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help with voltage divider

Discussion in 'General Electronics Discussion' started by Madsalts, Feb 25, 2015.

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  1. Madsalts

    Madsalts

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    Sep 25, 2014
    I'm building a simple circuit based on a 12V DC power supply. I want two motors to run at the same time. One motor is rated for 12V, and the other is rated for 6. I understand that I can have a circuit that runs from the power supply, to a resistor, which leads to another resistor, and finally to the 12V motor. Between the two resistors would be tap (leading to the 6V motor). I will also have a fuse prior to each motor, which will be of a value of about 1/2 stall current of the motor.

    First, is my assumption correct that I could make the circuit so simple? Second, how would I determine the values of the resistors? In accordance with the voltage division formula, it seems that each resistor should be rated for the same impedance if I want the tap to draw off 6V. I'm also making the assumption (and tell me if I'm right) that the Amperage rating for the resistors should be at minimum, the value of the fuse used.

    So, what I don't know is what values the resistors should be. Does it matter that much? I guess there's a threshold point (current draw of the motor under standard operating conditions?) above which I should keep the current. Is this correct? Am I overlooking anything? Thanks.
     
  2. BobK

    BobK

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    Jan 5, 2010
    You cannot use a voltage divider to supply power. The are used only to set a voltage very small currents are drawn from.

    You can put a resistor in series with a 6V motor and connect it to 12V, but this is not ideal either, because the resistor would need to use as much power as the motor. Also, the resistor would have to be sized to the current drawn by the motor, but that can vary by as much as a factor of 5 depending on load (a motor draws the most current when it is stalled). So this really does not work well.

    What you need is a voltage regulator to reduce the voltage from 12V to 6V. There are two kinds. Linear regulators basically act like a variable resistor and they would, once again, waste half the power going to the motor.

    The best solution is a switching regulator, which will reduce the voltage to 6V while only taking may 10 to 20% of the power. These are also called DC to DC converters. You would need one that can output up to the stall current of the motor.

    Bob
     
    KrisBlueNZ and Arouse1973 like this.
  3. Madsalts

    Madsalts

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    Sep 25, 2014
    Thanks for the prompt reply. On Amazon, there are quite a few switching regulators. Beyond supplying current of at least "stall current level" to the motor, and being able to convert voltage from 12V to 6V, what other factors should I look for in a switching regulator? Are any brands recommended? Am I right to assume that I should create two circuits from the power supply in parallel: one to the 12V motor, and one to the 6V motor? How much head do these give off? Is it OK to put them in an enclosed project box? I'd like to do this since they'd potentially be in a dirty environment. Is there a fire risk with these? They'd be running unattended. If there is a fire risk, should a fuse be used? Thanks.
     
  4. BobK

    BobK

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    1,688
    Jan 5, 2010
    If you operate the converter well below its limits it should not be a fire hazard. Using a fuse is always a good idea, though.

    How much current are we talking about?

    Bob
     
  5. Madsalts

    Madsalts

    73
    2
    Sep 25, 2014
    stall current = 850 mA
     
  6. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    You can get buck converters rated for 3A with adjustable output voltage for less than $2 on eBay. Search on eBay for LM2596.
     
  7. Madsalts

    Madsalts

    73
    2
    Sep 25, 2014
    thanks, I appreciate it.
     
  8. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    May 8, 2012
    It's amusing how a single character typo can totally change a sentence. I don't know how this reads in Australia, NZ, GB and Germany but here in the U.S. it's,.. let's say,.. uh... not rated G. :D

    Cheers,
    Chris
     
    davenn likes this.
  9. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    Yeah I had a chuckle too. But then, I have a sick mind.
     
  10. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    May 8, 2012
    But we're Men and women expect this of us! :p

    Chris
     
    davenn likes this.
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