Constant current source using an op amp

[ijmbarr]

I'm currently in the process of building a constant current source using an op amp.

Circuit diagram:
http://upload.wikimedia.org/wikipedia/en/9/97/Op-amp_current_source_with_pass_transistor.png

I'm using a voltage reference instead of a zener diode. Currently the load resistance is 0 ohms (shorted) and the "sense" resistor is one ohm. The voltage reference is providing one volt to the non inverting input of the op amp.

Everything seems to be working except the input pins of the op amp are currently at different voltages. The voltage reference is providing 1V, and the other is at 1.1V. This is going to be an issue for my design.

At first I thought it might be because the op amp couldn't provide enough current to the transistor, but by varying the "sense" resistor I can change the current and the input pins are at different voltages at a range of currents, from 0.5 to 2A.

I've tried replacing the components in case they were damaged, no change.

Does anyone have any idea where I could look next? I get the feeling it might have something to do with the imperfect properties of either the op amp or transistor, but I'm not sure what to look at. Any suggestions?

Apologies if I've explained the situation badly, if your not sure just ask more questions.

Thanks for your help
Iain

 

[vk6kro]

I think your first idea that the opamp couldn't drive the transistor was probably right.

At high currents, the current gain of transistors drops a lot. It may be as low as 10.

So, at 1 amp emitter current, you may need 100 mA of base current, which might be a lot for your opamp to supply on its own.

You might need a driver circuit between the opamp and the transistor. I think I have one somewhere which could be modified to suit, or you may already have a favourite circuit.

You could try a Darlington transistor. These have more gain and hence would need less current from the opamp.

 

[Mike_In_Plano]

Hello, Iain,

A common mistake with this circuit is that it introduces phase-lag and the op amp oscillates. If you're not checking with a scope, all you see are wrong operating points.

Try putting a .01 - .1uF cap between the output and inverting input of the op amp. Then add a 47 ohm resistor between the base of the transistor and output of the op amp this usually clears it up.

- Mike

 

[Bob S]

Take your op-amp and set it to one side. Take two NPN's and make a current source like the one in the attachment. This one regulates the current in R3 (100 ohms) to 27 mA (2.7 volts). This means that the voltage drop in the regulator resistor R1 (39 ohms) is ~1.05 volts.

Bob S

 

[ijmbarr]

Mike_In_Plano, thank you. It became obvious as soon as I connected the scope that oscillations were occurring.

Lesson learned: sometimes its worth getting the oscilloscope out rather than struggling along with a multimeter.

Thanks again.
Iain