Power question?

Keith May 26, 2016

  1. Keith

    Keith TrainBoard Supporter

    Looking to install a couple of small(30mm/30mm/10mm) 12 volt fans into
    a pair of Aristo Craft G Scale GP40's and a USAT G Scale NW-2 for starters.
    I've had them fail, when decoder went into thermal shutdown. Decoder then
    began speaking, stating, Temperature 98*C. Would do this about every 2 minutes
    or so, until decoder reset, after sitting for up to 30 minutes or more, to cool down.
    Finally restarting around 80*C.

    Looking to use the Smoke switch on the locomotives to power the fans, since I don't
    use the smoke units. Just need to find out what the voltage is at the On/Off switch.
    Thought I'd ask here first, before asking Dad where his meter is, so I can have voltage checked.

    Have been wanting to do this project for several years now!
    Hope to order the fans shortly. I'll do the same to the K-27
    IF I can get the counterweight problem fixed. And maybe a
    couple of other units later. Namely, the SD70MAC and
    eventually, the PA, once I get Airwire and Sound added!

    Any information appreciated.
  2. BoxcabE50

    BoxcabE50 HOn30 & N Scales Staff Member TrainBoard Supporter

    How much space is available? I would think you might be able to find a fan amongst computer accessories. Such as used for cooling a microprocessor.
  3. TwinDad

    TwinDad TrainBoard Member

    OK, I'm a little unfamiliar with G scale decoders... I take it there is a SMOKE output from the decoder that is intended to power/control a smoke generator? And you want to re-purpose that as an on/off control and power for these 30mm fans you have?

    Can you point me to the specific decoder(s) you are using? And track voltage and type (DC? DCC? AC?)

    Basically speaking you'll want to tie the + fan wire (red, usually) to a common voltage, and the - fan wire (black, usually) to the control IO of the decoder. You'll need to make sure the IO can handle the current load of the fan, and that the common voltage doesn't exceed 12V. You can run a 12V fan on 5V, it will just run slower (5/12 of max speed). But if you want more airflow and your next higher common voltage is above 12V you'll need to use a resistor or a diode or regulator or something to drop the unwanted voltage and deliver 12V to the fan. You could use a 7812 regulator wired between the common voltage and the fan to supply the 12V. Just tie the "Ground" pin of the 7812 to the decoder Ground, the black wire of the fan to your decoder IO, the "Vin" pin to Track Common, and "Vout" to the red wire of the fan. This will supply 12V to the fan when the IO is on, and shut the fan off when the IO is off. The regulator will be active at all times, but will draw negligible current when the fan is off.

    It's doubtful that the fan will draw enough current to overheat the 7812, but it wouldn't hurt to bolt or glue (with conductive glue) the tab of the 7812 to some grounded metal piece in the loco. If your loco uses the frame to transmit track voltage (and if the track voltage is AC or DCC-like), then don't use the frame for this.

    I took a guess and looked at the QSI Titan decoder to get an idea... looks like there is a Track Common that is your track voltage - 1.5V, and there is a 5V common. The Titan instructions say to tie your fan to 5V common and J3 Pin 8, which is "Port 11".
  4. Keith

    Keith TrainBoard Supporter

    Found 30mm/30mm/10mm, or about a 1.6", fan on Amazon, that should fit perfectly.
    These are what I'm looking at right now:


    Just need to make a fan mount out of a piece of styrene. Planning to have
    one fan blowing up, through rear radiator fan. The other, blowing
    horizontal, across sound board.

    The dynamic brake blister covers:
    Smoke unit output
    On/Off slide switches for:

    My thought was to use the smoke unit switch. By disconnecting smoke unit
    and wiring in fans. Should be a constant voltage, and does NOT change with track polarity.
    This is why, in part, I need to know voltage at slide switch.

    Power - DC track power
    Power supply - MTH Z-4000 A/C transformer, with 12 Amp bridge rectifier added.
    Control - NCE G-Wire throttle, with QSI sound board and Airwire receivers in locomotives.
    DCC control also possible, if Airwire reciever disconnected. Tried briefly, with Digitrax system.
    Circuit breaker tripped after maybe 10 minutes, with single locomotive!

