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

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 data*** that
the best CTR is going to occur when IF is around 10ma. The data
*** indicates that the CTR for the 4N27 ranges from 10% (min) to 30%
(typical).

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
.

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