I'm using a PIC16F873 to generate a PWM that drive a NMOS transistor that drive a motor DC. But now i need read the current that flow throw the motor and i don't want to use a shunt resistor. Coul you tell another cheap sugestion?
There are some indirect ways that measure the voltage drop across any of the parasitic resistances. You can also infer the current from the drain-source drop of the driving FET if it is in saturation. Another way which is more expensive is to use a Hall effect sensor.
using anti-saturation ckt. (checking drain to source voltage drop to see if it is in saturation region) is a common solution specially at high current which shunt resistor is very lossy or the voltage drop is very low.
it is also used for igbt switches.
You don't need any application note. Simply you are using parasitic resitor that resides on drain-source reginon. Usally you can see drain source region as resistor with constant resistance. Measuring Usd voltage you can "see" what currenti is flowing from drain to source. Becase this voltage is usually small you'll have to aplify it.
Simce
Yes. You should measure the voltage drop across the transistors and the current with a meter temporarily in series. From this you can get the equivalent saturation resistance of the transistors and set the scale factor for your calculations.
U could use a current transformer (a single loop running through a ferrite ring with multyturn secondary, if your current is AC, else U need a DC current transformer, who besides the AC scheme described also contains additional excitation coil.
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yes flatulent is guide you in right way i have done this with centrifuge motor to measure the current used a linear hall effect sensor allgerio
now lanch the new series of hall effect current sensor
Im using a 3515 allegro hall effect sensor to measure current with the pic. It works perfect once you could determinate the losses in the magnetic circuit.