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Need help with optoisolator theory, CTR, If ,???

Discussion in 'Electronic Basics' started by [email protected], Jun 3, 2008.

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  1. Guest

    I am trying to optically isolate the high voltage (0-170VDC)
    relaxation oscillator part of an EDM circuit from the servo motor
    control part (12VDC). The voltage between the tool and work piece is
    zero when the tool is touching the work piece (no sparks). When the
    tool is too high above the work piece, the voltage is 170 VDC (no
    sparks). When the tool is in the right place for sparks, the voltage
    is somewhere between these two extremes. I am wanting the
    optoisolator to map the 0-170VDC to roughly a 0-12VDC range.

    I am trying to use a 4N27 optoisolator. I picked this one because I
    already have a bunch of them on hand. I don’t know much about
    optoisolators but it appears to me after reading the datasheet that
    the best CTR is going to occur when IF is around 10ma. The data
    sheet indicates that the CTR for the 4N27 ranges from 10% (min) to 30%

    I put together a test circuit to try to map the 0-170 VDC to the 0-12
    VDC. I am using 5 -30VDC wall warts wired in series to give me 0, 37,
    75, 112, 150, and 187 VDC. I have a 10K resistor (R1) wired in series
    with the diode on the input side. On the output side, I have +12VDC
    connected to the collector, and a 4.7K resistor (R2) connected between
    the emitter and ground. I am measuring the voltage (Vout) across this
    resistor 4.7K resistor, hoping that it will vary between 0-12VDC in a
    predictable way.

    Here are the experimental results:
    At 37V (input), IF was 4ma, and Vout was 5.26, I out was 1.2 ma,
    making the CTR 30%
    At 75V (input), IF was 2.25 ma, and Vout was 11V, I out was 2.25 ma,
    making the CTR 100%
    At 112V (input), IF was 3.3 ma, and Vout was 11V, I out was 2.32ma,
    making the CTR 70%
    At 150V, 187V, Vout was at 11V.

    Not really what I was expecting. I am looking for a lot more
    variation in Vout, especially in the Vin between 75VDC and 170VDC
    window. What values for R1 and R2 should I use to make this circuit
    behave more like I want it to? Also why is the circuit behaving the
    way it is and not the way I want it to?

    Any help would be greatly appreciated. Thanks
  2. neon


    Oct 21, 2006
    any isolator are to mean one source to another like 0-5vdc to a voltage 0-5vdc sitting at 200 dc.
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