12V

The 12V supply should be adequate, however the 20Amp rating is what concerns me.  Most electronic devices are current sensitive and not so much voltage sensitive.  You will NEED to install circuit breakers into your system that should trip IMO no more than 5Amps.  Damage to your decoders and other electronic devices can be damaged.  PC power supplies are designed to supply power in many different areas of the PC.  The 12V circuits are for powering the hard drives, cd rom drives, fans ect.  Almost all the other areas are driven by 3.5 to 5v circuits.  What will you be using for a DCC system?

breakers a good idea, TQ

Currently (no pun intended) the 5v output is powering all the turnout servos.

Any commercial DCC starter system provides its own power, and I will not use the computer power supply with that.  As I move to DCC++ a power supply will be needed.

2 Things

Your 12v may not be enough. As a DCC power supply, the raw DC voltage is changed to an AC voltage with the signal that goes to the tracks. In order for the motor to run on DC there is a bridge rectifier on the decoder. This will drop your voltage to less than 12v to the motor. If you look here:

https://sites.google.com/site/markgurries/home/part-sources/power-supplies-xmfrs

You will see that the commercial power supplies for DCC are all above 12v, the lowest is 13.8.

NMRA S-9 Electrical says:
The minimum DC track voltage is 12VDC.  The maximum DC track voltage is set by the voltage rating of the motor and must not be allowed to overheat.
NMRA S-9.1 DCC electrical Section C says:
The DCC voltage is based on the DC voltage PLUS 2 volts.  12VDC + 2V =>  14V DCC. 

See a detailed explanation here:

https://sites.google.com/site/markgurries/home/technical-discussions/boosters/nmra-track-voltages

There are special circumstances with high amperage supplies. Read about that here:

https://sites.google.com/site/markgurries/home/technical-discussions/boosters/10amp-booster-ho

If it were me, I'd be looking for a power supply that could do between 13.8 and 15 volts at 5amp max.

Something like this would work:

http://www.ebay.ca/itm/Pyramid-PS9KX-5-Amp-Power-Supply-w-Cigarette-Lighter-Plug-110v-ac-to-12v-DC-/321519841593?hash=item4adc137139:g:8q0AAOSwVFlUEgOL

It is designed to replace an automobile source. Even though the industry says cars run on 12v the actual voltage is 13.8v.

Should be fine

The Digitrax systems have a switch for O, HO, or N scale, I set mine for N scale even though I am running HO (and On30). I believe that puts around 12 volts to the rails. More voltage just gives you a faster top speed, something that we don't need!

Power Supplies for DCC

Hi All,

This is not quite as straightforward as many think, so please bear with me for a bit.

First, the NMRA standards actually specify that the DCC voltage on the rails should not be less than 14V. The appropriate standards are NMRA standard S-9 Electrical and NMRA standard S-9.1 Electrical Standards for Digital Command Control. Strangely enough the standards do not differentiate for scale.

To me this is really not the point. I use the Digirax DCC system, and I have some direct use of 2-3 other systems, but my comments are from personal experience with Digitxax over more than15-20 years.The real question is what do base stations need/expect for operating power to function properly. Early on, I successfully operated a DCS100 base station with at least 10 different locos and decoders with a 12V power supply! I upgraded later to a 15 volt supply (for a better current rating. With new sound decoders I found I was having intermittent problems programming and then in later years running newer locos with newer decoders. I invested in a PTB-100 programming booster with good results. However, I continued to get ever more increasing weird, intermittent, and/or nonfunctional results. I was getting ready to dissect my command station when I gave consideration to the DC power source I was using. The Digitrax specs indicated it would tolerate a DC input up to about 18 VDC. I hooked up a lab grade power supply that I could monitor and control closely, and to my amazement all the odd behavior stopped-- and I mean stopped -- when I hit a 16.1 Volt DC input! I changed my Power supply to one just under 17 Volts and never looked back.

Base station electronics takes the DC power you feed it and shapes & forms the DCC command packets it places on the rails. The reason NMRA standard specifies a minimum of 14 V for DCC on  the rails is to allow the electronics in a decoder to form the (usually) pulse width modulation speed control signal for DC motors in the loco. That added 2V gives the electronics the margins that are needed. Likewise consider the base stations need the same kind of margin to form the 14 V DCC command packets they send to the decoders. While 16 V may not fit all base stations it will give you the indication that something more than 12 volts is likely needed for their proper care and feeding.

