current to voltage convertor problem

Discussion in 'General Electronics Chat' started by dileepchacko, Dec 30, 2008.

  1. dileepchacko

    Thread Starter Active Member

    May 13, 2008
    102
    1
    Dear All

    I want to design a current to voltage converter for the following specification
    Output current: 1mA to 200mA. I have used op-amp current to voltage converter using LM324, but after 40mA the output of the op-amp is going to clip. There is no proper output after 40mA. I have simulated various op-amps such as LM324, OP27, OP4177 etc... the result is same. so please suggest me a good current to voltage converter.
     
  2. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    Your post didn't contain the most important piece of information.

    What output voltage you want from the converter for 1mA to 200mA input current?

    Added: Please also post an image of any of your circuit
     
    Last edited: Dec 30, 2008
  3. dileepchacko

    Thread Starter Active Member

    May 13, 2008
    102
    1
    The output voltage should be from ±1 to ±10V. all these voltage are in peak to peak value. I have attached the current to voltage converter using op-amp.
     
  4. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    You are expecting too much current from the Opamp output which it can't provide.

    Try this in your simulation. If the 1Ω is too high, you can use 0.5Ω and increase R1 to 100K instead. I would not go lower than 0.5Ω though.

    [​IMG]

    The present values will give you -10V with 200mA input current.
     
  5. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    If you want more output current than 40ma then use a power opamp with a much higher output current.
     
  6. dileepchacko

    Thread Starter Active Member

    May 13, 2008
    102
    1
    I am not worry about maximum output current, I am worry about maximum input current, because none of the op-amp is not working after 40mA of input.
     
  7. dileepchacko

    Thread Starter Active Member

    May 13, 2008
    102
    1
    Thanks LC. This circuit can be used for my application. One thing is here is 1 ohms shunt resistance. Is it possible to get an same output without using 1 ohms shunt resistance.
     
  8. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    You have to have some kind of shunt, it can be a lot smaller, but there needs to be something. Part of the problem is the original op amp circuit was creating a virtual ground, which means it has to provide a matching current. As you have discovered past a certain limit this isn't practical. The shunt is a passive way to measure current, a voltmeter and a resistor is all you need. I don't know what a DVM uses for it's resistance, but they use a shunt for the higher current.

    You could get 1Ω resistors, measure them if possible, and parallel 10 of them to get 0.1 Ω, which would be less interactive with your circuit. At 1A it would drop .1V. The advantage of precision measurements is you can increase the apparent accuracy of your measurements.
     
  9. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    It seems you are still having trouble on visualizing your current-to-voltage converter.

    In your original design, you are feeding input current directly into the INV input pin of the opamp. This current cannot go inside the opamp but have to flow via the 50Ω resistor to the opamp output.

    Therefore the opamp must provide this current at its output. While below a few tens of mA is fine, ordinary signal opamp are not designed to output such a large current. Therefore your circuit stop working at 40mA because the opamp just can't cope.

    That's is also why I said earlier that you are expecting too much current from an opamp which it cannot provide.

    I don't know why the 1Ω or 0.5Ω resistor is causing you concern because your previous circuit arrangement use a virtual earth approach and is low impedance anyway. Anyway, in this case you have two options, either:

    1. follow Audioguru's advice and use a power opamp which is capable of high output current, you need one with at least 250mA output. I tried the LT1970 and it works nicely.

    [​IMG]

    2. build additional buffer stage after the output of the opamp to increase the current capacity of the opamp
     
    Last edited: Dec 31, 2008
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