Reducing current to 4-20mA for measurement

Hi,

I want to acquire current data range between 3-5A (according to motor's load), at 24VDC applied to a motor. How to reduce the current to 4-20mA range so it can be read in the NI DAQ card? Many thanks.

I use a hall effect sensor similar to these. They are pretty common so the link is just an example. In this case the unit linked to will output 4 to 20 mA for a current of 0 to 5 ADC. THere are also ways to build your own but for commercial applications I go with off the shelf solutions.

Ron

some theory when you use op amp

if we measure the current using a resistor -R1,
Gain set to the op Amp G
input voltage clamp -Vc
out put resistance -R2
then you have

at 3A the out put current to load R2 must be 4mA
so we have ... G (3R1-Vc)/R2 =4
similar for 5A case then we have G (5R1-Vc)/R2 = 20

if we divide above equations the solution would be Vc= 2.5 R1
also we will get GR1 = 8R2

the values can be set to get required value of mA for a particular load R2 for the range 3-5A.

sorry i couldn't sketch a diagram for you.
if it for industrial application then always go for a ready made one.

Another option: place a low Ohms shunt resistor (~1mΩ) in series with the positive motor lead, and then use a "high-side-current-monitor" like a ZXCT1009 or similar to produce a ground-referenced voltage or current proportional to the current flowing to the motor...