Current transformer Wire wrapping

Discussion in 'Electronic Basics' started by Bordon, Sep 1, 2005.

1. BordonGuest

Im trying to build a current transformer from a U Bolt.

I am a novice so be gentle.

I want to detect 120 volts of current in a household line.

I wrapped a 1/4 inch u bolt in about 300 wraps of 30 Gage Magnet wire.
I have wrapped about 4 wraps of the house wire as the primary.

When I test it I only get about 1/2 volt of power out of the secondary.

I want it to light an led or light. I have one led that is about 9volts.

It is my understanding that I should be getting around 12 Volts with my set
up.

One question I have is does it matter what the ubolt is made out of.
Everybody talks Iron, but of course I assume that this ubolt is made out of
steel or who knows what, from home hardware. ~ Will this work? (I have no
Idea if anything is made out of Iron any more)

It also is zinc plated, and then I covered the bare ubolt in Teflon tape to
make the wire stay and in case of shorts. - is this a problem (eg Zinc)?

I wrapped it from the threaded part to the other end and stopped just before
the other threaded part. and back over the secondary a little because I was
not finished my wrappings. I tried to be neat but of course its not that
easy. I put one bolt on each threaded bolt and then the plate and then
another bolt to hold it on so it didn't move --o|o--- .

So far do you see any problems? And possibly why IM getting low voltage on
the Primary. I have tried to test it with a 2 amp drill. The voltage ranges
from .5 - .8.volts.

thanks.

2. BordonGuest

Woops mis-spoke on one point.
What I meant to say was Why am I getting low voltage on the Secondary.

3. John PopelishGuest

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:
http://dkc3.digikey.com/PDF/T053/1605.pdf

4. BordonGuest

Is this open circuit voltage? Current transformers are usually loaded
Yes I tried several resistors from 10R to about 360R.
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 www.tech-chat.de)

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

Regards

5. BordonGuest

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.

6. John PopelishGuest

Bordon wrote:
(snip)
I was thinking that you could pull the 16.5 volt winding out, one turn
at a time, without taking the core apart.
Most toroids are ferrite, which is not high enough permeability to
work well at 60 Hz. Many are powdered iron (dispersed on insulating
material) that are even lower permeability. If they have the specs
for the core, you would want to get one that has a permeability of
5,000 or 10,000. Most high frequency materials have a permeability
between 10 and 1000. Most powdered iron cores have permeability below
100.
I guess it would work better than your U-bolt (the ferrite has a much
higher resistance than iron, so it won't short out all your secondary
energy), but it would be hard to get 300 turns in there plus the line
wire.

7. John PopelishGuest

Then you have built a very low efficiency current transformer.

8. Jasen BettsGuest

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

9. Jasen BettsGuest

espeically the unemployed ones
sometimes if you remove the aluminium (or steel) bracket the I sections are

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

10. John PopelishGuest

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.

11. BordonGuest

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.

12. BordonGuest

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.

13. John PopelishGuest

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.

14. Jasen BettsGuest

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

15. Gordon WGuest

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

You might also like to visit http://www.coilcraft.com/pwrsense.cfm for more
info. They also supply samples if you are interested in a professional
product.

HTH

Gordon

16. BordonGuest

If you are doing this as an interesting exercise you might like to visit a
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.

17. Jasen BettsGuest

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

18. BordonGuest

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.