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Re: [beam] What happens when motor load outstrips drivers?

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  • databasement@conninc.com
    ... your motors are gonna try to draw all the current they can, (even to their own demise) - try hooking a LED to five volts without a current limiting
    Message 1 of 2 , Apr 24, 2000
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      Justin wrote:

      > What happens when your motor drivers (eg 74HC based) can supply 50mA,
      > and the load on your motors would prefer 100mA - does the motor just
      > work with the 50mA and apply less torque to the load, or do Bad Things
      > happen (eg chip damage, or power losses)?
      > (I have a situation where I'd like to be able to limit the current the
      > motors can draw, but I'm guessing the obvious way is never the best way
      > :-)

      your motors are gonna try to draw all the current they can, (even to their
      own demise) - try hooking a LED to five volts without a current limiting
      resitor....what happens? 8^)
      Anyway, you will eventually overheat your chip, try a passive heat sink, (if
      solar powered, there may be some cooling between solar engine firings)
      but to solve your 'problem'....if you want to limit current, use a resistor,
      do the math, or experiment to find the value you need.

      Remember Ohms Law
      E=I*R voltage=current*resistance

      here's a little tutorial:
      http://www.lerc.nasa.gov/Other_Groups/K-12/Sample_Projects/Ohms_Law/ohmslaw.html
    • Wilf Rigter
      It s a combination of things and it depends on your definitions: 1. Motor driver supplies 50mA but at what voltage drop? 2. Motor prefers 100mA but under what
      Message 2 of 2 , Apr 26, 2000
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        It's a combination of things and it depends on your definitions:

        1. Motor driver supplies 50mA but at what voltage drop?
        2. Motor prefers 100mA but under what load?

        There is no standard for motor drivers but measuring output voltage and
        current under load provides a good indication of performance.
        ie Vsupply and the current it can deliver to a motor with Vsupply - 0.4V
        across the motor windings. In Tilden type h-bridges, the output
        transistors operate in the saturated mode with the maximum current h-bridge
        set by the rated collector current, the base currents and the gain of the
        output transistors. The best transistors for this application are a high
        gain/low saturation voltage type (BC807-25 and BC817-25) but the common
        2N2907 and 2N2222 work fine at 100mA. 74HC or 74AC devices have internal
        resistance which can be safely measured by measuring the voltage across a
        100 ohm resistor (V resistor) connected to the HC output and to either +V
        or 0V with the HC output in the low and high state respectively. The output
        resistance is ( Vsupply / V resistor -1) x 100 ohm. For a HC bridge the high
        and low state resistance must be added to give the total series resistance
        of the driver. If the driver is overloaded and the chip heats up, this
        series resistance will increase protecting the HC bridge but reducing power
        to the motor.

        Motor specs are usually rated by voltage, current, torque and RPM but a
        simple and useful spec is winding resistance and no load current at the
        supply voltage. The resistance gives the maximum short circuit current when
        the motor is stalled and the no load current tells us something about the
        efficiency of the motor. A measurement of torque at half of short circuit
        current is a good measure of the efficiency under heavy load.

        regards

        wilf





        > -----Original Message-----
        > From: Justin [SMTP:jaf60@...]
        > Sent: Monday, April 24, 2000 11:27 AM
        > To: beam@...
        > Subject: [beam] What happens when motor load outstrips drivers?
        >
        > What happens when your motor drivers (eg 74HC based) can supply 50mA,
        > and the load on your motors would prefer 100mA - does the motor just
        > work with the 50mA and apply less torque to the load, or do Bad Things
        > happen (eg chip damage, or power losses)?
        > (I have a situation where I'd like to be able to limit the current the
        > motors can draw, but I'm guessing the obvious way is never the best way
        > :-)
        >
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