Current transformer Wire wrapping

Woops mis-spoke on one point.

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What I meant to say was Why am I getting low voltage on the Secondary.

A current transformer transforms current, so the voltage in the wire has nothing to do with the output. I perfect current transformer outputs (into a short circuit) a current that is proportional to the current in the primary by the ration of the primary turns count divided by the secondary turns count.

That gives you a turns count of 4/300=.0133. So, if the core were perfect, the transformer would output .0133 amperes for each ampere in the 120 volt line.

Is this open circuit voltage? Current transformers are usually loaded with very low resistance loads. What is the current in the line wire?

Bare (no series resistor) red LEDs usually need about 1.5 volts to make them glow.

Your understanding is pretty weak. ;-)

It sure does. The core needs to be magnetically soft and of high permeability. A steel U bolt is pretty poor on each count. Normally, current transformers are tape wound donuts made of silicon transformer steel or more exotic alloys ,like permalloy or other nickel iron alloys. There are also ones made with interleaved E and I stampings, like those used in all sorts of voltage transformers. Here is an example:

You can make an approximation of one of these by removing the transformer from a wall wart (small wall plug mount supply) and unwind the low voltage winding. Then add a turn so of heavy wire in that space and pass your line current through that wire, using the original line voltage primary as the secondary.

It will work to some extent as a very poor efficiency transformer core.

No. All that is important is that the core easily pass magnetic field, be insulated from the windings, and less important, not circulate current around the core, under the wire. That last point is why good cores are made of lots of layers of metal tape or stacked laminations. These layers are insulated from each other, to keep the core from also acting like a short circuited turn.

I suspect that this is reasonable since most of the energy put into the core by the primary is used up circulating current around the circumference of the bolt. Your .8 volts at the ends of 300 turns means that there is .8/300=2.7 millivolts generated in each turn. That doesn't sound like much, but think how much current the surface of the bolt must be carrying when you think of it as a sheet of metal, several inches wide, wrapped around the 1/4" rod. It must have a resistance well below a milliohm.

If you are willing to wrap a coil by wrapping the wire around a dowel rod that is notched on the ends (like the end of an arrow) so you can pass the wire through a hole, you can do better by stacking up a bunch of washers (with a layer of tape between each for insulation) and wrapping your 30 gauge wire through the hole. Then pass your power lead through, also. But by the time you do all that, you could have made a better one from a voltage transformer with a split bobbin as I described, above. Or you could buy a commercial unit like the one I listed above, or one of the ones at the bottom of this page:

Yes I tried several resistors from 10R to about 360R. Its my understadning you must have a load on the secondary.

Well really I tried a couple of LED's some of lower values as you mentioned. But obviously even for a small LED it was not enough.

If all else fails I may try this, as AC/DC adaptors are cheap. I may get back to you on this. I have have one actually open on my desk right now. Its .25 A and 16.5v. However if you look at my diagram I think I will need to un solder the botom of the lamenated metal Pieces to get at the Primary. May be other tranformers dont have this problem. (I think its from an old modem or something)

.----------. ,´--------- /| ,´-----------/ | /------------/ | +-------------+ | | | o-------o Lamenated Metal Pieces | +---------+ | | | |========o------------o Secondary (16.5v) | |=========| | | | |XXXXXXXXX| | | | |========o------------o Primary (120V) | |=========| | | |#+---------+#----------o Solder | | ' I guess where they put the +-------------+´ bottom part of Transformer in after they have put in winding asembly. (created by AACircuit v1.28.4 beta 13/12/04

Interesting idea.

I can get from my local Electronic store the a "Toroid" Dounut. They don't seem to have anything else eg Current transformers. Nearby anyway.

Can I wrap one of these up. And would I get better response with this ? If So how many wraps would I use. The dounut is probably a 3/4 inch hole and about 1 3/4" across.

Regards

Woops reading this again I think what you mean is was there a load on the line 120v line. Yes I used a 2 Amp drill starting and stopping to draw current. The volt meter certainly showed that there was "action" on the secondary. So it works. It just isn't very much. Of course when you stop the drill the voltage drops to almost nothing on the secondary.

Bordon wrote: (snip)

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I was thinking that you could pull the 16.5 volt winding out, one turn=20 at a time, without taking the core apart.

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Most toroids are ferrite, which is not high enough permeability to=20 work well at 60 Hz. Many are powdered iron (dispersed on insulating=20 material) that are even lower permeability. If they have the specs=20 for the core, you would want to get one that has a permeability of=20

5,000 or 10,000. Most high frequency materials have a permeability=20 between 10 and 1000. Most powdered iron cores have permeability below=20 100.

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I guess it would work better than your U-bolt (the ferrite has a much=20 higher resistance than iron, so it won't short out all your secondary=20 energy), but it would be hard to get 300 turns in there plus the line=20 wire.

