DCC with 4.5 V motors: resistors needed?

Ottergoose Sep 2, 2014

  1. Ottergoose

    Ottergoose TrainBoard Member

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    I'm jumping into my first scratch-build project (3D printed Bombardier Flexity Swift - one of the light rail vehicles used in Minneapolis / St. Paul) and recently ordered a motorized chassis from Japan, a TU-KIHA40000, which has a pair of 4.5 volt motors.

    Is there a need to put resistors in place between the DCC decoder and the motors in order to drop the voltage, or can the power output be handled entirely by the decoder? Looking at the literature for the TCS M4 decoder, CV5 is "voltage when the throttle is at top speed," is that all there is to it, or is there some electronic wizardry going on that would be fine for a regular motor, but fry my humble 4.5s?

    No need/requirement to run pure DC, so no worries there.
     
  2. Ottergoose

    Ottergoose TrainBoard Member

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    Asked TCS directly via their email system, and got a response less than 30 minutes later - nice!

    They say:

     
    Last edited by a moderator: Sep 2, 2014
  3. James Norris

    James Norris TrainBoard Supporter

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    Hi Nick,

    I agree that setting the CV's so the max voltage is lower would be best, you can also adjust the speed tables which will have a similar affect.

    Putting resistors in line with the motor will generate heat and that may not be so desirable. As the power from the decoder to the motor is DC (as apposed to AC going into the decoder) you can drop the voltage using diodes. As a diode works it causes a voltage drop of roughly 1.5 volts. You will need to put a put a pair of diodes in, one facing each way, other wise the motor will only run in one direction. This can also be useful if the two motors run at different speeds. I have used this very successfully to put one decoder into a pair of mismatched locos. You can read more in a post on my blog here entilteld 'Cost Effective DCC Installs for Consisted Locomotives'.

    -James
     
  4. RT_Coker

    RT_Coker TrainBoard Supporter

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    If the motors are the same, it may be possible to put the two 4.5 volt motors in series to get a maximum voltage of 9 volts and ~half the maximum current draw.
    Bob
     
  5. Ottergoose

    Ottergoose TrainBoard Member

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    Thanks for the suggestions guys, I appreciate it.

    Not sure at this point if I'll use both motors in the same vehicle or not, so it might not be possible to connect them in series. What's the normal top end voltage anyways, something like 13-14?

    Now I have to figure out how LED's work... this should be a fun project!
     
  6. mathi

    mathi TrainBoard Member

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    Hi!

    Since there are problems with usual decoders and mini-motors (the german KÖF by Minitrix with a 6V motor) the german decoder manufact. Doehler&Haass came with a DH06 decoder which is in this famly state of the art in my eyes, even in DCC.
    Although it was developed for the tiny KÖF it's doing fine in other small engines as well. So far there is no longer lasting way to cut down the problems with mini motors and decoders to my knowledge and I used this small decoder myself with fully success.
    The only problem would be to get by such stuff in the US but I'm sure that they will ship so here is the link, but just in German:
    http://doehler-haass.de/cms/pages/produkte/fahrzeugdecoder/dh06a.php

    Best regards
    Mathi
     
  7. Ottergoose

    Ottergoose TrainBoard Member

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    Thanks for the suggestion! Despite three years of German in high school, I'm afraid "mein Deutsche ist schlecht," and attempting to setup a decoder with documentation and support immediately available in English might be difficult for me :)
     
  8. SJ Z-man

    SJ Z-man TrainBoard Member

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    Couple of comments here:

    CV5 does *not* change or limit the 'Voltage' to the motor! Only the 'effective' (apparent) voltage. The same Decoder voltage (essentially the track voltage) is applied with each motor on pulse. If Speed Step 1, that full near track voltage is on 1/128 of the time so the motor still sees a full voltage but not for very long and therefor runs slower. At higher Speed Steps, that same near track voltage is applied but 'on' a greater percentage of the time. Fact. There is no adjustable voltage regulator in the decoder. By setting a lower CV5 value, the maximum 'effective' (apparent) voltage to the motor is reduced so it runs slower. Reducing CV5 now means you may only have about 22, 43, 67 or less Steps to run with in (less fine speed control) and many times these lower volt motors will easily run quite fast even at Speed Step 1, *especially* if BEMF is enabled! (Typical of any decoder today).

    True about the resistors but diodes don't change the rules. They too have a voltage drop across them and with current flowing thru them, that's power lost too. o.1 Amps with ~1.5 volt drop is o.15 Watt, the equivalent of using a 1/4 Watt resistor in practice. You need 2 diodes back to back to run in either direction, 2, 3 or 4 depending on how much you want drop.

    Getting the motors in series would help and have negligible issues. Running a single motor at the high an or HO won't generally damage it, *** until it stalls ***. Then, the heat factor in the motor goes *** way *** up (compare to a 12 Volt motor).


    Sent from my iPad using Tapatalk
     
  9. Ottergoose

    Ottergoose TrainBoard Member

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    For any "mechanical" means of dropping the voltage (diode or resistor) the same amount of heat will be getting kicked out, no? Just a matter of spreading it out, right?

    The prototype equipment I'm modeling has a roof covered with electrical equipment; I might be able to have the resistors / diodes exposed to air to help dissipate the heat. Sounds like I'd need quite a few diodes to drop the voltage down to 4.5.
     
  10. SJ Z-man

    SJ Z-man TrainBoard Member

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    True. But you can still direct connect to your 4.5 volt motor. Just be careful that it either does not stall (motor lock up) or pull heavy loads as that will significantly increase motor heating (same with an external resistor or diode strings - they will get hotter under same conditions). Plus, with direct to 4.5 volt motors, you will have less effective Speed Steps as the DCC system 'interpolates' 0-128 down to say 0-43 (<--- the probable CV5 setting).
     
  11. Ottergoose

    Ottergoose TrainBoard Member

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    Appreciate the input; I'll give it a whirl with a direct connection and see what sort of results I can get with the TCS M4T which should be here in the next couple of weeks. I'm hoping to do one motor per light rail vehicle, along with Kato caboose trucks (thank goodness they're coming out again, perfect timing for me), so hopefully they'll be pretty free-rolling.

    In any event, I'll follow-up with a report once I've got my ducks in a row.
     

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