$2 Carabiner AC Amp Sensor (aka Current Transducer, CT Sensor, Amp Meter, Split Core Clamp-on Ammeter)

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Introduction: $2 Carabiner AC Amp Sensor (aka Current Transducer, CT Sensor, Amp Meter, Split Core Clamp-on Ammeter)

 Impressed by the usefulness of AC current sensors but put off by the prices? Me too! So I decided to make one.  Purchasing a new split-core current transducers costs around $25. Here is a $2 option.

Here are some creative uses of current sensors:
Real-time Web Based Household Power Usage Monitor
Tweet-a-watt


What you need:
-cheap carabiner (what other use for those fake caribiners!)
-or use a $1 c-clamp for better results
-3' of small gauge wire
-a store bought clamp-on current sensor

Once complete, read your sensor with a millivolt meter ; such as, a multi meter, micro controller, arduino, etc 

Step 1: Wrap the Carabiner and Calibrate

 Wrap your carabiner with enough turns to achieve the required resolutions. I initially used 30 turns with no resolution so I doubled to 60 turns as shown below.

Next, slice/separate the wires of an extension cord in order to test/calibrate your CT sensor.  If you try to sense both the load line and neutral line, the magnetic fields cancel and no current is induced.  

Next, plug something into the extension cord.  I used a space heater. Measure the amperage with a store bought clamp-on meter.  

Calibration
I measured 11.8 amps with my Field Piece meter.

The field piece has been calibrated so that 1amp AC is equal to 1milivolt AC (1AAC/1mVAC).  My Caribiner sensor measured 0.3 milivolt.  

11.8 amps / 0.3 millivolts = 39 AAC/mVAC

We're done! 

If you are considering putting this around any high voltage wiring, consider wrapping all exposed metal parts in electrical tape.




Step 2: Ferrous Core Materials

On further investigation, an aluminum carabiner does not encourage inductance.  A ferrous material is needed. According to this inductor manufacturer, a "core material with a higher permeability than air confines the magnetic field closely to the inductor, thereby increasing the inductance." Classically described, iron has unpaired electrons in its orbital shells that create a polarized atom and these atoms easily arrange and encourage inductance whereas aluminum's electrons are not organized to create a polarized atom and hence the atoms do not arrange about a magnetic field.

Steel alloys are mostly iron.  The results were outstanding. 

Starting with the highest resolution:

1-1/2" C-Clamp ~$1.00
0.45 AAC/mVAC consistent within 0.21 amps between 5 to 12 amps

5/16" steel/zinc U-Lock ~$1.00
1.0 AAC/mVAC consistent within 0.50 amps between 5 to 12 amps

1/4" x 2" zinc U-bolt with nuts $0.75
1.70 AAC/mVAC consistent within 0.65 amps between 5 to 12 amps

1.5" Key Ring ~$0.70
5.9 AAC/mVAC 

Lastly,  results may improve with tighter wrapping of the wire to the core material.



Step 3: More Carabiners

The 16 gauge wire on the large carabiner yielded the best results for aluminum but poor compared to steel at 13.3 AAC/mVAC.

The thin gauge red wire on the large carabiner yielded 30 AAC/mVAC.

Next I tried the cheap $0.50 carabiners.  

The yellow and black wire on the green carabiner yielded 40 AAC/mVAC. 

The red wire on the blue caribiner yielded 24 AAC/mVAC.


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35 Comments

ok i'm starting to get angry with this thing... i haven't been able to get a reading... when I use my multimeter I get 0. using the arduino with this circuit (http://openenergymonitor.org/emon/sites/default/files/current.png) Burden = 27 ohm, C1 = 10 uf, Rvd = 10k ohm
the analog reading is always 510.

any ideas?

This will only work for AC current, not DC current.

Would magnet wire work better than the hookup wire you are using?

I'm not familiar with magnetic wire.... I tried bare copper wire and it seemed it did not function as well as insulated wire.

right, bare copper would short on the clamp and not work any better than just a plain-old clamp.

magnet wire is an insulted wire used in electric motors and electromagnets, so it's designed for inductance. I'm no EE, but it seems like this may improve the resolution of your readings, or make reading smaller currents easier. I could be wrong.

Magnet wire is just copper wire with laquer or enamel on it. :)

lacquer or enamel insulator, yes. the insulation is thinner than the plastic insulation so that coils can be more efficient. that's was my point.

You wrote that it was designed for inductance. I guess you meant without plastic insulation they could be wound tighter and make smaller inductors. :)

 I really want to do this, but how would I attach it to my computer for real-time analysis of current use??

 yea... its been a month... sorry i have no good excuse for you...

probably the easiest way to plug this into your computer is to use the above link: 
www.instructables.com/id/Real_time_Web_Based_Household_Power_Usage_Monitor/

and use the op-amp described (AD8820, AD627 or equivalent) but instead of plugging into the ioBridge you can plug it into an Arduino or something like this.. and use that to connect to the PC. hows that sound? fun... you go! :) Let me know.