Difference between revisions of "Current sense"

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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.
+
If something blocks an electric motor 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
+
* resistive shunt
* high-side current shunt
+
** low-side current shunt
 +
** high-side current shunt
 
* magnetic field sense
 
* magnetic field sense
 
* MOSFET voltage
 
* MOSFET voltage
Line 12: Line 13:
 
* ... ''(any others I missed?)''
 
* ... ''(any others I missed?)''
  
== low-side current shunt ==
+
== resistive shunt ==
 +
 
 +
With the resistive shunt technique,
 +
all the charge flowing through the the load also flows through a resistor (the shunt resistor) in series with the load.
 +
 
 +
[http://electronicdesign.com/article/articles/what-s-all-this-shunt-stuff-anyhow-2144.aspx "What's All This Shunt Stuff, Anyhow?"]
 +
by Bob Pease 2002
 +
describes one way to custom-build a high-current shunt.
 +
 
 +
[http://electronicdesign.com/article/test-and-measurement/what-s-all-this-wattmeter-stuff-anyhow-2190.aspx "What’s All This Wattmeter Stuff, Anyhow?"]
 +
by Bob Pease 2002
 +
describes how to use the previous shunt in a circuit to be spliced in the middle of an extension cord to measure true RMS watts flowing from a power outlet to a device under test.
 +
Even though a copper shunt has a pretty bad resistance variation with temperature (tempco), other parts in this circuit compensate for it.
 +
 
 +
=== low-side current shunt ===
 +
 
 +
      +-----------------+
 +
      |                |
 +
      |  +            (load)
 +
    (power)              |
 +
      |  -            (shunt R)
 +
      |                |
 +
      +-----------------+
 +
      |
 +
      GND
 +
 
 
Low-side is (electrically) the simplest.
 
Low-side is (electrically) the simplest.
  
Line 19: Line 45:
 
and measuring the voltage across the resistor.
 
and measuring the voltage across the resistor.
  
== high-side current shunt ==
+
=== high-side current shunt ===
 +
 
 +
      +-----------------+
 +
      |                |
 +
      |  +            (shunt R)
 +
    (power)              |
 +
      |  -            (load)
 +
      |                |
 +
      +-----------------+
 +
      |
 +
      GND
  
 
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 TI INA117, which when powered by +/-15 V, can handle a common-mode voltage of +/-200 V.
+
[https://www.silabs.com/products/power/currentsensors/ Silicon Labs' current sensors].
 +
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 31: Line 68:
  
 
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 (often 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
* indirectly measuring the magnetic field with a "one-loop current transformer" (which can only measure AC current).
+
** [http://www.hvwtech.com/products_view.asp?ProductID=881 30 Amp Current Sensor AC&DC] A commercial sensor based on the Allegro ACS712.
 +
* 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 "Rogowski Coil" (which can only measure AC current) and an integrator circuit. [http://en.wikipedia.org/wiki/Rogowski_coil Wikipedia: Rogowski Coil].
  
 
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"]. )
 +
 +
Douglas W. Jones
 +
"Current Limiting for Stepping Motors"
 +
mostly discusses resistive shunts,
 +
but it has some information on magnetic field current sensors in the
 +
[http://www.divms.uiowa.edu/~jones/step/current.html#sensors "Other Current Sensing Technologies"] section.
  
 
== 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://en.wikipedia.org/wiki/Current_mirror current mirror] isn't useful for motors, right?'')
+
[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 [[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.
 +
 +
== current estimation ==
 +
 +
Rather than directly measuring the actual current, many people measure something else that (they hope) is close enough to being proportional to the current.
 +
 +
==== lossless average inductor current sensing ====
 +
The "lossless average inductor current sensing" technique:
 +
 +
    (switched)--------+-----+
 +
    (power)          |    |
 +
                      |    Rs
 +
                      |    |
 +
                (L1 + DCR)  +--- Isense+
 +
                      |    |
 +
                      |    C
 +
                      |    |
 +
                      +-----+--- Isense-
 +
                      |
 +
                    (load)
 +
                      |
 +
                    GND
 +
 +
The current through an inductor with some known internal parasitic resistance DCR can be sensed with a series resistor (Rs) and capacitor network connected in parallel with the inductor-resistor network.
 +
The designer picks the sense components Rs and C such that Rs*C -- the time constant of the RC network -- is close enough to L/DCR -- the time constant of the inductor resistor network. Then the voltage across C is proportional to the current through L.
 +
(IR3508Z data sheet
 +
[http://www.irf.com/product-info/datasheets/data/ir3508zmpbf.pdf])
 +
    current through L == voltage across C * (1/DCR).
 +
 +
<ref>
 +
Dr. F.A.E.
 +
[http://blog.fairchildsemi.com/2015/navigating-dcr-currents "Navigating DCR Currents"].
 +
2015.
 +
</ref>
  
 
----
 
----

Revision as of 20:47, 19 April 2015

If something blocks an electric motor 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:

  • resistive shunt
    • 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?)

resistive shunt

With the resistive shunt technique, all the charge flowing through the the load also flows through a resistor (the shunt resistor) in series with the load.

"What's All This Shunt Stuff, Anyhow?" by Bob Pease 2002 describes one way to custom-build a high-current shunt.

"What’s All This Wattmeter Stuff, Anyhow?" by Bob Pease 2002 describes how to use the previous shunt in a circuit to be spliced in the middle of an extension cord to measure true RMS watts flowing from a power outlet to a device under test. Even though a copper shunt has a pretty bad resistance variation with temperature (tempco), other parts in this circuit compensate for it.

low-side current shunt

      +-----------------+
      |                 |
      |  +            (load)
   (power)              |
      |  -            (shunt R)
      |                 |
      +-----------------+
      |
     GND

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

      +-----------------+
      |                 |
      |  +            (shunt R)
   (power)              |
      |  -            (load)
      |                 |
      +-----------------+
      |
     GND

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
  • indirectly measuring the magnetic field with a "Rogowski Coil" (which can only measure AC current) and an integrator circuit. Wikipedia: Rogowski Coil.

Because magnetic field sensing is inherently non-contact, it works just as well high-side as low-side. ( "Closed-Loop Magnetic Current Sensor". )

Douglas W. Jones "Current Limiting for Stepping Motors" mostly discusses resistive shunts, but it has some information on magnetic field current sensors in the "Other Current Sensing Technologies" section.

MOSFET voltage

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.

current estimation

Rather than directly measuring the actual current, many people measure something else that (they hope) is close enough to being proportional to the current.

lossless average inductor current sensing

The "lossless average inductor current sensing" technique:

   (switched)--------+-----+
   (power)           |     |
                     |     Rs
                     |     |
               (L1 + DCR)  +--- Isense+
                     |     |
                     |     C
                     |     |
                     +-----+--- Isense-
                     |
                   (load)
                     |
                    GND

The current through an inductor with some known internal parasitic resistance DCR can be sensed with a series resistor (Rs) and capacitor network connected in parallel with the inductor-resistor network. The designer picks the sense components Rs and C such that Rs*C -- the time constant of the RC network -- is close enough to L/DCR -- the time constant of the inductor resistor network. Then the voltage across C is proportional to the current through L. (IR3508Z data sheet [1])

   current through L == voltage across C * (1/DCR).

<ref> Dr. F.A.E. "Navigating DCR Currents". 2015. </ref>