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Transitioning from the classroom

  1. Jul 27, 2013 #1
    I was thinking lately of taking a few battery-operated devices and converting them such that they could run off other energy sources (like outlets). But, I was wondering what concepts should be taken into consideration?

    For example, I have a fan that runs off of two D 1.5V batteries; this is all I know. I do not know the current draw from the batteries (and consequentially the power limitations of the fan's internal circuitry). I know the limitations of the batteries, in terms of mA-h, but that is all and gives you little information. I was thinking perhaps attempting to come up with a Thevenin Eq. circuit and going from there, but with the electric motor to turn the fan I am not sure where to start.
    --I was thinking 5v from an arduino, and then a transistor for current drive. Although, I am not sure how much current.

    Or perhaps I want to run a small DC motor that I have. The only information on the datasheet is that it's 1.5-3.0V motor with 300mA draw at no load. What about when there is a load? Power dissipation limitations? Do you just arbitrarily design a circuit, perhaps a transistor, with an output of ~500mA at 2V?

    Similarly, I was looking at converting a laser-pointer. The device uses three 1.5V batteries and the specifications are ~<5mW. This gives me a starting point much better than the other two projects mentioned above, but still leaves some uncertainty. Is this circuit as simple as using the power limitations of 5mW, and voltage of 4.5V, designing a circuit that will supply ~1.11mA? I have never seen an LED that draws such low current.

    I am fully able to work through problems with the constraints and requirements given, but how to do you figure out such constraints to begin with?
    Last edited: Jul 27, 2013
  2. jcsd
  3. Jul 29, 2013 #2
    To answer your last question, how we figure out the constraints: We look at the datasheets and calculate from there :) The datasheets will most likely show you the "Absolute Maximum Ratings" which provide you with the knowledge of the absolute constraints, and then you design from there.

    The fan:
    If the batteries are in series and they thus provide 3 volts, the next step would be to figure out the current draw. If you dont have a measuring apparatus, or the datasheet, the only other way I can imagine doing it would be to measure the time it takes for the fan to use up all the juice for the fully charged batteries.
    if they have for example 10 mA-h and it takes 1 hour, then you know that the fan has been drawing 10 milliamps. this would be an experimental estimate of the current draw. if you're not going to change the voltage, then you would be able to model the fan as a fixed resistor.

    The DC motor:
    The current draw would be greater when you have it loaded. The maximum power dissipation is not possible to know unless it tells you on the datasheet. So here you would need to be on the safe side. I would believe that supplying 400mA would still not be a problem. However, think about this. The current drawn by the DC motor provides a torque, given by the "torque constant" of the motor. If you wanna design a driving circuit for the motor, you would need to make sure that the transistor and other components can handle the current draw.

    The Laser pointer:
    If you wanna replace the battery, the yes.
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