Model Railway Forum

Just a curiosity one on how can a Hornby controller work at around 17.5 volts yet an NEC Power Cab work at 13.4 Volts giving around a 25% difference. I cant quite work out the compatibility to work the same thing.
If this was done with 240 volts you would be damaging things with that sort of % difference.

DCC decoders and controllers are allowed a wide operating voltage. See https://www.nmra.org/sites/default/file ... s_2006.pdf

The two Hornby DCC Power supplies do not output 17.5 volts!
The recommended rail volts for OO DCC is between 13.5 and 15 volts. NMRA Standards recommendations are linked to below. The actual rail volts will depend on the mains to low voltage power supply feeding the system.
As sold the NCE PowerCab comes with a regulated 13 ish volt DC PSU. The Hornby DCC range of power supplies are regulated to 15volts DC output on both their 1.0Amp and 4.0Amp PSUs. Hence the different rail volts.
The NMRA recommendation for OO/HO maximum is around 15 volts https://www.nmra.org/sites/default/file ... s_2006.pdf Edit... Rog (RJ) has beaten me to the Standards
Roughly, lower maximum rail volts equals slightly slower top speed.

Please remember if using a multi meter to read rail volts it MUST BE a True reading RMS meter, oscilloscope or RRAmp meter to obtain accuragte readings. A domestic multimeter used on its AC volts range will not give a true reading, due to the rail volts not being conventional sine wave AC.

If you have a system running a high voltage there are two ways of reducing the voltage, either replace the transformer a with spare laptop power supply with a more appropriate voltage ensuring it is of sufficient amperage, or assemble two strings of didoes (1N540x type) - each diode in the string will cause a voltage drop of 0.6 or 0.7v. Connect the ends of the two strings together with one set of diodes letting current pass in one direction and the other set in the other direction, then connect the pair of strings in one leg of the output from your dcc system.

Funny thing is I've recently been looking at this. The default setting for my Lenz system is 16 volts but it can go down to 11v and up to 22v and mine can be altered at increments of half a volt anywhere in between these two extremes as my Lenz voltage settings are programmable.