Layout Power... Help Me Decide?

rray Feb 4, 2023

  1. rray

    rray Staff Member

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    So recently I had a look under a layout and in the bird's nest of wiring, I seen a power strip of wall warts, transformers, AC bricks and daisy chained AC Power Strips. Yeah...

    Well I am in the process of wiring up a 12' x 30" T Track modular layout in my garage. I DO NOT want wall warts and power strips all over. I want ONE 12V power supply. 1 only. So let me tell you what I need to power.

    *Digitrax DCS210 Evolution 5-8 amp command station.
    *Kangaroo 12V mini windows 10 PC for JMRI.
    *Surroundtraxx DSP-80 multi train sound system (Z Scale no onboard sound decoders).
    *Digitrax TWO BXP88 LocoNet Occupancy Detectors powered off the DCC booster.
    *Digitrax TWO DS52 Solenoid Turnout decoders powered off the DCC booster.
    *12 Tortoises with Hare accessory decoders powered off the DCC booster.
    *20-30 Arduino and Attiny microcontroller projects powered off the 12V power supply through generous use of 5V 1.5A buck converters scattered around the layouts modules.
    *500 or so LED's current limited to no more than 6mA each scattered around the layouts modules.

    So I can add up all the maximum current usage and come up with near 30 amps at 12V, but it would be near impossible that I actually used that much. All functions on all microcontrollers at full activity, and maximum amount of trains running, sound up full blast and I might go near 20-25 amps, but what realistically would happen is I run 6 locomotives at a time at 30 milliamps each, sound is tracking the blocks at realistic low scale volume, and lots of lights and animations on drawing maybe 5 amps max other than initial layout power up surge.

    My thinking is to get a 20A 12V 240W open frame regulated switching supply for $20 from Amazon like this. What's everyone's opinion, and I would really appreciate hearing from the microcontroller and electronics geeks of you.
    power supply.jpg


    Thanks,
    Robert
     
  2. MK

    MK TrainBoard Member

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    Do you have or have access to a surplus PC power supply? Get one of these and done!

    https://www.amazon.com/XH-M229-Desktop-Computer-Transfer-Terminal/dp/B07SV4KZ4B

    You 12V current supply needs can be spec'ed to how big of a PC power supply you use. A small 500 watt PC power supply can get you 41-42 amps, I think.

    With this, the 3V and 5V comes along free for the ride in case you need other voltages.
     
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  3. rray

    rray Staff Member

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    I don't have a PC power supply, other than the laptop brick style. But I did find an Amazon Choice 360W 12V open frame supply for $21. I don't know if you are familiar with T Trak modules, but they use Anderson Powerpole connectors, and I plan on using a fuse block with fuses rated for each cable harness. So the DCC system will be on one fuse, the accessory power on another fuse, the sound system on a third, and a couple spares:
    Fuse Block.jpg
     
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  4. MK

    MK TrainBoard Member

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    Yes, I'm familiar with T-Trak. My club runs them all the time at shows. We were part of the Combined T-Trak layout at Amherst.

    That looks good with the fuse box. We just can't use all Anderson PPs in public because there will be someone who will haphazardly connect the 12VDC to the DCC bus and blow the command station! For your own personal layout it may be fine. We have to use unique, unmistakenable connectors to eliminate oopsies which would cost major $$$.
     
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  5. french_guy

    french_guy TrainBoard Member

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    With a PC PSU, make sure the 12V is single rail......
     
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  6. rray

    rray Staff Member

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    So I did not get much feedback on this thread, but thanks for those that did chime in, I went ahead and ordered this 30A single rail power supply for $21 this morning. My Anderson Powerpole connectors came in and i am going to start making cables up today, and there is only one day delivery on the power supply and meter.
    01.jpg


    I also ordered this power meter for $16, to go between the power supply and the power distribution fuse block, not that i need it, but I want to have a better idea of my actual needs and usage. It will also be handy as allowing me to adjust the trimmer voltage level with all loads powered:
    02.jpg
     
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  7. rray

    rray Staff Member

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    So for T-Trak-Z we have a block of 4 powerpole connectors for track with the red and yellow shells, and 2 for accessories with the white shell. The pins are top to bottom for the DCC loops and accessories are left to right. That would blow the fuse on the accessory line. At least there are colors so a guy would have to be pretty inattentive to mess that up. But yes, it could happen.
    03.jpg

    Maybe we need a electrical coordinator who is the only one who makes the module connections, just in case, and he follows a protocol of connecting all the accessory connectors first, then powers it up and measures for voltage leaks across rails, and if OK, proceeds to connect DCC connectors?
     
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  8. BigJake

    BigJake TrainBoard Member

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    Let's take a step back for a minute...

    Regardless of the apparent mess of having separate, isolated supplies, they do serve a valuable purpose. They restrict power supply return currents to travel directly back to their supply by dedicated paths. When power supply return currents are forced (intentionally or not) to mix with signal returns, the signals suffer, and malfunctions ensue.

    Do you need an isolated supply for each individual powered device? No, but some strategic supply-return isolation boundaries are better than none. Even/especially when using non-isolated interfaces (e.g. USB, TTL, etc.) between them. Having each end of the interface supplied by separate, isolated supplies means that no supply return currents will travel on the USB power return/shields, signal returns, etc., thus reducing ground noise between subsystems, and improving reliability.

    Note, virtually all wall-wart and encapsulated, in-line brick supplies (like laptop supplies) are isolated.

    I would consider the following:
    1. A separate isolated supply for the PC.
    2. Separate isolated supply(s) for the PC display(s) not directly powered by AC line cords (their internal supplies will be isolated from their respective AC returns)
    3. A separate isolated supply for the DCC system and DCC accessories.
    4. A separate, isolated supply for the Surroundtrax system
    5. One or more isolated supplies for all the Arduino systems together (closely co-located ones might share a supply.)
    I spent my entire career developing mission-critical embedded electronics systems. Isolation is your friend. It keeps (especially noisy) return currents from going where you don't want them, causing problems you don't need.
     
  9. rray

    rray Staff Member

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    All the Arduinos have their own buck converters to take the 12v down to 5v, are those considered isolated?
     
  10. BigJake

    BigJake TrainBoard Member

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    Buck converters are not isolated. They can be combined with an isolated front-end, but a that's really a different topology. The input and output for a buck converter share a single common/ground/return.

    I don't think you need a separate isolated supply for each Arduino, especially if multiple Arduinos are closely located to each other, but an isolated supply for them (and/or their associated buck converters) as a group is a good idea.

    Even when isolated supply returns are combined at a single point in order to insure common reference voltages for signals between them, the return currents are automatically routed back to their respective supply's return, minimizing interference with signals in other isolated domains.

    BTW, any two-pronged wall-wart is an isolated supply, because the AC line neutral is NOT considered a safe ground connection.

    Three-pronged wall-wart supplies are usually isolated, but not necessarily. The plug's third (round-ish) prong is for ground, which should only/ever carry current in adverse conditions, even though house-hold neutral is grounded* at the main circuit breaker panel.

    *Rant-on: Don't get me started on the utter stupidity of the North American 220/110VAC power system. The Europeans got it right, where everything runs on 220VAC, and there ain't no stinkin' neutral! How safe is your household ground system if it's connection to earth also carries any imbalance in the total load currents on each of the two phases? If everything were 220V, there would be no imbalance between phases! *Rant-off.
     
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  11. Erik84750

    Erik84750 TrainBoard Member

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    Not only that; also a lot less copper is required for the same wattage.
     
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