You might note that I use my own electronic designs at times for DCC control. My "base stations" or DFGs (DCC Function Generators as I call them) typically use only 12V battery supplies when I demo at clinics. I can attest that there are decoders out there that will respond to a 9 Volt DCC signal, and those that will not always respond to a 12V DCC signal.

Best advice: check your DC base station manufacturer's specification for DC power into your station and adhere to it. The warnings given previously about using very high current power supplies are well taken. If not heeded, you may find yourself in possession of an excellent DCC bus welder!

'Hope this helps. Have fun! 

Best regards,

Geoff Bunza

TQ, I did not account for the losses

Now I understand the slightly higher voltage helps with  expected voltage drops through the electronics, both the base station and the decoder.  Diodes in bridge rectifiers - the light goes on! (pun intended)

Geoff, as always you have testing and actual numbers to help explain the situation.

Thanks, all.

I think it was a 16V supply

I think it was a 16V supply kit I bought from Spring Mill years ago for my Digitrax Chief.  I just checked the NCE branded power supply and it's listed as 15 volts.  Hopefully it would not give strange anomalies like described by Geoff above.  Never heard any reported.  Maybe Digitrax likes a solid 16 volts.

Zephyr Power Supply

My Digitrax Zephyr uses a 14V supply.

Regards,

My power supply ...

For my NCE system, I am using this power supply, rated at 15V output ...

When I measure the track voltage, I get 14 volts. To measure track voltage with a regular volt-ohm-meter, just put a decoder in a loco and before you connect up the function leads, read the voltage across the blue and white function wire leads when you turn on F0. This will generally be within about half a volt of your track voltage - mine reads a perfect 14 volts.

Power supply option

You can connect two outputs in series if your power supply has a -12v output with its separate ground, connect it in series with the 3.3v so you could have 15.3.  Of course the max current of the -12 supply is usually pretty low. You would connect the grounds together and then between the -12v and +3.3v would be 15.3v, of course you could use the +5v and get 17v.  Of course there may be other issues why this doesn't work, like loading and sense issues on the +3.3v

My initial plan was to use an

My initial plan was to use an ATX computer power supply to power my layout, but it's not working out.

My Digitrax SuperChief won't power up with 12.0 VDC. Testing with a variable power supply and a digital multimeter it requires 13.5VDC as a bare minimum to power on correctly.

In the old days there were plenty of negative voltage leads available on power supplies. They have basically all disappeared. The only one you're still likely to find at all is -12V, but the current rating on that is minimal, anywhere from 0.3A to 0.8A is typical, even on 1000W+ power supplies. I spent some time at the local computer store looking through 55 different models of ATX power supply and not one of them exceeded 0.8A on -12V, and none had any negative rail other than -12V.

You may have 100 amps or more available on +12V (as noted, some form of circuit breaker/limiter is very important if you don't want melted rails and/or fire in the event of a short) but if you need more than 12VDC a computer power supply is no longer the viable option it was 10 years ago.

For what it's worth, I'll be powering my layout with a 15VDC supply from DigiKey.

Check out Mean Well

You can get a 15V 7Amp switching power supply from Mean Well for around $16 (plus the cost of a power cord). Jameco and others sell them.  Otherwise, since you have excess amperage, you could build a voltage regulator/converter to go from 12V to 15V at a decrease in amperage.  However, that would cost almost in parts what you spend on the above power supply.

Anoteher option

Here is a boost DC-DC convertor on ebay that would work as well, too bad you need something more than just the ATX power supply.  This is about $11

http://www.ebay.com/itm/DC-DC-600W-10-60V-to-12-80V-Boost-Converter-Step-up-Module-car-Power-Supply-LO-/161842093298?hash=item25ae8a5cf2:g:hc4AAOSwFnFWCoZl

There are smaller ones as well 150W and 250W versions, all depends on your power out you want/need.  The 150W is only about $3, of coruse these are all from China.