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.
There are 3(?) basic techniques: for measuring current:
- low-side current shunt
- high-side current shunt
- magnetic field sense
- MOSFET voltage
- the "non-dissipative overcurrent protection", a kind of current mirror used in the L6208N ...
- ... (any others I missed?)
low-side current shunt
Low-side is (electrically) the simplest.
For smaller motors, the current is usually measured by running the current through a shunt resistor to ground, and measuring the voltage across the resistor.
high-side current shunt
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.
Maxim application note 746: "High-Side Current-Sense Measurement: Circuits and Principles"; Newark: high side current sense; Digikey: high side current sense; Linear: current sense circuit collection (why doesn't this include the Linear LTC6103 ?); Texas Instruments: "Current Sensor", Silicon Labs' current sensors. A few op amps can handle common-mode voltage well outside its power supply -- such as the TI INA117, which when powered by +/-15 V, can handle a common-mode voltage of +/-200 V. This is useful for high-side current sense and also 4-20 mA current loops.
magnetic field sense
For large motors, the current is measured by running the power wires through a magnetic field sensor -- either
- directly measuring the magnetic field with a Hall effect sensor, for example, the Allegro ACS712 or other [Allegro Hall-effect current sensors]), which can measure DC and AC current, or
- directly measuring the magnetic field with a magneto-resistive effect sensor, for example, the 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
Because magnetic field sensing is inherently non-contact, it works just as well high-side as low-side. ( "Closed-Loop Magnetic Current Sensor". )
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 current mirror isn't useful for motors, right?)
This uses the turned-on resistance of the MOSFET as if it were a shunt resistor.