Cheap, easy solution to determine if AC current is is flowing or if ACV present

[viper]

We are working on a device and I simply need to know if/when a motor is running.  It pulls 7-12A AC.  I probably don't 'need' to know how much current is on the line, but it would be helpful.  Otherwise, simply knowing when it is on or not is really the main parameter. 

The constraints here is the line cannot be opened so something needs to clamp onto or around the wire.  A single conductor will be available with room to get something on it.  CTs are rarely cheap and since our requirement is low, I am hoping for a penny cheap solution here?  How to non contact V meters work? 

[bob91343]

Clamping on the wire wlll measure its magnetic field, indicating whether current is flowing.  You can't measure voltage that way.  The cheap clampon meters usually have low sensitivity, which can be increased by winding several turns of wire around it.

Basically it's a current transformer.  Google it.

[Benta]

I assume you're talking 60 Hz here?
Do you have the option of looping one wire into a pig tail?
In that case, stick a piece of soft iron through the loop and make a second loop on the iron with another wire. Now you have your current transformer.

[SmallCog]

We use a little current sensing transformer coil thing to do that.

The ones we use require the wire to be unhooked  and passed through the centre of it but I'm sure you could come up with a solution for that

In our case the signal goes into a module from Omega to turn it into a 4-20mA signal which is fed into a PLC

[Benta]

Not exactly a "penny cheap solution", eh?

[Ian.M]

I've been there, done that and got the tee-shirt in the days before digital energy metering, and if you cant break the circuit to thread the wire through the core of your improvised sensor^*,  you'll need a split core which pushes both the cost and mechanical complexity to secure it up significantly.  The only saving grace is you only need on/off indication, so as linearity isn't a concern, you can use a much smaller core and let it saturate.

Consumer grade clip-on current transformers (as used for energy monitors) are only a few bucks each on Aliexpress.  I'd bet the cost to improvise anything that's reasonably secure and reliable will exceed the cost of one off-the-shelf.

* I used a small mains transformer with the original secondary removed and a single turn of the wire round the center limb in place of it), with a bridge rectifier + burden resistor across the original primary.

[ledtester]

Some ideas for a diy current transformer:

http://www.kerrywong.com/2011/07/16/simple-current-transformer/

Making a Current Transformer from Washers - Leo's Bag of Tricks
https://youtu.be/ejfSI7v28vQ

http://www.seekic.com/circuit_diagram/Communication_Circuit/FERRITE_BEAD_CURRENT_TRANSFORMER.html

[langwadt]

https://youtu.be/P47pjVyPP3w

[ledtester]

https://youtu.be/P47pjVyPP3w

That's a good video - thanks for reminding me about that one!

The trick at the end about how to use a split current transformer without having to separate your mains wiring is neat. Another option is to get a "ac line splitter" such as:

https://www.amazon.com/Tekpower-M920-Line-Splitter-Separator/dp/B003A0MD48/

[ejeffrey]

Non-contact voltage sensors are just a metal plate for capacitive pickup and a ton of gain.  They will give you a present/not-present signal for AC voltages but won't tell you anything about the current flow.  For that you need a current transformer.

[robert.rozee]

give the attached a try.

cheers,
rob   :-)

[viper]

I assume you're talking 60 Hz here?
Do you have the option of looping one wire into a pig tail?
In that case, stick a piece of soft iron through the loop and make a second loop on the iron with another wire. Now you have your current transformer.

I really appreciate all the ideas and solutions guys!  I will certainly save the DIY CT provided above as we have a few other projects going. 

However, the thought from Benta is along the lines of something I was thinking, and yes, there is enough slack in the conductor where one or more loops can be created.  I am curious if you could better explain your idea?   

[Benta]

Well, take an iron bolt lying around and make one loop around it with one of the conductors.
Wind, say, 10 loops around it with another conductor (thinner) and connect a resistor across the ends of that.
That's a current transformer, and you'll be able to measure a voltage across the resistor.