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

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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

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## 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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## 32 Discussions

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?

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.

Is there a simpler way? I know just like controlling a relay from your computer, it's easy to control 1 or 2 relays from the parallel port of your computer, but it gets infinately more complicated as you add more. I was wondering if the same goes for these AC clamps, would it be possible to hook even just one up without too major of circuitry?

I have lots of spare/junk components laying around but nothing elaborate like apics or arduinos or even any useful I/C's.

try wiring to the tip and sleeve of a mono 3.5mm male plug, then plug that into your microphone port. this should give you a nice input, just a matter of writing of finding some software to graph it.... i used to have some......

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.

hey thats interesting stuff thanks for the links.

the digikey looks like ~\$11 for a 1" diameter ring.  I couldn't see the dimensions on the radio shack item but half the price. I'd really like to see how the performance matches up so I'll probably try one out eventually.... so you say there's ferrous in a monitor cable too? can you add to that, cause I didn't see any last time I checked.

You should get better results with iron than steel. whats the difference? from what I can remember, iron has more iron in it.  Impurities are added to steel to make it stronger (~ 1% carbon for instance).  Iron often appears rougher, darker, and rusts a lot easier.

regardless, I think you should get good results with the steel. what kind of AAC/mVAC are you getting?

most pc monitor cables have a ferrite choke on one end (see this).  You would have to cut open the cable to get the ring.

1" is probably bigger than necessary for most applications.  I was thinking about 495-3856-ND which has a inner diameter just under 1cm, which should be plenty to go around most any power cable I would measure, even with the thickness of the windings.

Ferrite is very brittle, so it is probably not feasible to cut it to make something that snaps together.  However, for my application it is probably okay to have a solid ring (I would have to break the circuit to put the ring around one wire.

The Radio Scrap snap together choke would be a pain to wrap, because of its length.  But, it does snap together - and is easily available. (Hard to justify a \$1 digikey order).

I saw no deflection on my analog voltmeter set to a 300 mA range. I may not have given it enough load, though - I think I was drawing less than an amp.

Awww I see.  I was pretty happy with the resolution reading my space heater in the range of 5-12 amps plugged into 120 volt outlet.

At ~1/2 amp you would only see 1 mVAC on your voltmeter using an arrangement similar to the C-clamp.  As you play around with materials you may also want to try more windings. Also, I tried once with uninsulated wires and it didn't work so don't go that route.

thanks for the lead on the monitor cables.... Sense I've been looking for ferrite it is kind of hilarious that it turned out to be so close.

what are you using to measure mVAC?  Even when I upped the current to 10A, the voltage was still too small for any of my cheapo multimeters.  I exchanged my c-clamp for a steel snap clip (something like this) with about 5 feet of wire wrapped around it - enough so the windings were snug against each other.  I think I need to build an op-amp based full wave rectifier that averages (so I get a steady DC voltage) with a capacitor, and also tweak it to amplify, so my voltages come out between 0 and 2.5 VDC.

By the way, Radio Shack has some toroids too, along with a different snap-together choke that might be much better for this application, although at \$8, it alone breaks the \$5 price point.  www.radioshack.com/family/index.jsp?categoryId=2032273 at the bottom of the page.

I'm using a Fieldpiece.  It measure VAC in the ranges of 600VAC, 200VAC and 200mVAC (the mVAC range is made for a c-clamp plug in). It is necessary to have resolution in the mVAC range, say around 1-20 mVAC based on the type of clamp you use.  The above setup sounds like you have plenty of windings and steel to get resolution in this range.

I got it working this weekend.  I used JasonT's schematic with a few modifications to amplify & rectify the signal to 0-2.5VDC.  I tested a bunch of core materials, including a c-clamp like yours, the stainless steel snap, the solid toroid ferrite, and the radio shack snap-together ferrite.  The solid toroid ferrite was vastly superior to all the others.  It had over 10x the signal of the snap-together ferrite, which was about 10% better than the c-clamp, which had about twice the signal of the snap.  I noticed that the results of the snap-together ferrite are greatly reduced if the two parts are not firmly snapped together, which isn't surprising.

Getting it working with the hobo was a little tricky. I had hoped to use the Hobo's 2.5VDC supply, but it is pulsed, so it only provides power during the sampling (plus about 20ms beforehand), which is once per second (or less frequently), so I ended up powering it with a little dc transformer I had kicking around.

Given that the solid toroid ferrite is so much better and cheaper than anything else, I think I'll go ahead with making a bunch of them.  I don't really need a snap-together, since I plan on measuring circuits at the breaker box, and it is easy enough to turn off the breaker and remove the wire to slide the toroid around it.  My goal is to have 8 of these running simultaneously (two 4-channel Hobo U12's).

You can improve inductance by making your wrappings tighter around the core. You also should be able to just wrap a coil of wire around the current you  want to measure and still be able to measure current. It would still work the same since a electron moving in a wire will produce a magnetic field, and since that magnetic field changes, it will induce a voltage in the coil of wire you wrapped around it, proportional to the amount of current flowing in that measured peice of wire.

iirc, electrical steel > ferrite > steel > Aluminum in terms of permeability

"You can improve inductance by making your wrappings tighter around the core."

Why would that be?

Inductance = (DiameterOfCore*NumberOfTurns^2)/(Length/Diameter OfCore  + 0.43)

Nothing in the equation mentions the diameter of the wrapping, nor the distance between the wraps and the core.