Electrical question.

John Moore Jan 30, 2023

  1. John Moore

    John Moore TrainBoard Supporter

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    In a few days some new motive power will arrive. It uses this power chassis which is a 4.5 volt motor.


    The loco is quite small less than 45 mm long so space is at a premium within the shell. So the object is to operate this at my normal track voltage 12 volts. Doing some research on the internet I come up with needing a 4.7 K ohm resister to drop the incoming voltage.

    First question is this right?

    Second question do I need one for each wire to the motor or will one do?

    And the next issue is going to be the physical size of the resister since again space is at a premium so the smallest one possible is my best option.

    Note that my electrical knowledge is limited to flip the switch and do not stick your finger in an empty light socket so keep any answers in simple English this old mind can absorb.
     
    gjslsffan and mtntrainman like this.
  2. CSX Robert

    CSX Robert TrainBoard Member

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    Would have to know the current draw of the motor to know the correct resistor value, but 4.7k would certainly be too large. Current = volts / resistance = 12 / 4700 = 0.0026, so a 4.7k resistor would limit the entire circuit to 0.0026 amps (2.6 milliamps), so the motor would have to draw significantly less than that to work. You need a maximum 7.5 volt voltage drop (12-4.5). Voltage drop = resistance x amps, resistance = voltage drop / amps. If the motor draw 100 milliamps, you would need 7.5/0.1 = 75 ohms. If the amp draw is 10 milliamps you would need 7.5/0.01 = 750 ohms, etc.
     
  3. Shortround

    Shortround TrainBoard Member

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    I would just use an old DC controller. Or use a 9volt or lower battery and a small rheostat to power the rails. Much easier and I have several to use.
     
  4. jbonkowski

    jbonkowski TrainBoard Member

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    I'll add that the current you want to know is the stall current, so push down on the engine (while it is on) hard enough to stop it from moving.

    Measuring the current will require a meter, usually a hand-held digital multimeter (DMM). One of the two wires coming out of the power supply will need to be interrupted between the power supply and the track, and the meter inserted. You will likely have to rotate the DMM knob to a current setting, and maybe also change where the probes plug into the meter (usually labeled "A" for amps). The knob on the DMM will likely have choices for the range of current, so you will have to use that info in making sense of the current reading value.

    Per your question, only one wire to the motor will need a resistor. You can get a pretty small resistor. Resistors are rated in power (Watts), so a lower Watt rating will be a smaller resistor. If the engine stall current is 40 milliamps (mA), that would be 1/2 Watt (power=voltage*current so 0.48 W =12 V * 0.04 Amps). If the stall current is 20 mA or less, then you can use a 1/4 W resistor.

    Jim
     

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