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− | If something blocks an electric | + | If something blocks an electric from turning ("stalled"), the current rapidly increases far above normal levels -- then the [[motor driver]] needs to turn off the power before the motor, or the motor driver MOSFET, or both are destroyed. |
Systems that use a 4-20 mA current loop also need to measure current. | Systems that use a 4-20 mA current loop also need to measure current. | ||
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There are 3(?) basic techniques: | There are 3(?) basic techniques: | ||
for measuring current: | for measuring current: | ||
− | + | * low-side current shunt | |
− | + | * high-side current shunt | |
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* magnetic field sense | * magnetic field sense | ||
* MOSFET voltage | * MOSFET voltage | ||
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* ... ''(any others I missed?)'' | * ... ''(any others I missed?)'' | ||
− | + | == low-side current shunt == | |
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Low-side is (electrically) the simplest. | Low-side is (electrically) the simplest. | ||
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and measuring the voltage across the resistor. | and measuring the voltage across the resistor. | ||
− | + | == high-side current shunt == | |
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In situations where low-side sensing is difficult ( automobile electronics bonded to the "GND" car frame; other systems where it is inconvenient to put a resistor on the "lo" power wire), we turn to high-side sensing. | In situations where low-side sensing is difficult ( automobile electronics bonded to the "GND" car frame; other systems where it is inconvenient to put a resistor on the "lo" power wire), we turn to high-side sensing. | ||
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* directly measuring the magnetic field with a magneto-resistive effect sensor, for example, the [http://www.zetex.com/3.0/3-3-2b.asp?rid=76 Zetex magneto-resistive current sensor], which can measure DC and AC current, or | * directly measuring the magnetic field with a magneto-resistive effect sensor, for example, the [http://www.zetex.com/3.0/3-3-2b.asp?rid=76 Zetex magneto-resistive current sensor], which can measure DC and AC current, or | ||
* indirectly measuring the magnetic field with a "one-loop current transformer" (which can only measure AC current). [[AC current sensor]] | * indirectly measuring the magnetic field with a "one-loop current transformer" (which can only measure AC current). [[AC current sensor]] | ||
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Because magnetic field sensing is inherently non-contact, it works just as well high-side as low-side. | Because magnetic field sensing is inherently non-contact, it works just as well high-side as low-side. | ||
( [http://focus.ti.com/docs/prod/folders/print/drv401.html "Closed-Loop Magnetic Current Sensor"]. ) | ( [http://focus.ti.com/docs/prod/folders/print/drv401.html "Closed-Loop Magnetic Current Sensor"]. ) | ||
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== MOSFET voltage == | == MOSFET voltage == | ||
− | [http://www.4qdtec.com/mircl.html pseudo 'mirror' current sensing a MOSFET] -- sampling the voltage across a MOSFET while it is turned on. That voltage is linear with current but varies with temperature. If the purpose of measuring current is to turn off the MOSFET before it overheats, the variation with temperature doesn't matter. (''A true [ | + | [http://www.4qdtec.com/mircl.html pseudo 'mirror' current sensing a MOSFET] -- sampling the voltage across a MOSFET while it is turned on. That voltage is linear with current but varies with temperature. If the purpose of measuring current is to turn off the MOSFET before it overheats, the variation with temperature doesn't matter. (''A true [http://en.wikipedia.org/wiki/Current_mirror current mirror] isn't useful for motors, right?'') |
This uses the turned-on resistance of the MOSFET as if it were a shunt resistor. | This uses the turned-on resistance of the MOSFET as if it were a shunt resistor. | ||
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