Here is a simple circuit that you may want to consider. This will convert a 0-5volt input signal to a current flow in the output loop from 0-100 mA. To change the voltage to current ratio just change the value of the resistor. You must select the FET with suitable current rating and the resistor with suitable wattage. In the example shown the resistor should be 10 watts or better. A wire wound resistor is recommended. If you underrate the resistor its temperature rise will change the resistance and thus the voltage to current ratio.

Also keep in mind that this circuit basically regulates the voltage drop across the resistor. At 100 mA there will be a 5 volt drop across the resistor and the opamp output will be at a voltage above this level such that it turns the FET on hard enough to cause the 100 mA to flow. This is the reason that the 12 volt supply is chosen. The circuit will not work with a supply voltage that is too low (such as 5 volts).



Michael Karas