new dumb question, electronic noise and opto isolator

Discussion in 'General Electronics Chat' started by shortbus, May 24, 2015.

  1. shortbus

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    Sep 30, 2009
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    When using an opto isolator, to isolate two different circuit "sections". Do you put it on the 'noise producing' section and run wires to the 'quiet' section? Or do you run wires from the 'noisy' section and then to the opto on the 'quiet' section? These circuit "sections" are on two separate boards but belong to the same overall circuit. I know I'm probably not asking correctly, but that's nothing new.
     
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  2. Hypatia's Protege

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    Mar 1, 2015
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    Assuming the concept is that said noise cannot 'pass' the isolator:

    The signal drives the LED (or the driver of same) while the load is connected to the detector -- details depend upon the type and specifications of the output device...

    Best regards
    HP
     
  3. MikeML

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    Opto isolation does little if the two circuit sections share a common power supply, or a common ground connection.
     
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  4. shortbus

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    So it's not really beneficial in those type of circuits? I was planing on using one power supply in both areas. Don't you at least need a common ground to all voltages in a combined circuit?
     
  5. kubeek

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    Optocouplers are ideal when you have a large potential difference between the ground of the measuring circuit and the ground of the sensor.
    This is usually the case where you are sensing say voltage on a large motor, and the motor current is so large that the ground point near the motor is moving up and down some volts compared to the ground at the measuring side.
     
  6. crutschow

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    Mar 14, 2008
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    Yes, you need a common ground between the two circuits if you have only one power supply.

    Whether an isolator reduces noise depends upon how much noise there is between the two grounds.
    If there is significant noise between the two ground (as measured from one ground to the other) such as if one board is analog and the other digital, then an isolator would help reduce that.
    A differential receiver would also help reduce the ground noise effect on the signal if you carry the source ground along with the signal as as input to the differential receiver.

    As to which end of the opto goes where, it depends upon which direction the signal is going.

    What is the nature of the signal?
     
  7. MaxHeadRoom

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    Jul 18, 2013
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    An example where the opto is used for transition from one supply level to another but no galvanic isolation is in the PC controlled AC spindle, the PC .P.S. is at earth ground potential and so is the spindle by virtue of the grounded neutral, so using an opto control voltage transition takes place, but not galvanic isolation.
    Max. ​
     
  8. AnalogKid

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    As for where to locate the opto (assuming you still need one), it depends on the signals. Long wires radiate more than short ones. So if the driver signal is noisey, locating the opto close to the source should radiate less noise into the system. OTOH, it makes the wires to the receiver longer, and long wires also pickup noise. For a better answer we'll need more information about your signals.

    Sometimes it isn't about noise. If the opto is part of an isolation barrier between encrypted and non-encrypted information, it is kept close to the non-encrypted side to minimize radiation of the red signals.

    ak
     
  9. shortbus

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    This question was for my ongoing edm project. In another forum someone suggested that isolation would be needed between my logic circuit and the power/spark circuit. But the more I think about it, it probably wouldn't help. The 'noise' will be mostly radiated AM band, from the spark gap. A small version of the old spark radio. Since the noise is RF an optoisolator probably wouldn't be very effective. Extra decoupling caps might be more effective though?
     
  10. shortbus

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  11. kubeek

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    Look at page 18 in the datasheet - you see the output pair driving the gate relative to the source. In a regular driver you have the bootstrap diode that would charge the 0.1uF capacitor, so with the optocoupled driver the arrangement is the same, except for the 18V bootstrap voltage being tied to the ground of the output stage.
     
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  12. Stuntman

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    Mar 28, 2011
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    It sounds like you are proposing an optocoupler to relay analog signals. Have you done this successfully?

    Some quick internet research showed a few devices built specifically for this task. How interesting.

    It should probably be noted that optocouplers are historically used for digital signal transmission between HV/LV circuits, floating PSU's, AC/DC circuits, differing ground planes, etc.
     
  13. kubeek

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    I was thinking more along the lines of feeding digital gate drive to power transistors without disturbing the controller.
    However there are techniques to couple linear signals quite accurately.
     
  14. ian field

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    Oct 27, 2012
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    There's an appnote for that - can't remember who by, but you basically use a dual opto, one channel passes your analog signal to the isolated circuit, the 2 IRLEDS are put in series and the output transistor of the second channel is wired back to the feedback network of the amplifier driving the 2 IRLEDs.

    Probably not easy to get going right - and there's any number of off the shelf isolation amplifiers, some are opto, some use FM RF and any number of methods, but as long as your signal gets through intact and the device stands the rated isolation voltage - who cares what transmission medium goes on deep within.
     
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