Hi, i want to create voltage to current converter. V in should be from 0 to 1 V, current output from 4 to 20 mA. I found some scheme on google, could someone explain me how it works? http://electronicdesign.com/site-fi...ctronicdesign.com/files/29/2985/figure_01.gif

Thanks for help !

 

This may, fortuitously, provide some insight

 

Through feedback the opamp maintains the same voltage on the + and inverting input of the Opamp. To do this the voltage drop from the - input through the top 100K and the 100 ohm pot must equal the voltage drop from the + input through the bottom 100K. Since the resistances are fixed that means the top and bottom currents are fixed for a given ein and e1 and therefore the current differences are fixed. The difference in those currents must flow through the load to satisfy Kirchoff's current law. This current is independent of the resistance in the load.

P1 just sets the bias on the inverting input and therefore the zero offset, P2 changes the required current differences in the top and bottom branches and therefore current in the load. The transistor is there just to provide a higher current range into the load while allowing the circuit to have a high input impedance.

 

I built a similar circuit to drive a 4-20mA meter I had. Mine was a bit more complicated and used two op-amps on a single supply. One for offset (4mA at 0V) and one for gain (20mA at 1V).