Large Scale Power

[TJMac]

I'm currently running DC 2-rail O scale and looking to incorporate RailPro, but I have a question/concern before I spend the money.

POWER

Looking at the PWR-56 spec page - "The PWR-56 has enough power for 1 - 2 G scale locomotives with LM-3S-G modules installed". 1-2 locos at a time isn't enough, and I simply can't justify multiple $100 power supplies if there are other options available.

Any recommendations?

Thank you

Tom

[Dean]

I would think any quality power supply would work. I have Ho but use a power supply I bought off of E-Bay.

Similar to this https://www.ebay.com/itm/30V-5A-DC-Power-Supply-Adjustable-Variable-Dual-Digital-Lab-Test/152857635607?epid=25012678214&hash=item2397066b17:g:zfwAAOSwO~VaUtii

[Alan]

Dean is correct, virtually any DC regulated power supply will work. Estimate the total amperage you will need and then buy a power supply with that rating. Dean is using a lab supply. That's great but it is overkill. Lab power supplies are designed to have extremely low noise, now ripple output. Those fine specifications go unused as there is no need for or benefit from such a precision waveform on a model railroad.

Here is my collection of power supplies. Two 12V @ 30A units for upper deck lighting, a 12V @ 40A unit for lower deck lighting, a 12V @ 20A for accessories, and a little 15V @ 7A unit to run the trains. All were bought on eBay.



And to make sure the supplies are behaving:

[Dean]

I found a 12v 20A power supply on eBay for $21.89 and free shipping.  https://www.ebay.com/itm/Universal-12V-3-5-10-15-20-25-30-50A-Switching-Power-Supply-Driver-for-LED-Strip/122480488771?hash=item1c8467f143%3Am%3AmbHRn4DA1ANr7tBMGw1d2iA&var=422931826547

Yes my power supply ( 30v-5A ) is over kill. But priced under $30.00 and free shipping, I couldn't pass it up. I bought two so I had a spare.

[Alan]

Certainly nothing wrong with using a lab supply and yours was priced right. I mentioned it only so the OP knows he doesn't need to have a lab supply (which are more expensive than general purpose supplies) just to run trains.

I too have a lab supply but it is used at my workbench when I am building electronic do-dads. The variable voltage is essential when building stuff and the clean output doesn't cause odd operation of electronic circuits that have been breadboarded without all the usual protection and filtering that goes into the final assembly.

[TJMac]

Awesome info thank you! Another question, the instructions for the large scale module say not to exceed 12amps....is that the engine under load or the system as a whole (IE: the power supply should be less than 12amps)?

So something like this?

15-18 volts
10 amps
Watts will be 150-180

Electricity is NOT my thing 

[Alan]

12 amps is the most current a single LM3 can handle for a brief instant. 8 amps is the most current a single LM3 can handle for an extended period of time. You must use some sort of circuit protection to disconnect the power if the current exceeds 12 amps through a single LM3. That could be a circuit breaker or a fuse. I would think a 12A fuse on the locomotive would be the most cost effective solution.

The power supply rating should be (total motor stall current of all locomotives that will run simultaneously) * 2.

For example:

Loco 1 stall current = 5A
Loco 2 stall current = 8A
Loco 3 stall current = 6A
Total stall current = 19A
x 2 = 38A
Use a 40A power supply

[TJMac]

I think I got it....I think. How do I calculate the stall current of my locos....or can I?

Thanks again!

Tom

[Alan]

Sooner or later you have to set the loco stall current on the HC so that it will control the loco correctly. Follow the instructions in the HC manual. The stall current will display on the screen.

If you do not yet have power you can directly measure the loco motor resistance with an ohmmeter by placing the test leads on the drive wheels or on the motor leads. Then use Ohm's Law to determine what the amperage will be at whatever voltage power supply you plan to use. Amps = Volts / Ohms. You may need to spin the motor armature slightly by hand to find the lowest resistance reading (motor brushes spanning two commutator strips).

