How can i convert 0-5 Voltage source to 0-20mA steady current source using op amps?

Question

The problem is i cant produce a steady current. it changes when i add different load.
The supply voltage for op amps is 24V and 0V
Potentiometer will be use change the voltage from 0 - 5V .

Answer 1

You can do it by inputting the 0 to 5 volts to the non inverting input of the op amp. Connect the output of the op amp to your 0 to 20 mA load, the return of the op amp will go to the inverting input of the op amp. Then connect a 250 ohm resistor (series 220 and 33 ohm resistors) from the op amp to ground.



However the op amp power supply should be ± 12 volts not 24 and 0 volts. If you have to use 24 and 0 then you'll have to use a second op amp to create "0" volts ( 12 volts above ground), You do this by using two 10k ohm resistors wired in series connected to the 24 and 0 volts of the op amp supply, the junction (12 volts) is wired to the non inverting input of the second op amp. The second op amp is connected as a voltage follower ( the inverting input is connected to its output). The output of this second op amp is "ground for your 0 to 5 volt input and the 250 ohm resistor.



The V to I op amp will not be able to provide 20 mA of current to a resistance load much above 300 ohm since it will hit the + 12 volt rail attempting to do so.

Answer 2

Use the 0-5V signal as the control voltage for a voltage to current converter (V-I converter).



The link below shows one using 0-1000mV as the input, though the input is easily changed to 0-5V. The first stage provides gain and offset adjustments so that the input 0-5V or whatever becomes 200mV to 1000mV. The second stage converts this voltage to current (it is a current sink, but floating load (not grounded). The formula is I = Vin/R22, so for 1V for 20mA with 50 ohms (2 x 100 in parallel). The load resistor at the receive end is often 100 ohms to give 100mV per milliamp. This is meant to give 4-20mA, so that a zero is 4mA, no confusion with broken wires.



The second link may be more like you need. It is for a single supply, and acts as a current source instead of sink, so the load can be grounded. There is an error due to base current in both of these circuits. Make sure the op-amp can handle the supply voltage, and that the supply is defined as 24, not 24 to 32 (battery) or such like. Note the op-amp must be rail to rail output and at least the ground side of the input to turn off the PNP transistor.



Search images for 'V-I converter circuit' and you may find one that uses a FET to overcome this, or otherwise better suits your purpose. Simulation is a good tool to test out your design.

Answer 3

See the reference for a single transistor current source. The way this works is R2 is selected so at the current you want the voltage across R2 will be 1.2 volts. Any more current draws current thur the diodes and starts turning off the transistor. The circuit as is could be used as a current sink. For a source connect the current source directly to V+ and using the ground as the current sink.

Answer 4

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Answer 5

4-20ma Current Source

Answer 6

You need to use a voltage dependent current source. So long as the current source is not saturated the current will be independent of the load resistance.