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Signal isolation - optocoupler

Discussion in 'General Electronics Discussion' started by Lord_grezington, Aug 29, 2013.

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  1. Lord_grezington

    Lord_grezington

    124
    2
    May 3, 2013
    Hello

    I am looking to protect my pcb using an optocoupler.

    I am looking to use low cost components, input into the optocoupler varies (could be 10, 12 or 24Vdc). Output needs to be a standard 5Vdc.

    However I have noticed that all the low cost variations of the optocouplers are only rated up to 6V revers for the revers bias diode. Am I looking at the wrong thing here? A high current is not needed so anything up to 50mA is ok. I will only need a frequency of around 11Khz.

    Optocouplers are good for digital signals, however I am also looking have a analogue 4-20mA signal which I am putting through a 249Ohm resistor to convert into a ~1-5V analogue signal. Is there any possible way of isolating this as well reliably? I only need around 8 bits and the sampling speed will be very low (does not need to be any higher than 100Hz)

    Thanks
     
  2. BobK

    BobK

    7,682
    1,688
    Jan 5, 2010
    Are you planning on putting a reverse voltage on the opto input? If not, you are indeed looking at the wrong thing. You drive an opto just like an LED, so you would just calculate the resistor based on the input voltage.

    Bob
     
  3. Lord_grezington

    Lord_grezington

    124
    2
    May 3, 2013
    ok, fair enough. I can probably put a 5.1V Zener to get the various input voltages needed.

    any idea's on the analogue isolation?
     
  4. Lord_grezington

    Lord_grezington

    124
    2
    May 3, 2013
    I have just had a thought, the output of the opto is a current limiter as its a transistor. If I convert the 4-20mA signal into a voltage through a resistor, the LED on the Opto will dim as the voltage (current) reduces, the photo sensor will then allow a proportional amount of current through- will this not then allow current flow though the transistor however with limited current?

    Is it not then possible to just measure the current through a differential amp and sense resistor and get the 0-5Vdc output?

    Just a thought, may be a flaw somewhere though.
     
  5. BobK

    BobK

    7,682
    1,688
    Jan 5, 2010
    Any opto coupler can be used for a varying output by varying the input current, but I believe some are specifically designed to do that accurately. I have no experience with them though.

    Bob
     
  6. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,271
    Nov 28, 2011
    There are "linear" optocouplers from Infineon (Siemens) and HP. I've had experience with the Infineon IL300. It contains one LED and two phototransistors. One phototransistor is used by the driving circuit to servo the LED current, and the other is used by the receiving circuit. Accuracy is determined by the matching of the two phototransistors, and is probably OK for 8-bit resolution. But you need circuitry on the isolated input side to drive the optocoupler and you need to provide power to that circuitry, using an isolated DC-DC converter, for example.

    I've re-read your first post several times and I can't figure out what you want to do. You say you want to use an optocoupler to protect your PCB, then you talk about frequencies and voltages. You really need to explain your requirements in detail before you will get any good suggestions.
     
  7. Lord_grezington

    Lord_grezington

    124
    2
    May 3, 2013
    Hi Kris - sorry, I can be fairly vague at times

    There are 2 isolated inputs that I need.

    The first is a simple digital on/off which will need to be pulsed at max rate of 11Khz. I have now sorted this one out.

    The second I have a 4-20mA current loop, I can convert this to a 1 to 5V analogue signal using a 249 ohm resistor to gnd. The resolution needed is only 8 bits, the frequency on this is how often I will be checking the analogue voltage on the isolated side to feed into a simple closed loop feedback system.

    I will do some research on the linear optocouplers, thank you for the suggestion.
     
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