Examples:

Power supply voltage = 18V
Meter reads 6 ohms across loco wheels
18 / 6 = 3
Stall current at 18 volts = 3 amps

Power supply voltage = 24V
Meter reads 6 ohms across loco wheels
24 / 6 = 4
Stall current at 24 volts = 4 amps

[atsfguy]

Alan,
  Are you saying to measure resistance between drivers to obtain the wattage to feed into Ohm’s law?
I played with electronics way back in the 60s but my Army career interveened and I missed out on lots of good stuff coming on in the 70s.
  Correct me if I am mistaken but I thought one had to actually stalll the loco in order to measure stall current. Your method, if I understood correctly, is much simpler and easier. Easy is better!
  Thanks for everything you do in this forum, it is much appreciated.

Cecil Collum
atsfguy
US Army, retired

[Alan]

Yes, measure the resistance between drivers. Would be better to measure right at the motor terminals to eliminate any wheel wiper influence. You aren't directly measuring wattage although Ohm's Law would allow you to calculate it from the measurements. Wattage is a power measurement - amps x volts. Knowing the wattage is not necessary for your purposes. You are trying to determine only one half of that equation - amperage.

When a DC motor is stalled it effectively becomes nothing more than a coil of wire that has resistance (the winding + brush contact resistance). The resistance of the winding and the voltage applied determines how much current will flow. By measuring the resistance of the winding with an ohmmeter while the motor is at rest you are measuring the winding and brush contact resistance the same as if the motor were stalled under power. Ohm's Law then tells you the amperage at any specified voltage. At this point you could then calculate wattage if you are curious however there is no need to do so. Your power supply choice will be based on the desired voltage and the required amperage. The power supply will have a wattage specification but that is used more to indicate the load it places on the mains (wall plug).

Take several measurements slightly turning the motor shaft between measurements. Make sure the shaft is still when measuring. Average the readings. Plug the averaged resistance measurement into Ohm's Law along with whatever voltage you have selected for your trains. The result will be the stall current of the locomotive. Be aware, if the shaft is turned while taking a measurement you will get an incorrect reading because the motor acts as a generator when the shaft is turned. Sensitive digital multi-meters will detect the small generated voltage and give an erroneous resistance reading.

There is no need to be ultra-precise with this measurement process. Close is certainly good enough. Notice in the earlier post it was mentioned to double the needed amperage when selecting the power supply. There are several reasons for that. First, power supplies wear out quickly when continuously operated at or near 100% rating. Heat makes components fail sooner. Second, your track, layout wiring, and the motors connected to it will have natural capacitance and inductance. The power supply voltage regulation response time will be affected by such especially so when a power supply is operating near its rated output. Lastly, you do not want normal operation of the trains to use enough current to be anywhere close to the power supply internal circuit breaker setting. Nuisance trips may result. In summary, ample power supply headroom is a good thing. It is also why precise loco measurements aren't required. Close is good enough.

[atsfguy]

Thank you for the explanation, Alan. It makes more sense than my first reading with my limited electronics knowledge.
The forum is extremely fortunate to have you and several other electronics experts on board.
Cecil
atsfguy

[Dean]

In RailPro the 'stall current' is when the wheels are spinning and the train is not moving. This current reading will be lower than the calculated value. Actually, I think it is almost impossible to get a model train to stall with the wheels not turning.

Because I run HO, these power supplies for large-scale give me the willies. My 225 Amp Lincoln arc welder has an output of 12V to 50V DC and a current output from 40A DC to 130A DC.

Like Alan mentioned, be sure to have circuit breakers or fuses in your locomotives and protection for the layout. Be sure the fuses and circuit breakers you choose are rated for DC.

[Alan]

In RailPro the 'stall current' is when the wheels are spinning and the train is not moving. This current reading will be lower than the calculated value. Actually, I think it is almost impossible to get a model train to stall with the wheels not turning.

Because I run HO, these power supplies for large-scale give me the willies. My 225 Amp Lincoln arc welder has an output of 12V to 50V DC and a current output from 40A DC to 130A DC.

Like Alan mentioned, be sure to have circuit breakers or fuses in your locomotives and protection for the layout. Be sure the fuses and circuit breakers you choose are rated for DC.

Exactly right. The reason for the manual measurement is to help select a suitable power supply, not to set the RP stall current setting. To the best of my knowledge there is no means of manually entering the RP stall current.

[Dean]

You can manually set the stall current for a locomotive. Without the hand controller in front of me, I couldn't tell you how. But I have done it while experimenting with all the capabilities of the RailPro system.