause any problems to their equipment, they do not warrant that the automatic cutout on their command stations will operate properly in the event of a short. Therefore, if you are using the light bulb solution and any kind of damage occurs, Lenz will (rightly) disclaim all responsibility.

Digitrax do not support the solution at all and instead, have developed their own product to solve the problem properly - the PM42:


How does the PM42 work ?


We will discuss the PM42 here, however, we acknowledge that there are similar products provided by other manufacturers which work the same way.
The PM42 acts as a 'hub' and is connected to the output of a command station.
It has 4 separate outputs which can each be connected to separate districts.
Each output is individually protected using the same technology as the cutout on the command station. A short in one district will cause a cutout for that PM42 output only.
The PM42 effectively races the cutout of the command station.
The important factor to note is that current is completely removed from a shorted district.


General Comments

At US$79.95 (less than $20 per sub-district), the Digitrax PM42 provides a solution which properly removes current from shorted districts while allowing other districts to operate.
The Model Electronics Railway Group (MERG) are also developing a cutout device similar to the PM42 for around GBP 10.00.

While the use of light bulbs gives the required operational effect, it does not provide proper electrical protection because it allows 2 amps of current to continue flowing which is sufficient to cause damage. Proper and safe protection should remove current completely by breaking a circuit.

The use of lighbulbs is not advocated by command station manufacturers and is a very old solution which originated on Hornby Dublo and similar DC controllers in the 1950's, long before devices such as the PM42 were available. Today, the lightbulb solution is obsolete but it seems to still retain a following in North America.

The author has no connection with any of the manufacturers or products mentioned in this article other than as a satisfied user.


Graham Plowman (Updated 9/10/2020 4:07:39 PM +11:00)


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