Isolating 0 to 4 volts

Discussion in 'The Projects Forum' started by paulepc, May 5, 2014.

  1. paulepc

    Thread Starter Member

    Apr 2, 2014
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    I am using a micro controller that outputs 0 to 4 volts. The output is then fed into some op amps running +/- 12 volts.
    What is the best way to isolate the output of the micro controller, I want to make sure that no 12 volt comes back at the M/C as it runs at 3.3 volts.

    Thanks
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    What do you intend to accomplish with the 0-4V output from the microcontroller?
     
  3. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    How can a MCU running at 3.3 Volts put out 0 - 4 Volts?
    I would expect a max of 3.3 Volts.

    Bertus
     
  4. paulepc

    Thread Starter Member

    Apr 2, 2014
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    The 0 to 3.3 volts goes into an op amp to deliver +/- 10 volt to drive a motion controller .

    You are correct I can only get 3.3 in fact I get 3.2 I rounded it up to 4.

    do you have any suggestions?
     
  5. MrChips

    Moderator

    Oct 2, 2009
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    The input impedance of an op-amp circuit can be designed to be very high and thus prevent high voltage feedback. You can add an additional 100kΩ resistor in series with the MCU output for additional protection depending on the configuration of the op-amp.

    If you truly require galvanic isolation you can use an opto-isolator. You can go with linear coupling or you can use PWM output from the MCU.

    You can also use a digital pot.

    There are many ways to skin this cat.
     
  6. paulepc

    Thread Starter Member

    Apr 2, 2014
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    Have not heard of a digital pot can you give more info please.
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    A digital pot accepts a digital input (like binary or BCD) and has an output similar to a traditional mechanical pot, however they are generally limited to 5v for supply voltage, so this won't solve your problem.

    I'd simply use a 1.2k resistor between the uC output and opamp input; that way even if the opamp got fried and internally shorted to +12v, the resistor would limit current to 10mA maximum (assuming the uC output was 0v, and Vcc was +12v and Vee is -12v).
     
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  8. crutschow

    Expert

    Mar 14, 2008
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    Better to round it down to 3V to avoid confusion.;)

    If the only thing you are concerned about is keeping the 12V from damaging the MCU then just add a resistor in series with the output (or a resistor at the input to the op amp) to limit any current, such as MrChips suggested. The MCU output has protection diodes to limit the voltage as long as the current is limited. In any event it's quite unlikely that an op amp input will fail such as to apply 12V at their inputs.

    You could using a inverting op amp to generate the ±10V with a 20kΩ input resistor to the MCU from the inverting input. Use a 133kΩ feedback resistor to give a 20V output span for a 0 to 3V input. Use a 200kΩ pot connected as a rheostat from the -12V supply to the inverting input to get a +10V offset for the output so 0 to +3V input will give +10V to -10V output. Note that this gives a signal inversion but you can easily invert the signal in the MCU software to compensate.
     
  9. paulepc

    Thread Starter Member

    Apr 2, 2014
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    Thank you all. I have been told that the Mc has no protection, and I already lost 1!

    I have used pots in the region of 100K but it is difficult to fine tune a nice centre zero volts.

    I will try crutschow's suggestion.
    Paul
     
  10. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    A micro running on 3.3V should be able to work for all eternity driving the input of any op amp running any supply: an op amp input should not drive any back voltage.

    If your circuit does supply such a back voltage it has either failed or is incorrectly designed, or something else.

    Best for you to post that portion of your circuit (schematic) so may eyes can give it a look-see.

    Even if the back voltage is a fault condition some of us have designed for fault conditions to prevent secondary failures, or even to keep on working despite failures.
     
  11. paulepc

    Thread Starter Member

    Apr 2, 2014
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    Here is the circuit. problem is if the microprocessor is switched off the end output goes to -15 Volts which must not happen because the motion controller will go flat out.
    the circuit needs to go to zero volts if there's no input.
     
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