Then you have built a very low efficiency current transformer.

use more wraps of magnet wire and houuse wire

most LEDs will work at 2.5V or less

it depends on the U bolt and how much current the AC mains circuit is passing

Iron is better than mild steel (which you probably have) mild steel is better than high tensile, bigger is better.

not really, a bigger problem is that solid metal transformer cores are inefficient, if you can replace your U bolt with a proper transformer core you'll get better results.

that's about as good as you're likely to get. more turns of magnet wire will give more voltage if there's room for them. more turns of house wire will also help.

one way to get more efficiency would be to take a "wall-wart" type plugpack powersupply apart, and take a chisel to the transformer secondary (the bit with the thick wires) and remove them, wind a few turns of your house main in their place and take the output from the primary (originally connected to the plug pins) -- something like that could produce upto 100V... using fewer turns of house wire will reduce the voltage...

Try something that uses more power like a clothes iron, hairdryer, heat gun, or electric jug.

Bye. Jasen

espeically the unemployed ones :)

sometimes if you remove the aluminium (or steel) bracket the I sections are all together and come off easily allowing access to the windings.

sometimes they are interleaved and that doesn't happen.

possibly welded, I jabe a microwave transformer that's welded. it's probably not worth the bother trying to undo the welds (or solder joins) all that Iron is going to require a large soldering iron to unsolder.

much easier is to (carefully) cut the secondary using a sharp knife or chisel as pull it out

that would work, if you get a pre-wound toroid you could just make a few turns of mains wire through the centre, use the high voltage winding for output and ignore the low voltage winding.

Toroids are typically more efficient than sheet-metal transformers. use as many turns as you can.

Bye. Jasen

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Thanks Jasen,

Didn't see your post till today because I was away. I have taken apart my Ac/Dc adaptor and as you and John suggested cut out the secondary. I used pointed wire cutters and just started cutting. I have not wrapped my hot line yet, as some other things have got in the way but I hope to try it out in the next day or so. I may also try salvaging the Ubolt with more wraps and see what happens. If you are still out there I have one other question. I'm assuming that I should wrap using the (white) neutral(AC) (not to be confused with the green ground) wire rather than the (black) hot. This is the way I have been doing it anyway.

Thanks.

Since the neutral wire has very little voltage with respect to ground, but carries the load current, it is the safest conductor to use with your transformer. Remember that one turn around the center leg of the E-I core is made by passing the wire down through one core hole and back up through the other one. It is a shame you didn't pull the low voltage winding out, a turn at a time (while counting turns). By the ratio of no load primary to secondary voltage, this would have told you about how many turns the original primary (now the current transformer secondary) has. But the core should be efficient enough that you should be able to connect an AC milliampere meter across it and, with a known primary current (from, say, the .83 ampere current of a 100 watt, 120 volt light bulb), measure the effective turns ratio of the CT.

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Ouch, no I didn't count the wraps. ( I guess I'm not into music that kind of music. ;))

Yes thanks for the confirmation. White I assumed would be the safest conductor to use .

Wrapping, I suppose what you are doing is re-wrapping what you have taken out. Accept a lot less. I will try one wrap at a time and test it to see what I get with each wrap. I will also test it with different loads.

Bordon wrote: (snip)

That's right. You are replacing a winding that probably had at least a hundred turns with a winding of one to several turns. But it is still a winding.

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If you are doing this as an interesting exercise you might like to visit a discussion on sci.electronics.design back in April this year titled 'current transformer winding wire help please'.

You might also like to visit

for more info. They also supply samples if you are interested in a professional product.

HTH

Gordon

neutral or hot it makes little difference electrically as the current I'm assuming you want to detect will be flowing in both of those conductors.

using the neutral may result in a slightly safer design

nevertheless for a little extra safety it's probably a good idea to earth the metal transformer core

Bye. Jasen

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Yes I will do that. Im a little depressed now because my last two experiments failed. The U bolt (although I may try adding more wire to it) and the transformer Idea. It didn't work either. I believe it was because the wire in the primary winding of the transformer must have been broken. It was a good transformer before I started, but once I went to all the trouble of removing the secondary, etc etc. it didn't seem to work as a current Transformer. I put an ohm meter on the ends of the transformer coil and got nothing mmm that is no continuity. The wire must have got broken some how. Well how the heck to you fix a break in a transformer ?? I tried various tests thinking that maybe it was bad soldering etc. but no luck. Soooo I have taken another transformer apart. But this one is smaller and it looks like taking the wires out will be real fun. It also has some diodes and a capacitor in it. Which I assume is to convert the power to DC and cushion things a bit.

Thanks. I will persevere.

You can sometimes disassemble the laminations, unwind the core using a take-up spool driven by a hand drill held in a vice. solder the break, paint it with some insulating compound (eg nail polish) and re-wind it (driving the core spool with the drill this time) repairing any further breaks caused by snags during the re-winding. if your time is worth more than about $2 an hour don't bother unless you want the experience.

I tried various

a job like this really needs a vice without it it's too easy to stress the terminls or leads going to the primary while you're prodding and poking at the wire in the secondary, and that primary typically has very fine wire - and that smaller one will have even finer wire.

Bye. Jasen

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Thanks.

Im my case because we are just talking a cheap ac/dc adaptor I just put it aside and started on another ac adaptor. But thanks. Its all a learning experience.

Here ya go!

Cheers! Rich