- #1
ijmbarr
- 10
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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
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
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