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Home Made Current Transformers
By: Dave Johnson  
July 8, 2012


A while back I got call from someone who wanted to monitor the AC current going through a power line, using a current transformer.  He found some nice commercial snap-on current transformers but they were all selling for about $25 each.  He wanted to know if he could somehow build his own transformer.  Some time back I had to do just that.  I needed to light an LED when AC current was flowing through a wire.  I looked around in my lab inventory and found a few snap-on split ferrite cores like the photos below.  These components are normally used as RF filtering devices.  They are often used on video cables, USB cables and some power supply cables.  They help to suppress RF, which might leak out of or leak into sensitive circuits.  Split ferrite cores are more convenient since they can easily be clipped onto an insulated wire carrying 50Hz or 60Hz AC.  This allows some AC voltage, generated by the AC current flowing through a wire, without actually touching the copper conductor.  This is a much safer way to monitor AC current.

The cores I like to use are about 1 inch in diameter and about a half inch thick. Find one with the highest RF filtering specification, meaning the highest impedance at some frequency.  These should have a high permeability.  The winding factor ranges from 0.5 to 2 microhenries per turn squared.  This means that the inductance L in microhenries is equal to WT^2 where W is the winding factor and T is the number of turns around the core.  If the winding factor of the core is 1, with two core halves held together, then 1000 turns will yield an inductance of 1 henry.  Magnet wire around 34 to 36 gage works well.  You can use finer wire to get more turns on the core.  First solder the end of the magnet wire to a 24ga insulated stranded wire, several inches long.  Wind the stranded wire around the core first, then begin winding the magnet wire around the core.  The insulated leader wire will make the assembly more robust and easier to solder later.  Finish the assembly with another insulated wire section.  A few wraps of mylar tape around the wire will help protect the winding.   Some people like to dab some 5 minute epoxy around the wires, to finish the winding.  In the old days, they often used wax to cover the wires.

 The number of turns needed will depend on the application.  A resistor will need to be placed in parallel with the multi-turn winding, to achieve good linearity of X number of AC volts to Y Amps of AC current.  The resistor value will have to be determined experimentally.   A target voltage might be about 100mv AC per Amp.  Commercial current transformers have an inductance of several henries but you may not need that much wire on the core if all you need is a fraction of a volt per Amp.
If your goal is to harvest some energy from the flowing AC current using a current transformer, this small split ferrite core sensing approach is not the most efficient.  Even with 10 Amps of AC current, the harvested power may only be 50 milliwatts or so.  That may be enough to light an LED but not much more.  A better option for energy harvesting is a small iron core transformer with a split bobbin, as shown below.  One of the windings is cut out, leaving an empty slot in the bobbin for the heavy current carrying wire.  However, the iron core method will not allow you to clip onto an insulated wire.  The wire has to be cut and pushed through the core and routed around the bobbin at least once.  Multiple turns will produce a higher secondary voltage.  In theory, with heavy current flowing, you can easily harvest a watt or more of power.
Commercial Clip-on Current Transformer Split Ferrite Core RF Suppression Device
Hand Wound Transformer Core Split Bobbin Transformer

Split ferrite cores and split bobbin transformers are available from these sources:  www.digikey.com, www.mouser.com, www.newark.com, www.jameco.com, www.alleletronics.com, www.goldmineelec.com.

Please send comments to me

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