Combining or Reducing Power Supplies
I have a Kitwood Hill Models Test Track Box into which I’ll be fitting the Digitrax Zephyr DCS50 it was designed for plus a Sprog II USB interface and I’d like to put one combined power supply into my Loco Testing Station, for use by both devices separately, not simultanseously.
The intention is to be able to switch between the two devices, so only one is live at a time – either one or other but never both. So I purchased from RS Components a 870-3271 C14 Panel Mount IEC Connector with 5 x 30mm Fuse Block and a 161-8280 RS PRO Switching Power Supply 15VDC 51W. I haven’t purchased switches yet as I haven’t decided on the aesthetic requirements – there’s no shortage of suitable options.
If you dig deep enough on the Digitrax website, you find:
Digitrax recommends the use of a dedicated PS315 power supply or equivalent for use with the DCS50. PS315 output is rated at 15VAC RMS 60Hz and allows up to 3Amps to enable correct operation of the unit. A DC supply of 18VDC and 3Amps will ensure correct track voltage.
Similarly, on the Sprog DCC website, you find:
Recommended Input Voltage 12-15V DC Regulated
As I’m a member of the Model Electronics Railway Group, I asked on their forum:
Will that RS PRO power supply comfortably and reliably but separately power both devices, do I need to do something a bit cleverer or am I attempting something that is impractical, inadvisable or impossible?
Without copying the entire thread, responses from various obviously knowledgeable people including Andrew Crosland of Sprog-DCC suggested that I was over-enginering the solution. I was focused on the desired outcome rather than examining my starting points. It would be better and safer to use the DC input capability of the DCS50 to see what the output at the track was if the input voltage was safe for the Sprog.
I found among my Stash of Oneday Useful Random Stuff (SOURS) a universal laptop power supply that is variable between 15 and 24 Volts DC which ticks two important boxes in the forum discussion; no mains voltage inside the test box and supply the voltage that best suits the Sprog or DCS50, as neither would be in use at the same time. Power into this supply is a standard IEC C13 “kettle” lead.
As suggested, I connected the power supply to the DCS50 and tested the track voltage with a multimeter at various input voltages, starting at 15VDC. Per the clear instructions on the transformer, I unplugged the mains supply before changing the voltage setting each time. What I found was:
| DC In | DCC* Out |
|---|---|
| 15.0V | 12.3V |
| 16.0V | 12.6V |
| 18.0V | 13.7V |
| 20.0V | 13.7V |
| 22.0V | 13.7V |
Above 18.0VDC input, which was confirmed to be within the capability of the Sprog but probably undesirable, the output from the DCS50 did not increase at the track.
Conclusion? There’s no need for more than 18VDC, which would not adversely affect the Sprog, though it’s preferable to turn it down to the recommended 15VDC. Or use 16VDC as a happy (and safe) medium!
The next step is to return the original power supply and C14 socket to RS Components, or try at least as I’ve had them some months, and purchase a suitable panel mount 12VDC socket to plug that variable power supply into, installed on the end of the test box.
Then, from that power socket to a centre off two-way rocker switch to choose between the Sprog and the DCS50. On the DCS50 side, a second such switch to choose between programming and controlling. All of that running at no more than 18VDC inside the box, so red and black cables.