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Voltage Regulator Circuit Calculations

  1. May 6, 2017 #1
    Hey Guys, so I'm just in need of some help in relation to finding the output current after the voltage has been dropped to 6V. The circuit has an input voltage of 24V and I've attached a photo of my circuit diagram. Hopefully I haven't made any errors in that, but I've tested the circuit and it does work effectively. The purpose of the circuit is to be able to flick between motors and also change their direction of rotation. The reason I need to know the output current is so I can find the power that each motor will require but I'm just a bit confused. Thanks.
  2. jcsd
  3. May 7, 2017 #2


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    You would have to measure the currents.

    Unless the motor comes with information about how much current it draws when supplied with 6 volts and with different loads, the only way you can find out is to measure it.

    You could put an ammeter in series with each motor, but this might cause the current to be reduced due to the resistance of the meter.

    Or, you could put the meter directly after the 24 volt supply and measure the current there.

    You could measure without the motors to work out how much is being used by the resistors and the regulator. Then subtract this from the figure you get with the motors running.
    Last edited: May 7, 2017
  4. May 7, 2017 #3
    Thanks very much. The motors are micro metal gear motors so I'll have a look and see if they have that information.
  5. May 7, 2017 #4


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    As VK6KRO has said, just measure it

    Be aware that the LM317 like most other linear regulators have inbuilt current limiting --- check the specifics in the datasheet
    it's somewhere around 1.5A
  6. May 7, 2017 #5
    How much current is drawn depends on motor characteristics as VK6KRO and davenn have said, but also upon how much mechanical load is present at the motor shaft. The motors will have a full load current rating, and (possibly) a locked rotor rating.

    Keep in mind that when flipping the direction switch the motor will pull LRA (Locked Rotor Amps) as it spins down before reversing itself. This can be quite a bit more than full load current, and may be enough to put the LM317 into overload limiting. If operating anywhere near the 1.5 amp rating bolt a fairly hefty heat sink to the regulator, or chances are good that it'll fry.
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