Technical

Many thanks to Phil Goodman for writing up his conversion of an A4 OOLS to DCC. The document left some further work to do regarding the different heater settings. I doubt it will be possible to exactly duplicate the way Hornby control the heaters, but it should be possible to do something similar, perhaps better.

The Hornby controller measures the current flowing through the loco heaters (and associated track/wiring) and varies the on/off duty cycle to maintain a target current. The voltage is supplied at 17V, but is switched on/off at roughly15kHz to vary the average voltage and therefore control the current flowing through the heaters. The reason for this is to keep the heating of the water/steam at a fixed level, regardless of the external variations in resistance of the track wiring, fishplates, tender wheel pick-ups or anything else.

From some rough measurements using a cheapo current/voltage meter added to a LiveDrive, I think the three current levels associated with the three heat settings are: 5.75A, 6.0A and 6.25A. Given the overall heater resistance of 2 Ohms (4 Ohms each) and maybe 0.25 Ohm of pick-up/track/wiring resistance, that looks about right as I see average voltages of 12.5V, 13.5V and 14.5V on both the Hornby controller and my extra readout. Given the 17V starting point, they correspond to approximately75%, 80% and 85% ON times. All these numbers are approximations as the heater resistance changes during warm up and we know the pick-ups can be variable as well as track wiring.

If the DCC system were to cycle the heater outputs off and on at a similar ratio - job done! It doesn't need to be quick, even switching every second or so would be fine. If the DCC control can manage the on/off switching and the operator provides the duty cycle percentage, then you have a fully controllable heating system (better than the original).

I hope this information is useful - I'm keen to see how a DCC control system might develop.

David