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Monitor low AC current of a 24 VAC circuit.


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Is there a Shelly device that can measure from 100 to around 600 milliamps of AC in a 22 to 28 VAC circuit?  The device would be powered with either a 10 to 15 VDC source or a 24 VAC source.  

The device would not be required to switch anything.   I just need to be able to monitor the AC current via a WiFi connection.

I want to monitor sprinkler valve current to know when one of them does not pass current.  For example when a coil opens or when the wiring to one fails.  I would monitor the current of the 24 VAC leg which is common to all of the valves.  When the system is operating normally, there would be two 24 VAC valves turned on with each drawing 190 to 300 mills. 

While the device wouldn't be required to switch anything, an added bonus would be to be able to open the circuit to the valves if a current higher than ~500* mills was measured indicating more than two valves were turned on by some unusual condition.   *(The holding current per valve, 190 mills, is lower than the pickup current, 300 mills, required)

Thanks!

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So not directly answering to your request, I use a standard Shelly (in my case Shelly 1PM, but any PM Shelly will work) and I measure the AC current on the input of the 24V AC transformer instead of the output.

Here is an example for power graph in Home Assistant:

image.png.d9ff5573a0bfc7465d99eb98d7c4beb0.png

The power here (about 3.3W) includes some constant power since I measure other components, but if for example a coil is disconnected the power will go down by 2W to 1.4W.

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Yes....  I wondered whether that would work.  I have a Shelly plug that can monitor current.  My 24VAC transformer powers some other ancillary items too.  But as you illustrate, the current draw by a valve should be detectable.

I also considered placing a 1 or 2 ohm resistor in series with the common 24VAC leg feeding the valves and using a bridge rectifier and capacitor to convert the AC voltage across the resistor to DC to be read by a Shelly uni.  I wasn't sure whether the UNI would be the best Shelly device to be used for that.

As you point out, measuring the current draw of the transformer would be cleaner and workable.

Thank you Shay!

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Providing power to the 110VAC transformer through the Shelly Plug works well.  Readings are useable.

0 valves on, the current is .069 amps and 5.1 watts

1 valve   on, the current is  .116 amps and  8.2 watts

2 valves on, the current is .158 amps and 11.0 watts

 

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