How to output analog current more than 2mA

[cli21]

Hi, I designed a simple PID loop to control the input voltage of a device. My device has a resistance of Ohm. I found that the input voltage being controlled can't exceed 0.6V, then I realized that the maximum output current is 2mA of my DAQ NI 6211. But I really need current more than 2mA(Max is 10mA). I tried using voltage follower since it has large imput impedance and small output impedance, but it doesn't work. Could somebody tell me one method to to solve the problem?

Thanks.

CJL

[johnsold]

You need some kind of amplifier. Your post omitted the input resistance of the device you are controlling.

A voltage follower should work, although if you build a simple one with a bipolar junction transistor, you will get an offset of about 0.6 V from the base-emitter voltage. A unity gain op amp circuit should work also.

Post an image of your schematic. Then we may be able to see why it did not work.

Lynn

[Henrik_Volkers]

Sink impedance?

Output voltage range?  (0-10V) or (1-9V) or ??  Just to know if you need R2R at 10mA !

If it is in a control loop, a simple BJT (as johnsold already mentioned) migth do the job.

[cli21]

Hi johnsold,

The resistance of my device is about 300Ohm. It's nothing but a themal actuator.  Attachment is my circuit. It's very simple but just doesn't work. 

[Henrik_Volkers]

Your circuit need a ground reference from AOGND and your actuator_GND  to GND of the OP power supply (-15v,GND,+15v)

How should the current flow otherwise?

[cli21]

Hi Henrik,

What I did is first I found that the due to the PID control(actually the integral part), the controlled voltage will increase during some time, but each time the value reached around 0.6V(2mA correspondingly), it will stop increasing. Then I got rid of the PID controller, just increased the analog output voltage manually, still the voltage of device will keep at 0.6V although I set the analog output value larger than that. 

By the way, my DAQ analog output range is -10 to 10V.

Thanks.

CJL

[Henrik_Volkers · Accepted solution]

caps 100nF || with 10µF ,

R optional..

what OP are you actually using? 

Without that GND connection no current can flow 😄 so you will stick at 0.6V like without driver.

[cli21]

Hi Henrik,

I am using UA741, the most ordinary op amp I guess, does it make any difference?

Thanks,

CJL

[Henrik_Volkers]

751 10V/.4k 25mA... should work... however where did you get this from 😄

[johnsold]

One other thing occurs to me. I have seen devices with 0-10 V control inputs where the reference line of the device was not at ground. Connecting a grounded source (such as your DAQ + op amp) would not work. The 0.6 V sounds suspiciously like a protection diode is conducting.

What is the device you are controlling? Does the specification for the device indicate the allowable voltage range with respect to ground for the control input reference (or common)?

Lynn