inductor design

[glen]

im trying to make a clamp meter to measure current (ac). the sensor is going to consist of a toroidal core wound with copper. the current carrying wire will pass through the core, inducing a current in the winding. i need some help on the inductor design. what type of toroid, what size, type of ferrite, how many turns, wire type, turn spacing.....
the current that i will be sensing will be the mains coming into my house. 240V, AC, 50Hz, 0.1 - 50 amps.
thanks for any help

 

[Phasor]

type of ferrite

I wouldn't use ferrite for a current transformer. Circular laminated steel is better.

how many turns, wire type, turn spacing.....

The number of turns depends on the transformation ratio. Eg, if you put in 100 turns, you get a transformation ratio of 100:1.
Enamel coated winding wire should be ok. I'm not sure about turn spacing.

Just a safety note - don't open circuit the secondary, while the primary (your mains) is energised. This can induce a high voltage in the secondary, which may destroy the insulation.

 

[john1]

Don't put both wires through your core.
Just use one of them.
Usually the live, just in case there are leaks.

 

[Klaus]

Glen, a word of caution: what you are trying to do might be illegal.
You may NOT connect anything into the mains before your electricity meter and the main fuse. And you MUST have a full electrician's licence (at least here in Australia) to connect anything to the mains wiring other than plugging things into a power socket.
As you need to disconnect something to insert the wire through the toroid you may be technically breaking some rules there.

Regarding making a current transformer, you might be much better off in buying a ready made one with a suitable ratio - and have an electrician wire it in if you are not licensed yourself to do so.
Ferrite is not suitable for 50Hz AC toroid transformers.

Klaus

 

[stevez]

I was pleasantly surprised to find that Radio Shack, here in the US, sells a clamp on meter for $20. It's only good for 30 amps but saved me the trouble of building my own. While not foolproof there is a measure of safety that comes with commercially available equipment.

Klaus is right on target with the warnings. Be careful and follow the rules.

 

[john1]

Klaus, with a 'Clamp-on' type there is no connection to
the circuit being measured. It opens out like a lobster
claw and closes round the wire you want to check.
You are right of course about the ferrite being
unsuitable.

Glen,
I would suggest old transformer laminations.
Make sure theres adequate paper covering the metal
to reduce the chances of getting a short to it.
As i dont think you will want all that many turns for
a normal meter movement, i would suggest using normal
wire, with normal insulation, you may find that easier
to work with.

I agree with Stevez, a shop bought item has a degree of
safety built in.

 

[Klaus]

John1, I'm fully aware what a clamp on meter looks like and does :wink:

Glen was a bit confusing in his post , he first mentioned a clamp meter and then talked about toroid cores etc.
Since a toroid core looks like a donut, and he mentioned passing a wire THROUGH it one can safely assume its not a clamp on core he's trying to make.

Now, a toroid current transformer is something that's usually permanently installed, hence my suggestion to buy a commercial one and have it professionally installed.

Clamp on meters are for instantaneous measurements, one would not normally leave those clamped on permanently and they usually cannot be monitored remotely as the display is built in.
Making one of these, while possible, is bound to disappoint as the opening cores have to align quite well when closed to avoid an air gap which would affect the reading. And he wanted quite a large range, 0.1 to 50 Amp.
Klaus[/code]

 

[john1]

Hi Klaus,

I suppose Glen could have been clearer,
but he hasn't been back so maybe he has
given up on this project.

I would have gone for 0 to 50 if it was me,
i wonder what the first 100mA were for ?

Cheers, John

 

[glen]

thanks guys

thanks for all the ideas. im aware of the legal side of things and would get it properly installed. i just want to monitor the power my house is using. if there is a better way than a toroid then let me know. its just my first idea. i dont want anything that has to break the circuit. i guess it doesnt have to measure as low as 0.1A. say 1-50A would be ok. i know u can buy them already made but the only ones ive seen are $120AUS! prefer to make my own anyway.

 

[lavenatti]

I've seen snap-on transformer cores, you wouldn't have to un-wire anything. You could even assemble the plates from an old transformer around your hot wire and then wind it. Might be a little more labor intensive, but i don't think you'll need that many turns if you amplify the signal before sending it to a meter. I'd start with 10-20 turns and an op-amp based amplifier with a gain of around 100 (or adjustable 10-1000), run it to my voltmeter and see what I got.

 

[Phasor]

run it to my voltmeter and see what I got.

I think you mean ammeter... a CT is a current source, not a voltage source.