# inductor design

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#### glen

##### New Member
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

##### Member
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

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

#### Klaus

##### New Member
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

#### lavenatti

##### Member
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

##### Member
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.

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