Measuring current in independient circuits
That spike is likely the compressor/fan/whatever turning on.  Most HVAC units (and anything with a motor in it, really) will draw an immense amount of current when it turns on.  That's why your house light dim when your heat pump kicks on.

I would do this:  place a zener diode that's rated at whatever voltage you would expect to be at max current, and this will clamp the voltage at that.  That way you will never see that spike, and you should just get a flat line on a graph if it tries to stay high current long enough (which would blow the AC breaker).

If 2.5A is the max current you expect, and at that current 5V are visible, then put a 5V zener (or maybe slightly higher) across the contacts like this:







simulate this circuit &ndash; Schematic created using CircuitLab

For AC sensing:







simulate this circuit

That should set you up so that you don't see that massive spike (which is perfectly normal).