LabVIEW
How to measure high currents accurately (upto 24A) using FPGA
The answer will depend on your specific situation, such as the voltage levels you are working with, the source of the current, the influence some additional components such as a shunt resistor may have, etc. Can you provide more details about the application, the source of the current, the voltage levels, how quickly the current will change, the stimulus for a current change, etc.
The most common method to measure a current is to add a shunt resistor in the current loop and measure the voltage across the shunt to calculate the current based on the voltage and resistance. You would need to make sure the absolute voltage levels (relative to the ground of the FPGA board) do not exceed its specifications (+-10V). The value of the shunt will depend on the current and the accuracy you need in the measurement.
If you use a 5 milliohm shunt, then you will have a 0.12V drop across the shunt resistor at 24A. The 7831R has a relative accuracy of about 300uV, this would translate into a relative accuracy of about 61mA for a 5 milliohm shunt (absolute accuracy will be less). If you cannot tolerate a 0.12V drop at 24A, then you will have to use a smaller shunt resistor which will give youless accuracy in the current measurement.
The shunt resistor value will have to be precisely measured ahead of time using a good DMM and a 4-wire resistance measurement. There will be some error associated with this measurement. The resistance will also change due to self-heating as the current through it changes (0.12V * 24A = 2.88W) adding some non-linearity to the measurement.
This is probably the simplest approach. If this setup is not feasible (e.g. the voltage in not within the 7831R input range) then you will need some additional signal conditioning. There are some off-the-shelf components for measuring larger currents that would translate the current into a voltage that you can measure with the FPGA board. Look at the following component as a place to get started.
A closed loop current sensor like the following may work. It runs off a 5 V supply, and is relatively easy to wire up:
http://rocky.digikey.com/scripts/ProductInfo.dll?Site=US&V=102&M=SCD20PUN
Christian L
NI Consulting Services
The most common method to measure a current is to add a shunt resistor in the current loop and measure the voltage across the shunt to calculate the current based on the voltage and resistance. You would need to make sure the absolute voltage levels (relative to the ground of the FPGA board) do not exceed its specifications (+-10V). The value of the shunt will depend on the current and the accuracy you need in the measurement.
If you use a 5 milliohm shunt, then you will have a 0.12V drop across the shunt resistor at 24A. The 7831R has a relative accuracy of about 300uV, this would translate into a relative accuracy of about 61mA for a 5 milliohm shunt (absolute accuracy will be less). If you cannot tolerate a 0.12V drop at 24A, then you will have to use a smaller shunt resistor which will give youless accuracy in the current measurement.
The shunt resistor value will have to be precisely measured ahead of time using a good DMM and a 4-wire resistance measurement. There will be some error associated with this measurement. The resistance will also change due to self-heating as the current through it changes (0.12V * 24A = 2.88W) adding some non-linearity to the measurement.
This is probably the simplest approach. If this setup is not feasible (e.g. the voltage in not within the 7831R input range) then you will need some additional signal conditioning. There are some off-the-shelf components for measuring larger currents that would translate the current into a voltage that you can measure with the FPGA board. Look at the following component as a place to get started.
A closed loop current sensor like the following may work. It runs off a 5 V supply, and is relatively easy to wire up:
http://rocky.digikey.com/scripts/ProductInfo.dll?Site=US&V=102&M=SCD20PUN
Christian L
NI Consulting Services
Do you know what time resolution you need to acquire your signal. Will the 200 kHz (5 microsecond) rate of the analog inputs be adequate?
We don't have any specific hardware for injector currents, but the previous information is applicable. You may also want to contact your local sales engineer to dicuss your application further. Another idea is to search the NI web site for key words on your application to learn about other similar applications.
Diesel Engine Fuel-Injection Measurement System
http://www.ni.com/niglobal/
Christian L
We don't have any specific hardware for injector currents, but the previous information is applicable. You may also want to contact your local sales engineer to dicuss your application further. Another idea is to search the NI web site for key words on your application to learn about other similar applications.
Diesel Engine Fuel-Injection Measurement System
http://www.ni.com/niglobal/
Christian L
