DCC and Relco Units

What is a Relco Unit ?

Relco units have been available since the 1980's and sold as 'track cleaning' devices.
This description is unfortunately, misleading, as they are actually electrical continuity maintaining devices.

Relco units are used on DC layouts. They constantly monitor for a circuit through the track and a loco. When a break in the circuit occurs due to the loco loosing contact, the Relco sends out a high voltage (50V) high frequency pulse which 'ionises' any offending gaps such that a circuit can be made again and the loco moves off.

Relcos do not clean track, however, they do delay the need for cleaning by allowing running on dirty track.

The picture above shows the typical wiring installation for a Relco unit. In this instance, a Gaugemaster DS100 controller is used. Its 16VAC supply is used to power the Relco. The track output (two lowest terminals in the picture, allowing for the angle from which the picture was taken) from the controller has a (blue) capacitor fitted across it (supplied with Relco) and one wire passes through the Relco on its way to the track.

When first released in the 1980's Relco's could be wired in one of two different ways:

Diagram A allowed the Relco to obtain its power supply from the controller whereas Diagram B required a separate 16VAC for the Relco. Later versions could only be wired using Diagram B. Our experience was that Diagram A was ineffective and B was much more reliable. If in doubt, wire using Diagram B.
The connections on the Relco are marked 'To Track' and 'To Controller', the latter being replaced with '16 VAC' on later models. In the diagrams above, 'To Track' is on the left end of the Relco and 'To Controller' or '16 VAC' is on the right end.

Today, there are other similar products such as the one made by Gaugemaster Controls. For the purposes of further discussion, we will use the term 'Relco' to generically mean the original Relco product, the Gaugemaster product and any similar products made by other manufacturers.

With the advent of more and more locos having DCC decoders fitted, we would strongly advocate inserting a switch in the Relco 16VAC supply (as marked in Diagram B) so that it can be turned off while decoder fitted locos are present - a Relco MUST ON NO ACCOUNT be switched on while decoder fitted locos are placed on the track because it is capable of outputting 50V which is much higher than the specifications for which decoders are required to accomodate, therefore, there is a risk of damaging decoders with Relcos.

What layouts can Relcos be used on ?

Relco's are particulary helpful in the smaller scales such as N and 009 where continuity can be a problem due to lack of weight.
They can also be helpful on layouts with dead-frog turnouts which can cause continuity problems when locos pass over them - loss of contact across the crossing.
DC layouts as a whole usually benefit from Relcos, particularly when locos are run slowly because it is at this time that continuity is easily lost due to the low voltage on the track.

DCC layouts have full voltage (usually 16VAC) on the track at all times. Therefore, DCC control is not as susceptable to loss of contact in the way that DC control is.
When a loco runs slowly in DCC there is full track voltage and the decoder inside the loco reduces the voltage to the loco motor.
DCC layouts do not gain from Relcos like DC layouts, due to the full track voltage making Relcos unnecessary.

Can Relco's be used on DCC layouts ?

Relco's MUST NOT UNDER ANY CIRCUMSTANCES be used on DCC layouts. They have the potential to damage decoders and command stations. Our accidental error found that a DCC command station actually destroys a Relco such that the Relco is completely unserviceable.
The author's personal experience has found Relco's not to be necessary on DCC anyway.

Our investigations into the technical aspects found the following issues:

  • A Relco sends out 50V, high frequency pulses which can be misread by decoders as instructions. These pulses may be ignored (best case) or could trigger undesired functionality (worst case) in the decoder (ie reprogramming, triggering a loco to 'take off' at random speed etc)
  • The voltage level output of a Relco can be up to 50V which exceeds the NMRA S9 standard of +/- 22 volts RMS. A Relco can therefore damage a decoder
  • Some decoders have in-built protection in the form of a zener diode which switches on when excess voltage is applied. This effectively causes the decoder to appear as a short circuit and this triggers the command station to cut track power. The command station would effectively cut out every time the Relco triggered and the layout would be unusable
  • Since a DCC layout nearly always has multiple locos drawing current in parallel at any one time (same applies when double-heading in DC), it is very unlikely that the Relco will ever detect a broken circuit and will therefore never trigger. It is therefore of no use
  • All the decoder and command station manufacturers we contacted strongly indicated against fitting Relcos to a DCC layout and/or running decoder fitted locos on a Relco fitted DC layout, sighting damage to equipment

What about decoder fitted locos running on a Relco fitted DC layout ?

As indicated above, it is very likely that a Relco will damage a decoder.
On a DC layout it is good practice to have a switch per Relco unit which enables them to be switched off individually.
If a decoder fitted loco is to be run, all Relco units should we switched off or disconnected at their power source.

Graham Plowman

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