DC: Multiple units vs amp draw

Discussion in 'DC' started by trainchaser007, Aug 11, 2012.

  1. klrwhizkid

    klrwhizkid Administrator Staff Member Administrator Frisco.org Supporter

    Yes, about 2.5 times as much current.
     
  2. trainchaser007

    trainchaser007 Passed away September 22, 2017

    OK, so I guess I want the highest voltage output possible and the highest amp output possible for that voltage.

    So...

    1. What is the highest voltage that typical (1990-present) DC locomotives can handle safely? 14v? 16v? 18v? (Seems like someone told me 18v???)

    2. On average, how many VA does a single, typical (1990 - present) DC locomotive draw at full throttle?

    -Brandon
     
  3. klrwhizkid

    klrwhizkid Administrator Staff Member Administrator Frisco.org Supporter

    Brandon, the highest voltage and current should be of little interest unless you are running rocket sleds. The main concern is the amount of current necessary to move a train at the voltage necessary to make the train run a prototypical speed. I am going to guess that voltage would be about 8 volts. The typical HO loco with can motors will pull about 1/2 amp at full stall.
     
  4. trainchaser007

    trainchaser007 Passed away September 22, 2017

    Keith, I was simply thinking that 6VA x 3 locomotives = 18VA, or 12V x 1.5A, or 14V x 1.286A, or 16V x 1.12A, or 18V x 1A... but perhaps as long as the total VA is sufficient, the volts-to-amps ratio really doesn't matter. With my limited knowledge of electricity, I could certainly be wrong. Anyway, the bottom line is that I want to run my 3 Frisco locomotives (an Athern SD40-2, an Athern GP40, and a Walthers GP15) together with the same overall performance-per-loco that a single locomotive has with a 6VA power pack. In order to get that result, when shopping, do I need to multiply the total output by the number of locos I will be running? In this case, 6VA x 3 locos = 18VA? If not, please tell me the minimum output I need to run them as desired? Thanks.
    - Brandon
     
    Last edited by a moderator: Jan 31, 2015
  5. klrwhizkid

    klrwhizkid Administrator Staff Member Administrator Frisco.org Supporter

    Brandon, I am pretty sure your 18VA pack will run those three locomotives you specify pulling cars on your layout.
     
  6. gstout

    gstout Member Frisco.org Supporter

    Unless your three locomotives are equipped with 1950s-era Pittman motors or some such, you should have no trouble running them together with the power pack you have.

    GS
     
  7. trainchaser007

    trainchaser007 Passed away September 22, 2017

    OK... I just read my last post which I admit was pretty confusing. To clarify, my best power pack is only 7VA. It sounds like 12V x 1.5A = 18VA would run 3 locomotives with no problem.

    In the meantime, I'm trying to understand how volts and amps impact a DC locomotive motor. What would be the difference if any? 12V @ .5A or 18V @ .33333A? Both are 6VA. Would the 18V run faster than the 12V... even with the same total output, or would they run at the same speed?

    - Brandon
     
  8. klrwhizkid

    klrwhizkid Administrator Staff Member Administrator Frisco.org Supporter

    Voltage determines speed. Since the motor has a fixed resistance, as voltage goes up, so does the current it draws. If a load is imposed on the motor, as load increases, so does the current if the supply voltage does not drop.
     
  9. trainchaser007

    trainchaser007 Passed away September 22, 2017

    I just wasn't getting it so I googled the difference between volts and amps. http://www.helcohi.com/sse/body/hp.html I may finally have a grasp on it. (Emphasis on "may.")

    It makes sense to me that, with an ample supply of water, a 1 1/2" hose would have a higher output rate (GPM) than a 1/2" hose. I imagined a 1/2" hose filling up a tank at a rate of 10 GPM. Even if the tank has an open drain with a rate of 9 GPM, the 10 GMP of the 1/2" hose is sufficient to keep the tank full. However, if 2 more drains were opened up, the total drain rate would be 27 GPM and the 1/2" hose couldn't possibly keep water in the tank. Sure you could increase pressure to the hose to get a higher output but that would increase the flow (speed) of the water. To increase the output without increasing the pressure, you would need a larger hose. If you were to connect a 1 1/2" hose with an output of 30 GPM to the same water supply, the 1 1/2" hose could fill the tank, even with all 3 drains (27 GPM) opened. (I think the tank would fill at 3 GPM.)

    Now if I have finally figured this out...

    The output of 12V x 1.5 and 18V x 1 are the same, but the pressure/speed of the current at 18V would have to be faster to get 18VA out of "a 1 amp hose"... the same way higher water pressure increases GPM but increased hose diameter increases the output without increasing the water pressure/speed. Each motor is like a "drain" but as long as the hose (amps) can supply enough GPM (watts), the tank won't run dry. Yes, you could increase the pressure (volts) with a small hose (amps) to get enough output, but unless you want the water (locomotive) to travel faster, you need to leave the spigot (volts) alone and get a larger hose (amps).

    Moral (I think): To get enough power to run multiple locomotives without increasing the speed, get a power pack with enough amps to offset the combined draw of the locomotives instead of getting a power pack with a higher voltage. (Am I right?)

    - Brandon
     
  10. klrwhizkid

    klrwhizkid Administrator Staff Member Administrator Frisco.org Supporter

    By Jove, I think he's got it!
     