    At full power, no load on track, I get 24-25 VDC. Adding a load, whether one locomotive, or 2-3
    locomotives, and lighted caboose, power drops to about a steady 21VDC. It may vary slightly, depending
    on how clean track and wheels are. Typically, a volt or two up or down. And because the Z-4000 is a
    fan cooled power supply, I can run both GP40's all day without a problem. Or the K-27. Unless the
    thermal breaker trips!
    Last edited: May 26, 2016
  5. Greg Elmassian

    Greg Elmassian TrainBoard Member

    I could tell from the first post these are QSI.

    1. are they the original QSI Revolution or the new Titan?
    2. what size digitrax system do you have? I think you need to verify the locos are running right, if they are not working right and drawing too much power, then that should be fixed first.
    3. what is the current the loco is drawing under load? (typical train that causes the overheating)

    Can give you much better (and accurate) answers with this data.

  6. Keith

    Keith TrainBoard Supporter


    1. Yes, original QSI decoders.
    Don't have any Titan boards at this time.
    Maybe one, when I can afford one, for a USAT ALCo PA.

    2. Have a 5 amp Digitrax system. Used primarily for the N Scale stuff.
    It was tested briefly on the G Scale. Although, midday, middle of summer
    and no shade probably didn't help the overheating situation!
    Primary power is an MTH Z4000 with a 12 amp bridge rectifier.
    NCE G-Wire throttle and Airwire receivers.
    With this set up, I can run both GP40's and 10-12 cars for up to 8 hours non stop
    with no trouble.

    3. Train can be anything from a single Aristo GP40 to a single unit, with 5-6 cars.
    Only time I have overheating problems is if I'm running in the middle of the day during
    summer, with temps in the 90's and direct sun in the early afternoon, 1:30-2:00 maybe.

    Longest run so far, has been 6 1/2 hours. But, temps in upper 60's to low 70's.
    Hasn't gotten extremely hot. Yet.
    With fans arriving today, I can look into how to try and install them.
  7. Greg Elmassian

    Greg Elmassian TrainBoard Member

    You are running unfiltered rectified AC... this will cause more heating than pure DC or DCC.

    DCC is a square wave and when rectified by the full wave bridge on the QSI it needs very little filtering, this is bccause the negative halves are "square" and when "flipped" to positive, you have pretty close to DC.

    But your AC is a sinusoidal curve, and when you rectify it, it is actually pulses of power:

    Top is AC, bottom is rectified AC... it's a bunch of pulses... BAD.... you need filtering to smooth it out:


    Even with filtering it's not great, but NOTE WELL: the more current you draw, the more filtering you need. You have none now. BAD.

    I would get an inexpensive, regulated switching DC power supply, or you can try filtering your old transformer with BFC's (Big Friggin' Capacitors)

    Here's a chart for filtering caps:


    Notice for 3 amps you need and 1 volt of ripple (still sucks) you need about 25, 000 microfarads, for 15 amps you need 5 times as much so about 125,000 microfarad, and you need the working voltage to be double what the nominal voltage, so assume 25v nominal... so a 50v 125,000 microfarad electrolytic capacitor.

    Here's one from Digikey that is close: http://www.digikey.com/product-deta...tronics-cde/CGS124U050X5L/338-4009-ND/1595820

    Notice it is $134....

    Meanwell 15amp 24v REGULATED filtered DC power supply: http://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_2219823_-1


    Buy a new power supply.

  8. Keith

    Keith TrainBoard Supporter

    Trains run fine and have, as long as temperatures stay cooler.
    As long as it cool, or I'm in the shade when I run, I have no problems
    Had sounds start acting up today, but only because it was near
    90* outside, and also in direct sun for 2-3 hours. Stopped train
    in shade and let sit for about 5 minutes. Sounds worked fine after.
    Parked in backyard for about minutes, as I needed to lay down for
    a bit, and sit down for dinner. No problems When I resumed running
    for a couple more hours. Have several small fans I want to install before
    too long, to provide internal cooling when it gets hot and during those days
    of intense heat. IF I choose to run.

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