I'm doing a project to monitor and control 5 separate air conditioner units in my house.

The AC Units run in 240V power (two 120V phases), each unit has its own breaker in the box.

I'm using five current transformers, as outlined here: http://openenergymonitor.org/emon/buildingblocks/ct-sensors-interface to detect the current flowing to the AC (<0.01 = off, between 0.03 and 0.8A just the fan, and 2A and up means full compressor running).

Everything mostly work, the only issue I have is that when I turn on a unit I get a current spike in another unit for a while, and that throw off my logs. I can confirm this reading with my clamp shell ammeter

I think this is because the cables are inducing current on each other (they all go through the same conduit).

Is there anything that I can do on the electrical side to get rid of this effect?

  • \$\begingroup\$ What signals are you trying to monitor? If you get a spike then that is either a reality you must live with OR it's a glitch that only exists in your measurement system. You have to decide which and then you can decide what you do to counter that glitch. \$\endgroup\$
    – Andy aka
    Oct 30, 2015 at 23:33

1 Answer 1


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 – 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).

  • \$\begingroup\$ Back to back Zeners are the way to go for AC current ,or you could put your single zener on the DC output terminals of a small bridge rectifier and tie the AC terminals of the bridge to the current transformer.So Daniels answer could work if set up right .What I always do is rectify the CT with a fullwave bridge and load the DC out with the appropriate burden resistor ,Then I use 1 or 2 simple RC filter sections to get rid of the ripple so the mocro wont get confused.Such filtering greatly reduces transients. \$\endgroup\$
    – Autistic
    Nov 4, 2015 at 9:57
  • \$\begingroup\$ Sorry, didn't realize we were sensing AC here, edit made \$\endgroup\$
    – Daniel
    Nov 4, 2015 at 14:18

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