  11. trainchaser007

    trainchaser007 Passed away September 22, 2017

    OK... I can already hear the DCC folks saying, "Oh no! Not this thread again!?!" I wouldn't bring it up again but I recently found a 12V, 12.1A power supply for free and I'm dying to know if I can use it to modify a power pack. As I've said before, I would like to run 2 or 3 locomotives on the same train but I need more than the current half amp output that each of my four power packs supply. I recently found a power supply to an Xbox 360 that someone was throwing away. It has an output of "DC 150W, 12V, 12.1A, 5Vsb 1A." I already know how to modify the 8-wire output cable for common 2-wire use.
    [​IMG]
    What I want to know is this: Can I replace the transformer in my MRC Railpower 1300...
    [​IMG][​IMG]
    ...with the Xbox 360 power supply in order to increase my amps from about 0.5 amps to 12.1 amps? The MRC Railpower 1300 has an output of "15VDC, 19VAC" and a "Total Ouput 7VA." I would only need to run trains with the modified power pack since I have 3 more small power packs for accessories and turnout remotes. Even if this would work, I think I would still need the rest of the MRC's electronics including the potentiometer and the DPDT switch. I'm concerned about something called a rectifier. Apparently rectifiers convert AC to DC. Assuming the 1300 already has a rectifier somewhere, and the Xbox 360 power supply already converts AC to DC, would this create a problem? I could get a MRC Railpower 1370 with an output of 18VA for $29.99 right now so if my "free" idea won't work, it's not a big deal. It's just another project I would enjoy if it will work. When I saw the power supply in a box of junk, I got curious so I thought I would ask. - Brandon
     

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  12. klrwhizkid

    klrwhizkid Administrator Staff Member Administrator Frisco.org Supporter

    Brandon, the issue may not be just the power capacity of the transformer in the power pack. If it was, then the output from the Xbox power supply may possibly be connected to that circuit board where the transformer connects. The transformer output is AC and it goes through a rectifier on the board. If the rectifier on the board is a bridge rectifier (may be a single four legged chip or four separate silicon diodes), then the output from the Xbox power supply can be connected where the transformer connects to the board. If you took a picture of the other side of the circuit board, I could probably make a judgement.

    I suspect the output control components of the power pack are also of limited capacity and would not handle the higher current capability of the Xbox supply.

    I highly recommend that you do some studying of basic electrical and beginning electronics as the background will be invaluable in your future railroading.
     
  13. trainchaser007

    trainchaser007 Passed away September 22, 2017

    Here you go, Keith. Hope these help. I was concerned that the DC power coming out of the Xbox 360 power supply won't work if their is a rectifier on these components that converts AC to DC. If only I could just connect 12 DC at 12A to a potentiometer for a throttle, a DPDT switch for direction, and then to the track, I would be set. I'm guessing it's not even close to that easy. - Brandon

    [​IMG][​IMG][​IMG][​IMG][​IMG]
     

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  14. bob_wintle

    bob_wintle Member Frisco.org Supporter

    Brandon: In short the answer is no. It is not nearly that easy. You would fry the Pot. There may be ways to make it work but it would take a little more money. I don't remember the name of the Old Kalmbach Electronics project book I have but there may be a very useful project in there for you.

    Bob Wintle MMR 419
     
    Last edited by a moderator: Aug 3, 2015
  15. klrwhizkid

    klrwhizkid Administrator Staff Member Administrator Frisco.org Supporter

    Brandon, in looking at all the pictures, I think I see the evidence of a bridge rectifier. You would have to disconnect the transformer from the circuit board and then you could use the AC output terminals as input terminals from the Xbox supply. There is a chance that I will work fine. The limiting factor is the current rating of the transistor that is visible. Can read the numbers on it? If it is a TIP102 as it appears, it has an 8 amp current rating and will work just fine.
     
  16. trainchaser007

    trainchaser007 Passed away September 22, 2017

    Keith,
    I can verify that that number is TIP102. So, all I have to do is cut the wires from the transformer and connect the Xbox 360 power to the AC outputs? If so, could I re-heat the solder where they are soldered to the circuit board and pull them loose from the circuit or should I just cut them and cap them? If this works, would I have a power pack with 12VDC @ 12.1A - theoretically anyway? That would be so awesome, especially since the mainline on my new layout is going to have about 7 more feet of track than the longest track on my last layout. More track = longer trains = more locomotives! Fingers crossed! Thanks so much for the help so far. - Brandon
     
  17. klrwhizkid

    klrwhizkid Administrator Staff Member Administrator Frisco.org Supporter

    Un-solder the transformer wires from the circuit board.

    Two cautions:

    1) You really should have something like a 10 amp fuse between the track and your power pack.

    2) I would highly recommend a power bus to feed every section of track; nickel-silver rail is actually not that good of a conductor and you will have voltage drop if you try to feed all the rail in one location.
     
  18. trainchaser007

    trainchaser007 Passed away September 22, 2017

    Keith,

    Will do on the un-soldering.

    I was already planning on running a bus like I did on the last layout. Actually there were 3 tracks on the last layout so I ran 3 bus wires with feeders to every section of track. I never want to do that much again. This one will only have one track/bus with 2 run-arounds (4 isolotated blocks). I am using as few sections of flex track as possible so that I don't have to run any more feeders than absolutely necessary. Another power pack will power lighting in the buildings later on.

    I'm guessing an in-line fuse is what you have in mind and that you don't think I should try to pull more than 10 amps?

    -Brandon
     
  19. bob_wintle

    bob_wintle Member Frisco.org Supporter

    Don't mean to butt in, however instead of a fuse I would insert an miniature breaker.
    They are readily available and if you have a derail can be reset instead of replacing a fuse. May be cheaper in the long run.
    Bob Wintle MMR 419
     
  20. trainchaser007

    trainchaser007 Passed away September 22, 2017

    Thanks, Bob. By the way, I know what you mean, but there's no "butting in" as far as I'm concerned. I always welcome any helpful advice. I had never heard of miniature breakers. That's something I will investigate. - Brandon
     

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