Editing Current sense
Jump to navigation
Jump to search
Warning: You are not logged in. Your IP address will be publicly visible if you make any edits. If you log in or create an account, your edits will be attributed to your username, along with other benefits.
The edit can be undone. Please check the comparison below to verify that this is what you want to do, and then save the changes below to finish undoing the edit.
Latest revision | Your text | ||
Line 1: | Line 1: | ||
− | 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. | ||
Line 5: | Line 5: | ||
There are 3(?) basic techniques: | There are 3(?) basic techniques: | ||
for measuring current: | for measuring current: | ||
− | + | * low-side current shunt | |
− | + | * high-side current shunt | |
− | |||
* magnetic field sense | * magnetic field sense | ||
* MOSFET voltage | * MOSFET voltage | ||
Line 13: | Line 12: | ||
* ... ''(any others I missed?)'' | * ... ''(any others I missed?)'' | ||
− | + | == low-side current shunt == | |
− | |||
− | |||
− | |||
− | |||
− | |||
− | |||
− | |||
− | |||
− | |||
− | |||
− | |||
− | |||
− | |||
− | |||
− | |||
− | |||
− | |||
− | |||
− | |||
− | |||
− | |||
− | |||
− | |||
− | |||
− | |||
Low-side is (electrically) the simplest. | Low-side is (electrically) the simplest. | ||
Line 45: | Line 19: | ||
and measuring the voltage across the resistor. | 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. | 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. | ||
[http://www.maxim-ic.com/appnotes.cfm/appnote_number/746/ Maxim application note 746: "High-Side Current-Sense Measurement: Circuits and Principles"]; | [http://www.maxim-ic.com/appnotes.cfm/appnote_number/746/ Maxim application note 746: "High-Side Current-Sense Measurement: Circuits and Principles"]; | ||
− | [http://www.newark.com/jsp/search/browse.jsp;Ntt=high+side+current+sense Newark: high side current sense]; [http://www.digikey.com/scripts/DkSearch/dksus.dll?KeywordSearch&site=US&keywords=high+side+current+sense Digikey: high side current sense]; [http://www.linear.com/ad/current_sense.jsp Linear: current sense circuit collection] (why doesn't this include the Linear LTC6103 ?); [http://focus.ti.com/analog/docs/gencontent.tsp?familyId=57&genContentId=28020 Texas Instruments: "Current Sensor" | + | [http://www.newark.com/jsp/search/browse.jsp;Ntt=high+side+current+sense Newark: high side current sense]; [http://www.digikey.com/scripts/DkSearch/dksus.dll?KeywordSearch&site=US&keywords=high+side+current+sense Digikey: high side current sense]; [http://www.linear.com/ad/current_sense.jsp Linear: current sense circuit collection] (why doesn't this include the Linear LTC6103 ?); [http://focus.ti.com/analog/docs/gencontent.tsp?familyId=57&genContentId=28020 Texas Instruments: "Current Sensor"]. |
− | |||
A few op amps can handle common-mode voltage well outside its power supply -- such as the [http://focus.ti.com/docs/prod/folders/print/ina117.html TI INA117], which when powered by +/-15 V, can handle a common-mode voltage of +/-200 V. | A few op amps can handle common-mode voltage well outside its power supply -- such as the [http://focus.ti.com/docs/prod/folders/print/ina117.html 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. | This is useful for high-side current sense and also 4-20 mA current loops. | ||
Line 69: | Line 32: | ||
For large motors, the current is measured by running the power wires through a magnetic field sensor -- either | 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 [[http://www.allegromicro.com/en/Products/Categories/Sensors/currentsensor.asp Allegro Hall-effect current sensors]]), which can measure DC and AC current, or | * directly measuring the magnetic field with a Hall effect sensor, for example, the Allegro ACS712 or other [[http://www.allegromicro.com/en/Products/Categories/Sensors/currentsensor.asp 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 [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) | + | * indirectly measuring the magnetic field with a "one-loop current transformer" (which can only measure AC current). |
− | |||
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"]. ) | ||
− | |||
− | |||
− | |||
− | |||
− | |||
− | |||
== 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. | ||
− | |||
− | |||
− | |||
− | |||
− | |||
− | |||
− | |||
− | |||
− | |||
− | |||
− | |||
− | |||
− | |||
− | |||
− | |||
− | |||
− | |||
− | |||
− | |||
− | |||
− | |||
− | |||
− | |||
− | |||
− | |||
− | |||
− | |||
− | |||
− | |||
− | |||
− | |||
− | |||
− | |||
---- | ---- |