problems with howland current source in book

Discussion in 'Homework Help' started by hunterage2000, Jun 5, 2012.

1. hunterage2000 Thread Starter Active Member

May 2, 2010
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Im trying to get the attached Howland current source to work but Im not getting the right results. For Vin = 550mV and R = 130 I get roughly -4mA which is correct but for Vin = 2.7 and R = 130 I get -6mA when I should get roughly 20mA. Can anyone suggest why?

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2. Jony130 AAC Fanatic!

Feb 17, 2009
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How do you measures the current? I real circuit?

3. hunterage2000 Thread Starter Active Member

May 2, 2010
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from the pic, I have a probe where the load is on my simulation and have ran a transient analysis.

4. Jony130 AAC Fanatic!

Feb 17, 2009
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Well you need to post actual schematic of the circuit that you use in the simulation.
What is the value of a load resistor ?

5. hunterage2000 Thread Starter Active Member

May 2, 2010
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the circuit is too big to post, but I have a rectified AC voltage that has been smoothed by a LPF at the input to the current source. not sure what a load resistor is?

6. Jony130 AAC Fanatic!

Feb 17, 2009
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Show full diagram of a current source.
What type op amp you have used? What supply voltage was? And which component is connect to your current source output. What is the value of this component?

7. hunterage2000 Thread Starter Active Member

May 2, 2010
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I have simulated the current source with no load just a multimeter connected to the node between the bottom 2 resistors. I have just used a dc source, but when I actually make it on a breadboard with a dc power supply of +10V and -10V and using another DC power supply as the input signal I get nothing out of the output at pin 3.

8. hunterage2000 Thread Starter Active Member

May 2, 2010
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I have used 4 150 ohm resistors and an input of 7V

9. Jony130 AAC Fanatic!

Feb 17, 2009
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You should be aware that in real life most op amps will not work properly with output current larger than >20mA. Also the input voltage 7V is too high for op amp supplied from a +/-10V power supply.

10. hunterage2000 Thread Starter Active Member

May 2, 2010
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the max voltage for what Im doing is about 2.5V, What did you mean by the component connected to the output?