How do I size an isolation transformer I'm using to emulate a variable reluctance sensor?

Question

I'm attempting to emulate a variable reluctance sensor on an engine. I know I need to produce a 600hz-6000hz zero-crossing +/-12volt waveform. The engine management controller's input measures 20okhm.

My first attempt used a Bourns SM-LP-5001 1:1 audio transformer for isolation. My LM2904 went into current limiting mode when I fed it any waveform on CKP1/CKP2. My assumption is that I mismatched something causing the current to exceed the LM2904's capacity. VIN is nominal automotive 12VDC, I am using 13.9VDC for my bench supply.

Here's what I'm thinking for my next attempt, but I need some understanding how to specify what transformer I need.

I found a Xicon 42TM018-RC audio transformer that has 1:1 10k CT winding and 550 ohms DCR on it's primary.

1. Since the ECM measured 20kohm at it's inputs with my DMM, does my transformer need to match 20kohm primary impedance?
2. Or will this circuit fail for a different reason?
3. Is there another approach that would work better?

Answer 1

It goes into saturation. When the flux rises above a saturation point, then the inductance of the transformer is equal to the air core inductance. So, except the series resistance nothing will stop the current rise as short circuit.

What you have to see is the volt second product. From the lowest frequency calculate half period, then multiply with voltage. Compare the volt second product with datasheet.

PS; Have you measured the output signal of the sensor? I don't believe it outputs +/-12V not nearly, you will burn the ECU!

EDIT:

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